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Lipid-anchored protein
Lipid-anchored protein
Lipid-anchored protein
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Lipid membrane with various proteins

Lipid-anchored proteins (also known as lipid-linked proteins) are proteins that are covalently attached to lipids embedded into biological membranes. The lipid-anchored protein can be located on either side of the cell membrane. Thus, the lipid serves to anchor the protein to the cell membrane.[1][2] Such proteins are a type of proteolipids.

The lipid groups contribute to the intracellular localization and the biological function of the protein to which they are attached.[2] The lipid serves as a mediator of the protein association with specific biological membranes and protein-protein interactions.[3][4] The lipidation can also sequester a protein away from its substrate to inactivate the protein and then activate it by substrate presentation.

Overall, there are three main types of lipid-anchored proteins which include prenylated proteins, fatty acylated proteins and glycosylphosphatidylinositol-linked proteins (GPI).[2][5] A protein can have multiple lipid groups covalently attached to specific amino acid residues.[2]

Prenylated proteins

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Isoprene unit

Prenylated proteins are proteins with covalently attached hydrophobic isoprene polymers (i.e. branched five-carbon hydrocarbon[6]) at cysteine residues of the protein.[2][3] More specifically, these isoprenoid groups, usually farnesyl (15-carbon) and geranylgeranyl (20-carbon) are attached to the protein via thioether linkages at cysteine residues near the C terminal of the protein.[3][4] This prenylation of lipid chains to proteins facilitate their interaction with the cell membrane.[1]

Caax Box

The prenylation motif "CaaX box" is the most common prenylation site in proteins, that is, the site where farnesyl or geranylgeranyl covalently attach.[2][3] In the CaaX box sequence, the C represents the cysteine that is prenylated, the A represents any aliphatic amino acid and the X determines the type of prenylation that will occur. If the X is an Ala, Met, Ser or Gln the protein will be farnesylated via the farnesyltransferase enzyme and if the X is a Leu then the protein will be geranylgeranylated via the geranylgeranyltransferase I enzyme.[3][4] Both of these enzymes are similar with each containing two subunits.[7]

Roles and function

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Prenylation chains (e.g. geranyl pyrophosphate)

Prenylated proteins are particularly important for eukaryotic cell growth, differentiation and morphology.[7] Furthermore, protein prenylation is a reversible post-translational modification to the cell membrane. This dynamic interaction of prenylated proteins with the cell membrane is important for their signalling functions and is often deregulated in disease processes such as cancer.[8] More specifically, Ras is the protein that undergoes prenylation via farnesyltransferase and when it is switched on it can turn on genes involved in cell growth and differentiation. Thus overactiving Ras signalling can lead to cancer.[9] An understanding of these prenylated proteins and their mechanisms have been important for the drug development efforts in combating cancer.[10] Other prenylated proteins include members of the Rab and Rho families as well as lamins.[7]

Some important prenylation chains that are involved in the HMG-CoA reductase metabolic pathway[1] are geranylgeraniol, farnesol and dolichol. These isoprene polymers (e.g. geranyl pyrophosphate and farnesyl pyrophosphate) are involved in the condensations via enzymes such as prenyltransferase that eventually cyclizes to form cholesterol.[2]

Fatty acylated proteins

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Fatty acylated proteins are proteins that have been post-translationally modified to include the covalent attachment of fatty acids at certain amino acid residues.[11][12] The most common fatty acids that are covalently attached to the protein are the saturated myristic (14-carbon) acid and palmitic acid (16-carbon). Proteins can be modified to contain either one or both of these fatty acids.[11]

Myristoylation

N-myristoylation

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N-myristoylation (i.e. attachment of myristic acid) is generally an irreversible protein modification that typically occurs during protein synthesis[11][13] in which the myrisitc acid is attached to the α-amino group of an N-terminal glycine residue through an amide linkage.[2][12] This reaction is facilitated by N-myristoyltransferase . These proteins usually begin with a Met-Gly sequence and with either a serine or threonine at position 5.[11] Proteins that have been myristoylated are involved in signal transduction cascade, protein-protein interactions and in mechanisms that regulate protein targeting and function.[13] An example in which the myristoylation of a protein is important is in apoptosis, programmed cell death. After the protein BH3 interacting-domain death agonist (Bid) has been myristoylated, it targets the protein to move to the mitochondrial membrane to release cytochrome c, which then ultimately leads to cell death.[14] Other proteins that are myristoylated and involved in the regulation of apoptosis are actin and gelsolin.

S-palmitoylation

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Palmitoylation

S-palmitoylation (i.e. attachment of palmitic acid) is a reversible protein modification in which a palmitic acid is attached to a specific cysteine residue via thioester linkage.[2][11] The term S-acylation can also be used when other medium and long fatty acids chains are also attached to palmitoylated proteins. No consensus sequence for protein palmitoylation has been identified.[11] Palmitoylated proteins are mainly found on the cytoplasmic side of the plasma membrane where they play a role in transmembrane signaling. The palmitoyl group can be removed by palmitoyl thioesterases. It is believed that this reverse palmitoylation may regulate the interaction of the protein with the membrane and thus have a role in signaling processes.[2] Furthermore, this allows for the regulation of protein subcellular localization, stability and trafficking.[15] An example in which palmitoylation of a protein plays a role in cell signaling pathways is in the clustering of proteins in the synapse. When the postsynaptic density protein 95 (PSD-95) is palmitoylated, it is restricted to the membrane and allows it to bind to and cluster ion channels in the postsynaptic membrane. Thus, palmitoylation can play a role in the regulation of neurotransmitter release.[16]

Palmitoylation mediates the affinity of a protein for lipid rafts and facilitates the clustering of proteins.[17] The clustering can increase the proximity of two molecules. Alternatively, clustering can sequester a protein away from a substrate. For example, palmitoylation of phospholipase D (PLD) sequesters the enzyme away from its substrate phosphatidylcholine. When cholesterol levels decrease or PIP2 levels increase the palmitate mediated localization is disrupted, the enzyme trafficks to PIP2 where it encounters its substrate and is active by substrate presentation.[18][19][20]

GPI proteins

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Structure of the glycophosphatidylinositol anchor in the plasma membrane of a eukaryotic cell

Glycosylphosphatidylinositol-anchored proteins (GPI-anchored proteins) are attached to a GPI complex molecular group via an amide linkage to the protein's C-terminal carboxyl group.[21] This GPI complex consists of several main components that are all interconnected: a phosphoethanolamine, a linear tetrasaccharide (composed of three mannose and a glucosaminyl) and a phosphatidylinositol.[22] The phosphatidylinositol group is glycosidically linked to the non-N-acetylated glucosamine of the tetrasaccharide. A phosphodiester bond is then formed between the mannose at the nonreducing end (of the tetrasaccaride) and the phosphoethanolamine. The phosphoethanolamine is then amide linked to the C-terminal of the carboxyl group of the respective protein.[2] The GPI attachment occurs through the action of GPI-transamidase complex.[22] The fatty acid chains of the phosphatidylinositol are inserted into the membrane and thus are what anchor the protein to the membrane.[23] These proteins are only located on the exterior surface of the plasma membrane.[2]

Roles and function

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The sugar residues in the tetrasaccaride and the fatty acid residues in the phosphatidylinositol group vary depending on the protein.[2] This great diversity is what allows the GPI proteins to have a wide range of functions including acting as hydrolytic enzymes, adhesion molecule, receptors, protease inhibitor and complement regulatory proteins.[24] Furthermore, GPI proteins play an important in embryogenesis, development, neurogenesis, the immune system and fertilization.[21] More specifically, the GPI protein IZUMO1R (also named JUNO after the Roman goddess of fertility) on the egg plasma has an essential role in sperm-egg fusion. Releasing the IZUMO1R (JUNO) GPI protein from the egg plasma membrane does not allow for sperm to fuse with the egg and it is suggested that this mechanism may contribute to the polyspermy block at the plasma membrane in eggs.[25] Other roles that GPI modification allows for is in the association with membrane microdomains, transient homodimerization or in apical sorting in polarized cells.[21]

References

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Lipid-anchored protein

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Lipid-anchored proteins are peripheral membrane proteins covalently modified by the attachment of lipid groups, such as fatty acids, prenyl groups, or (GPI), which embed them into cellular membranes and regulate their localization, interactions, and functions in processes like signaling and trafficking. This lipidation enhances protein hydrophobicity, enabling precise targeting to specific membrane domains and facilitating dynamic associations that are often reversible, unlike integral membrane proteins that span the bilayer. The primary types include N-myristoylation, where a 14-carbon myristoyl group attaches irreversibly to an N-terminal residue, commonly seen in Src family kinases; S-palmitoylation, a reversible linkage of a 16-carbon palmitoyl group to residues, as in Ras proteins; prenylation, involving irreversible thioether bonds with 15- or 20-carbon isoprenoids (farnesyl or geranylgeranyl) on C-terminal cysteines, critical for small like Rho and Rab; and GPI anchoring, a complex structure linking proteins to the outer leaflet of the plasma membrane, exemplified by . These modifications are catalyzed by specific enzymes, such as N-myristoyltransferases for myristoylation and DHHC family proteins for palmitoylation, and serve essential roles in eukaryotic , including (e.g., G-protein coupled receptor pathways), vesicular transport, and cytoskeletal organization. In humans, approximately 150 proteins are GPI-anchored, while thousands undergo fatty or , underscoring their prevalence across species from to mammals. Aberrant lipid anchoring contributes to pathologies, including oncogenic mutations in prenylated Ras leading to cancer and defective palmitoylation in neurodegenerative diseases like Alzheimer's.

Overview

Definition and characteristics

Lipid-anchored proteins, also known as lipid-modified proteins, are a class of peripheral membrane proteins that become covalently linked to molecules through co- or post-translational modifications, thereby facilitating their stable or reversible association with cellular s without the need for transmembrane domains. This lipidation increases the hydrophobicity of otherwise soluble proteins, allowing them to partition into the and achieve precise subcellular localization essential for their roles in cellular processes. The plasma membrane, primarily composed of a phospholipid bilayer with amphipathic phospholipids such as and forming the core structure, along with modulating fluidity, creates a hydrophobic barrier that separates intracellular compartments from the extracellular environment. Peripheral membrane proteins, which interact with the membrane peripherally rather than integrating fully across it, depend on lipid anchors to overcome the energetic barrier of membrane insertion and to target specific lipid environments, such as rafts or microdomains, thereby enabling efficient signaling and trafficking without spanning the bilayer. Key characteristics of these proteins include the diversity of lipid groups involved, such as isoprenoid moieties (e.g., farnesyl or geranylgeranyl), saturated fatty acids (e.g., myristate or palmitate), and complex glycolipids like (GPI). Attachment occurs either co-translationally, as seen in N-terminal myristoylation during nascent polypeptide synthesis, or post-translationally for modifications like , S-palmitoylation, and GPI anchoring in the or . While most anchors form irreversible covalent bonds—such as stable amide linkages in myristoylation or thioether bonds in —palmitoylation stands out as reversible due to its labile linkage, permitting dynamic cycling between membrane-bound and soluble states. These proteins are broadly classified into prenyl-anchored, acyl-anchored, and GPI-anchored categories based on the nature of the lipid modification.

Biological importance

Lipid-anchored proteins are evolutionarily conserved across eukaryotes and present in some prokaryotes, reflecting their fundamental role in cellular function since early eukaryotic evolution. Mechanisms such as , myristoylation, and GPI anchoring have been maintained through billions of years, with homologs of key enzymes like N-myristoyltransferase identified in diverse taxa from fungi to mammals, underscoring their ancient origins and essentiality for membrane-associated processes. These proteins play critical roles in maintaining cellular architecture by tethering soluble proteins to , thereby organizing domains and facilitating structural integrity. They enable rapid signaling through precise localization at the plasma or intracellular compartments, allowing efficient protein-protein interactions and cascades. For instance, they are vital in pathways involving small like Ras, which require membrane anchoring for activation, and Src family kinases, which depend on modifications for recruitment to signaling hubs. Lipid-anchored proteins represent a substantial portion of the eukaryotic , with S-palmitoylation alone affecting approximately 10% of human proteins, underscoring their widespread impact on cellular . This prevalence highlights their significance, as disruptions in lipid anchoring can impair membrane dynamics and signaling fidelity. The diversity of anchor types—ranging from prenyl and acyl groups to GPI moieties—further amplifies their functional versatility in eukaryotic cells. Their discovery traces back to the 1970s and 1980s, initially through studies on viral proteins and oncogenes, where modifications were linked to membrane association and transforming activity. Seminal work identified farnesylation in fungal mating factors in 1978 and extended to Ras oncoproteins in the early 1980s, revealing how these anchors drive pathological signaling in cancer. Subsequent characterizations of myristoylation in 1982 and palmitoylation in 1979 solidified the paradigm of anchoring as a key post-translational regulator.

Classification of lipid anchors

Prenyl anchors

Prenyl anchors are hydrophobic isoprenoid groups that covalently modify proteins to facilitate association. The two primary types are the farnesyl group, a 15-carbon chain composed of three units derived from (FPP), and the geranylgeranyl group, a 20-carbon chain consisting of four units derived from (GGPP). These anchors are attached via a thioether linkage to the sulfhydryl group of a residue, providing irreversible targeting distinct from the reversible of acyl anchors. The specificity of prenyl anchor attachment is determined by the C-terminal CAAX motif of the target protein, where C represents the modified residue, the two A's are typically aliphatic such as , , or , and X is a variable residue that influences the choice of prenyl group. Proteins with X as serine, , , , or are preferentially farnesylated, while those with X as , , , or are typically geranylgeranylated. This motif serves as the recognition signal for the prenyltransferases. The attachment process is catalyzed by specific prenyltransferases: farnesyltransferase (FTase) transfers the farnesyl group from FPP to the in the CAAX motif, while geranylgeranyltransferase I (GGTase I) transfers the geranylgeranyl group from GGPP to the same motif; GGTase II handles a subset of substrates but is primarily associated with non-CAAX motifs. Following , which occurs in the , the modified proteins are transported to the for further processing: the AAX tripeptide is cleaved by RAS-converting CAAX 1 (RCE1), exposing the carboxyl group of the prenylated , which is then methylated by isoprenylcysteine carboxyl methyltransferase (ICMT) to enhance hydrophobicity.

Acyl anchors

Acyl anchors consist of fatty acyl groups, typically derived from saturated or unsaturated fatty acids, that are covalently attached to proteins to facilitate membrane association. These anchors commonly involve chains such as the 14-carbon saturated myristoyl (C14:0) or the 16-carbon saturated palmitoyl (C16:0), though unsaturated variants can also occur. The fatty acyl moieties are linked to the protein via specific chemical bonds: an amide bond at the N-terminal residue for myristoylation, or a bond to the atom of a residue for palmitoylation. These linkages enable the hydrophobic acyl chain to interact with the , with chain length and saturation influencing the depth and stability of membrane partitioning. Shorter chains like myristoyl promote shallower insertion into the membrane, primarily through hydrophobic interactions at the interfacial region, while longer chains such as palmitoyl allow for deeper penetration into the acyl chain region of the bilayer, enhancing overall membrane affinity. Saturated chains generally provide more ordered and stable anchoring compared to unsaturated ones, which introduce kinks that may reduce partitioning efficiency. A key distinction among acyl anchors is their reversibility, which depends on the bond type. linkages, as in N-myristoylation, form stable, generally irreversible attachments that persist throughout the protein's lifetime unless specifically cleaved. In contrast, bonds in S-palmitoylation are dynamic and labile, permitting enzymatic cycling between acylated and non-acylated states through thioesterases, which allows for regulated binding and release.

GPI anchors

Glycosylphosphatidylinositol (GPI) anchors represent a distinct class of lipid modifications characterized by a composite structure that indirectly tethers proteins to the via a glycan linker. The conserved core of the GPI anchor features an ethanolamine-phosphate (EtNP) moiety amide-bonded to the protein, connected to a tetrasaccharide glycan chain consisting of three α1-2- and α1-6-linked residues () attached to a non-N-acetylated (GlcNα1-6), which links to the myo-inositol ring of (PI). This backbone structure, denoted as EtNP-P-6Manα1-2Manα1-6Manα1-4GlcNα1-6myo-inositol-P-PI, is evolutionarily preserved across diverse eukaryotes, providing a stable yet flexible association. Structural variations in GPI anchors arise across species and even within the same organism, enhancing functional diversity. In mammalian cells, the first (Man1) frequently carries an additional EtNP substituent at the 2-position, while some anchors incorporate a fourth mannose or branched extensions such as (GalNAc), , or on the glycan core. The PI moiety also exhibits heterogeneity, including diacylglycerol forms, alkyl-acyl variants, or ceramide-based phospholipids in organisms like and , which influence membrane integration and stability. Attachment of the GPI anchor to proteins occurs post-translationally at the C-terminus in the endoplasmic reticulum. A hydrophobic C-terminal signal peptide, typically 15–30 residues long, directs the process; the signal is cleaved between the omega (ω) site—a small, uncharged amino acid such as glycine, alanine, or serine—and the ω+1 position, allowing the exposed carboxyl group at the ω site to form an amide bond with the EtNP amino group of the preassembled GPI. This site-specific linkage ensures precise orientation, with the ω residue contributing to transamidase recognition without steric hindrance. In terms of membrane topology, GPI anchors predominantly localize to the outer leaflet of the plasma membrane, where the PI lipid tails embed into the bilayer's exoplasmic face, positioning the attached protein ectodomain extracellularly. This asymmetric distribution facilitates regulated release of GPI-anchored proteins through enzymatic cleavage by phospholipases, such as phosphatidylinositol-specific (PI-PLC) or (PLD), which sever the anchor at the inositol-phosphate linkage.

Prenylated proteins

Attachment mechanism

Protein prenylation is a involving the covalent attachment of isoprenoid lipids—either farnesyl (15-carbon) or geranylgeranyl (20-carbon)—to residues near the of target proteins via stable thioether bonds. This process is catalyzed by prenyltransferases: farnesyltransferase (FTase) and geranylgeranyltransferase type I (GGTase-I) recognize the CaaX motif, where C is , a are typically aliphatic , and X determines the isoprenoid type (e.g., serine, , , or favors farnesylation; favors geranylgeranylation). For Rab proteins, GGTase-II (also called Rab geranylgeranyltransferase) attaches two geranylgeranyl groups to cysteines in CXC or CC motifs, often with Rab escort protein (REP) as a cofactor. Following , the AAX tripeptide is cleaved by proteases such as Rce1 or Ste24, exposing the prenylated for by isoprenylcysteine carboxyl methyltransferase (ICMT), which enhances hydrophobicity and membrane association. This irreversible modification occurs co-translationally in the and is essential for proper protein localization, often complemented by polybasic regions or secondary palmitoylation in some cases like H-Ras.

Specific functions and examples

Prenylated proteins play crucial roles in membrane targeting, protein-protein interactions, and regulation of cellular processes including , cytoskeletal dynamics, and vesicular trafficking. The hydrophobic prenyl anchor facilitates insertion into lipid bilayers, enabling dynamic associations with specific domains like rafts or endomembranes. Key examples include the Ras family , which are farnesylated and central to mitogenic signaling; oncogenic mutations in Ras, coupled with , drive uncontrolled proliferation in cancers like pancreatic . Rho , geranylgeranylated, regulate cytoskeleton reorganization, , and adhesion, with implications in and cardiovascular diseases. Rab , doubly geranylgeranylated, coordinate vesicular transport between organelles, ensuring protein sorting and secretion; defects in Rab contribute to neuropathies like Charcot-Marie-Tooth disease. Other prenylated proteins include nuclear lamins, which support integrity (mutations linked to ), and G-protein gamma subunits, involved in heterotrimeric G-protein signaling.

N-myristoylated proteins

Attachment mechanism

N-myristoylation is the co-translational or post-translational covalent attachment of a 14-carbon saturated , myristate, to the alpha-amino group of an N-terminal residue via an irreversible amide bond. This modification occurs shortly after protein synthesis begins, typically within minutes, following the removal of the N-terminal initiator by 2 (MetAP2), exposing the for . The process is catalyzed by N-myristoyltransferases (NMTs), enzymes from the GCN5-related N-acetyltransferase superfamily that utilize myristoyl-CoA as the acyl donor. In humans, there are two NMT isoforms, NMT1 and NMT2, which share approximately 77% sequence identity but exhibit distinct substrate specificities and tissue distributions; NMT1 is more ubiquitously expressed, while NMT2 predominates in certain tissues like and testis. The reaction follows an ordered Bi-Bi mechanism, where myristoyl-CoA binds first to NMT, inducing a conformational change that creates the peptide-binding pocket for the protein substrate. Substrate recognition requires a consensus N-terminal , often Met-Gly followed by basic or hydrophobic residues (e.g., Gly-X-X-X-Ser/Thr), though no strict motif exists beyond the exposed . Post-translational myristoylation can occur in specific contexts, such as during when cleave proteins to expose a new N-terminal . Unlike reversible modifications like palmitoylation, N-myristoylation is generally irreversible, though rare demyristoylation has been reported via enzymes like SIRT2 (reversible deacylation) or bacterial effectors like IpaJ from , which cleaves the proximal to the myristoyl group. This modification enhances protein hydrophobicity, promoting association, often in concert with other signals like electrostatic interactions or additional acylations. involves cellular myristoyl-CoA levels and NMT localization, primarily in the and .

Specific functions and examples

N-myristoylated proteins play crucial roles in targeting, protein-protein interactions, and of cellular processes such as , vesicular trafficking, and . The myristoyl group facilitates anchoring to bilayers, often via "myristoyl switches" that allow dynamic binding and release through conformational changes, , or binding. In signaling, myristoylation enables recruitment to lipid rafts or specific domains, modulating activity and cycling. For trafficking, it directs proteins to organelles like the Golgi apparatus or mitochondria, supporting cargo transport and dynamics. Additionally, it influences protein stability by exposing or masking N-degron signals and contributes to host defense by regulating innate immune responses. Prominent examples include the Src family kinases (e.g., c-Src), where N-myristoylation at Gly2 promotes localization and dimerization, essential for activation in cell growth and motility pathways; disruption impairs oncogenic signaling. ADP-ribosylation factor 1 (ARF1), a , undergoes a GTP-myristoyl switch for reversible binding, critical for coat I (COPI) vesicle formation and Golgi trafficking. In , HIV-1 proteins like and Nef rely on myristoylation for assembly and immune evasion; Nef's modification targets it to the plasma to downregulate MHC class I. Other examples highlight diverse functions: recoverin, a neuronal calcium , uses myristoylation for phototransduction in retinal rods, switching between soluble and membrane-bound states upon calcium binding; and Bid, a pro-apoptotic member, is myristoylated post-caspase cleavage to translocate to mitochondria and induce release. In innate immunity, TIR-domain-containing adapter molecule (TRAM) requires myristoylation for Toll-like receptor 4 (TLR4) signaling, facilitating activation and inflammatory responses. Aberrant myristoylation is implicated in cancers (e.g., via NMT overexpression in tumors) and infectious diseases, making NMT a therapeutic target.

S-palmitoylated proteins

Attachment mechanism

S-palmitoylation occurs on residues, typically located near the , as in Src family kinases, or at the , as in H-Ras, though no strict exists; it often follows N-terminal myristoylation to enhance membrane affinity. The enzymatic addition of palmitoyl groups is catalyzed post-translationally by palmitoyl acyltransferases (PATs) from the DHHC family, comprising 23 members in humans, which transfer the palmitoyl moiety from palmitoyl-CoA to form a reversible bond with the cysteine thiol. Depalmitoylation, enabling dynamic cycling, is mediated by thioesterases such as acyl-protein ase 1 (APT1), which hydrolyzes the thioester linkage using a serine-histidine-aspartate . This reversibility distinguishes S-palmitoylation from irreversible anchors like . Regulation of the attachment mechanism relies on enzyme-substrate proximity, frequently at the Golgi apparatus or plasma membrane, where DHHC enzymes localize to facilitate targeted modification. The process is influenced by , with optimal activity at neutral to slightly alkaline conditions, and by acyl chain properties, where both saturated palmitate and unsaturated variants like oleate can be incorporated, affecting modification rates and membrane integration. Recent insights reveal autoacylation of DHHC enzymes, forming an acyl-enzyme intermediate that transfers the palmitoyl group to substrates, and high-throughput approaches in the have identified over 500 palmitoylated proteins across cells, highlighting the modification's prevalence.

Specific functions and examples

S-palmitoylated proteins primarily function in dynamic association, , protein trafficking, and interactions, often in lipid rafts to facilitate localized signaling. The reversibility of palmitoylation allows for rapid regulation of protein localization and activity, distinguishing it from static anchors. In , palmitoylation is crucial for recruiting proteins to membranes and enabling interactions. For instance, H-Ras and N-Ras, small , undergo palmitoylation near their following , which stabilizes plasma membrane localization and allows shuttling to the Golgi for depalmitoylation and recycling; this dynamic cycling regulates downstream signaling in pathways like MAPK/ERK for and survival. Src family kinases, such as Src, , and Lck, are dually myristoylated and palmitoylated near the , promoting targeting essential for T-cell activation and signaling upon receptor stimulation. Neuronal functions are exemplified by proteins like SNAP-25, which is palmitoylated at residues to anchor SNARE complexes at presynaptic membranes, facilitating release and fusion. PSD-95, a postsynaptic protein, relies on palmitoylation for targeting to synaptic membranes, where it clusters receptors and regulates and learning. Additional roles include G-protein signaling, where Gα subunits' palmitoylation enhances interactions with Gβγ and receptors, amplifying responses in pathways like activation. Endothelial nitric oxide synthase (eNOS) palmitoylation at the Golgi regulates its trafficking to caveolae, modulating vascular tone and endothelial function. Unique aspects of S-palmitoylation include its role in heterogeneous acylation, where unsaturated fatty acids can be incorporated (e.g., in Src), influencing and signaling specificity, and its involvement in immune responses, such as antiviral activity of IFITM3 proteins.

GPI-anchored proteins

Anchor structure and biosynthesis

The (GPI) anchor is a complex that tethers proteins to the via a covalent linkage at the protein's . Its core structure consists of a moiety linked to a glycan chain: ethanolamine (EtNP)-6Manα1-2Manα1-6Manα1-4GlcNα1-6-myo--phosphatidylinositol, where GlcN is and Man denotes residues. This structure enables membrane association while allowing lateral mobility in lipid rafts. Variations exist across species; for instance, in trypanosomes like , the inositol ring is acylated, and additional polylactosamine side chains may be present on the glycan core. GPI anchor biosynthesis occurs in the (ER) lumen and involves approximately 10 dedicated enzymes acting in a sequential pathway to assemble the pre-formed anchor before its attachment to the target protein. The process initiates on the cytoplasmic face of the ER with the transfer of (GlcNAc) from UDP-GlcNAc to (PI) by the PIG-A enzyme (also known as GPI-GlcNAc ), forming GlcNAc-PI; this step is rate-limiting and essential for subsequent additions. The GlcNAc is then de-N-acetylated by PIG-L to yield GlcN-PI, followed by flipping of the intermediate to the luminal side via an unidentified . In the lumen, inositol acylation occurs via PIG-W, and the conserved glycan core is built through mannose additions by PIG-M, PIG-V, and PIG-B (using dolichol-P-Man as donor), with EtNP side branches added by PIG-N, PIG-O, and PIG-F to complete the mature precursor (often termed H8). Lipid remodeling, such as conversion from diacylglycerol to 1-alkyl-2-acylglycerol, fine-tunes the anchor's hydrophobicity during or after assembly. Attachment of the GPI anchor to the protein is mediated by a transamidase complex comprising GPI8 (GAA1 in mammals), PIG-K, PIG-S, PIG-T, and PIG-U, which cleaves the C-terminal GPI signal peptide and forms an amide bond between the EtNP on the anchor and the ω-carboxyl group of the protein's C-terminal residue (typically glycine, asparagine, or aspartate at the ω or ω+2 site). Proteins destined for GPI anchoring possess an N-terminal ER-targeting signal sequence for translocation into the ER and a C-terminal GPI signal sequence, consisting of a spacer region, the cleavage/attachment site, a hydrophilic spacer, and a hydrophobic tail that is removed during attachment. This dual-signal system ensures specificity and efficiency in the ER. Quality control mechanisms in the ER monitor GPI-anchored protein (GPI-AP) maturation, involving inositol deacylation by PGAP1 and EtNP removal from the second mannose by PGAP5 to generate export-competent forms; defective intermediates are retained by the p24 receptor complex or directed to ER-associated degradation (ERAD). A significant portion of newly synthesized GPI-APs undergoes or degradation to maintain cellular . Genetic disruptions in this pathway, identified through screens of PIG mutants, underscore its importance; for example, somatic mutations in PIG-A abolish GPI synthesis, leading to (PNH), an acquired hemolytic disorder characterized by deficient GPI-APs on blood cells. Recent advances include comprehensive knockout libraries of GPI biosynthetic genes in cell lines, revealing nuanced roles in anchor maturation and disease modeling.

Specific functions and examples

GPI-anchored proteins primarily function in cell surface signaling, , and enzymatic activity, with additional roles in complement and evasion. These proteins, localized to the outer leaflet of the plasma membrane, enable interactions with extracellular and facilitate processes such as through clustering in lipid rafts. In signaling, GPI-anchored proteins like Thy-1 and recruit Src family kinases and activate Cγ upon binding, promoting cellular responses including proliferation and motility. is mediated by molecules such as contactins and glypicans, which interact with components or other cells to support tissue organization. Enzymatic activities are exemplified by ectoenzymes that perform or other reactions on the cell surface, contributing to uptake and . In complement regulation, GPI-anchored proteins protect host cells from immune-mediated damage. , known as protectin, binds to and inhibits the formation of the membrane attack complex (MAC) by C8 and C9 components, preventing complement-induced lysis. evasion is a key function in certain organisms, where GPI anchors allow rapid antigenic variation on the surface. For instance, the variant surface glycoprotein (VSG) of coats the parasite, shielding invariant proteins from host antibodies and enabling immune escape during infection. Representative examples highlight the diversity of GPI-anchored protein functions. , a GPI-anchored ectoenzyme, hydrolyzes esters to aid in transport and bone mineralization. The protein (PrP^C) serves in and binding but can misfold into PrP^Sc, contributing to neurodegenerative diseases like Creutzfeldt-Jakob disease. These examples underscore the extracellular orientation of GPI anchors, which positions proteins for environmental interactions. Unique aspects of GPI-anchored proteins include their susceptibility to shedding and enrichment in rafts. GPI-specific phospholipases, such as GDE2 and PGAP6, cleave the anchor to release soluble forms, regulating protein levels and generating signaling ectodomains. Additionally, these proteins concentrate in cholesterol-rich rafts, promoting formation that enhances for ligands and efficient signal propagation.

Common biological roles

Membrane targeting and trafficking

Lipid anchors facilitate the targeting of proteins to cellular membranes primarily through hydrophobic insertion of the lipid moiety into the , which is often insufficient alone for high-affinity binding and requires additional mechanisms for specificity. Electrostatic interactions between polybasic regions—clusters of positively charged and residues—and negatively charged phospholipids, such as , significantly enhance the membrane affinity of myristoylated proteins. For instance, in the Src family kinases, myristoylation provides initial hydrophobic anchoring, but the adjacent polybasic domain increases binding affinity up to 1000-fold in membranes containing 33% acidic lipids, as modeled by nonlinear Poisson-Boltzmann equations predicting optimal electrostatic partitioning. Similarly, in Ras isoforms, polybasic domains complement prenyl anchors to promote plasma membrane (PM) localization, with myristoylation serving as an alternative signal that restores targeting when combined with these regions. Specificity in membrane targeting arises from combinations of lipid anchors, which direct proteins to distinct compartments. The dual modification of N-myristoylation and S-palmitoylation, as seen in H-Ras and Src kinases, ensures robust association with the PM by increasing hydrophobicity and enabling partitioning into ordered lipid domains. In contrast, prenylated proteins like K-Ras4b rely on farnesylation paired with a polybasic region for PM targeting, while geranylgeranylation in Rab GTPases facilitates endosomal and Golgi localization. GPI anchors, through their structure, preferentially target proteins to - and sphingolipid-rich lipid rafts, influencing apical sorting in polarized cells. These combinations prevent mislocalization; for example, myristoylation alone directs proteins to intracellular membranes, but adding palmitoylation redirects them to the PM. Lipid anchors play crucial roles in protein trafficking, including , , and inter-organelle transport, by influencing sorting decisions at various cellular hubs. Prenylated anchors, such as farnesyl in H-Ras, promote through early endosomes via interaction with Rab11-positive compartments, enabling dynamic shuttling between the PM and endomembranes. GPI-anchored proteins are sorted into raft-associated vesicles at the trans-Golgi network, facilitating their exocytic delivery to the PM and resistance to non-raft pathways. In inter-organelle transport, myristoylated ARF proteins mediate vesicle budding from the Golgi, while anchor combinations regulate directionality; depalmitoylation of H-Ras, for instance, triggers endosomal release and . Experimental evidence from (FRAP) studies demonstrates that lipid-anchored proteins exhibit slower diffusion rates in raft domains compared to non-raft regions, with GPI-anchored proteins showing temperature-independent mobility around 0.1–0.2 μm²/s, reflecting constrained trafficking. Mutations disrupting anchors, such as farnesyl removal in Ras, cause accumulation in the Golgi and defective PM localization, underscoring their role in sorting fidelity.

Signal transduction and regulation

Lipid-anchored proteins facilitate by tethering signaling molecules to the plasma membrane, enabling spatial proximity that promotes efficient interactions and activation within cascades. Lipid modifications such as S-palmitoylation or GPI anchoring restrict and localize effectors like kinases near their substrates, allowing for allosteric conformational changes and rapid events. In neuronal contexts, for example, myristoylated or palmitoylated anchoring proteins, including A-kinase anchoring proteins (AKAPs), position adjacent to targets, ensuring localized cAMP-dependent responses with timescales from milliseconds to hours. Similarly, in immune signaling, acylated Src family kinases like Lyn associate with receptors in lipid rafts to phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs), initiating downstream pathways such as activation. Partitioning of lipid-anchored proteins into cholesterol- and sphingolipid-enriched further concentrates effectors, amplifying signal specificity and strength by segregating them from non-raft domains. This raft association enhances the recruitment of adaptors and enzymes, such as in (TLR4) signaling where GPI-anchored clusters with MyD88 in rafts to drive production. Rafts also modulate intrinsic protein activities through lipid-protein interactions, preventing aberrant signaling while supporting high-fidelity transduction. Regulation of lipid-anchored proteins occurs via dynamic anchor modifications, with palmitoylation cycles serving as reversible switches that control localization and activity. For Ras isoforms like H-Ras and N-Ras, palmitoylation at the Golgi apparatus, followed by depalmitoylation at the plasma membrane, allowing release into the and driving continuous cycling between the plasma membrane and internal compartments, ensuring only at appropriate sites for effector such as Raf in the MAPK pathway. This acylation-deacylation loop maintains Ras in a signaling-competent state at the membrane while preventing nonspecific retention elsewhere. Depalmitoylases provide feedback control; for instance, ABHD17 proteins catalyze N-Ras depalmitoylation at the plasma membrane, promoting its relocation to endosomes and attenuating oncogenic signaling. Lipid-anchored proteins enable cross-talk between GPCR and RTK pathways by integrating signals through common lipid-modified effectors. Palmitoylated Ras, activated downstream of RTKs like EGFR via Grb2-SOS complexes, can be modulated by GPCR-induced lipid changes, such as those affecting β2-adrenergic receptors, to fine-tune outputs like ERK activation. GPI-anchored proteins in rafts similarly bridge GPCR responses, as seen in their role in enhancing TLR-GPCR synergy for inflammatory signaling. These integrations allow coordinated cellular responses to diverse stimuli. The reliance on lipid anchors for such rapid, localized interactions reflects evolutionary adaptations that optimize signaling speed and precision, particularly in dynamic environments like immune or neuronal systems where quick effector redistribution in rafts initiates cascades.

Pathological and therapeutic aspects

Associations with diseases

Dysregulation of lipid-anchored proteins contributes to various diseases through aberrant membrane localization, signaling, and protein interactions. In , mutations in RAS genes, which encode prenylated proteins, occur in approximately 20-30% of human cancers, leading to hyperactive Ras signaling that promotes uncontrolled due to persistent membrane anchoring via . This prenylation is essential for oncogenic Ras activity, as inhibiting farnesyltransferase disrupts Ras localization and tumor growth. In neurodegenerative disorders, GPI-anchored protein (PrP) plays a central role in prion diseases, where misfolding of PrP^C into the pathogenic PrP^Sc isoform is facilitated by its GPI anchor, which influences sorting, raft association, and propagation. The GPI anchor also contributes to prion infectivity spread by enabling cell-to-cell transmission. In , altered S-palmitoylation of postsynaptic density protein 95 (PSD-95) disrupts synaptic integrity and trafficking, exacerbating amyloid-β-induced synaptic loss and cognitive decline. Similarly, dysregulated palmitoylation of PSD-95 substrates impairs neuronal plasticity in AD models. Other pathologies arise from defects in GPI anchoring, as seen in (PNH), an acquired disorder caused by somatic mutations in the PIGA gene, leading to deficient GPI synthesis and loss of GPI-anchored proteins like CD55 and on blood cells, resulting in complement-mediated . In Hutchinson-Gilford progeria syndrome (HGPS), a in LMNA produces , a permanently farnesylated prelamin A variant that accumulates at the , causing nuclear abnormalities and premature aging phenotypes. Viruses exploit anchoring for ; for instance, myristoylation of HIV-1 Nef protein enables its membrane binding and down-regulation, enhancing viral and immune evasion. Recent studies highlight palmitoylation of the , which is essential for its membrane fusion activity and viral entry, as S-acylation at cytosolic cysteines stabilizes the protein in rafts to facilitate .

Targeting strategies in medicine

Targeting strategies for lipid-anchored proteins in primarily focus on modulating the enzymes responsible for their attachment, such as farnesyltransferase (FTase), geranylgeranyltransferase (GGTase), and protein acyltransferases (PATs), as well as addressing defects in (GPI) anchoring. Inhibitors of FTase and GGTase block , preventing the membrane localization of small like Ras and Rho, which are implicated in oncogenic signaling. For instance, tipifarnib, an FTase inhibitor, has been evaluated in phase 2 clinical trials for poor-risk (AML), demonstrating a complete remission rate of 14% in older adults with untreated . Similarly, GGTI-2418, a selective GGTase I inhibitor, underwent phase 1 testing in patients with advanced solid tumors, establishing a maximum tolerated dose of 2060 mg/m² with manageable gastrointestinal toxicities, though no objective responses were observed. As of 2025, a phase 2a trial of GGTI-2418 (also known as PTX-100) is ongoing in advanced solid tumors. PAT inhibitors, which disrupt S-palmitoylation, offer another avenue for cancer by altering the trafficking and activity of palmitoylated oncoproteins. Analogs of 2-bromopalmitate (2-BP), a broad-spectrum PAT inhibitor, have shown promise in preclinical models; for example, 2-BP reduces proliferation and invasion in head and neck cells by inhibiting Ras palmitoylation and membrane association. More selective inhibitors targeting DHHC-family PATs are under development to enhance specificity and minimize . For GPI-anchored proteins, therapeutic strategies circumvent defects rather than directly restore anchoring. In (PNH), where GPI anchor biosynthesis is impaired, , a inhibiting complement protein C5, prevents intravascular by blocking the terminal complement pathway that targets GPI-deficient cells. This approach has transformed PNH management, reducing transfusion dependence and thrombotic events in clinical trials. Emerging therapies include proteolysis-targeting chimeras (PROTACs) designed to degrade enzymes, potentially overcoming resistance seen with traditional inhibitors by inducing ubiquitin-mediated proteasomal degradation. Although specific PROTACs for FTase remain preclinical, their application to related oncogenic targets highlights their potential for lipid modification pathways in cancer. Biomimetic approaches, such as proteolipid vesicles delivering GPI-anchored proteins, are being explored to restore surface expression in GPI-deficient cells like those in PNH. Challenges in these strategies include off-target effects, as FTase inhibitors can redirect substrates to alternative prenylation pathways, such as converting farnesylated RhoB to geranylgeranylated forms, which may paradoxically promote tumor growth in some contexts. Clinical translation has been mixed; while FTase inhibitors like tipifarnib showed limited efficacy in solid tumors due to such redundancies, , another FTase inhibitor, was FDA-approved in 2020 for , where it inhibits aberrant farnesylation of , improving vascular stiffness and survival. Ongoing trials aim to refine selectivity and combination regimens to mitigate these issues.

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