EEF2

Eukaryotic elongation factor 2 (eEF2) is a highly conserved GTPase encoded by the EEF2 gene on chromosome 19p13.3 in humans, essential for the translocation step during the elongation phase of protein synthesis in eukaryotic cells.^[1] This 858-amino-acid protein catalyzes the GTP-dependent movement of peptidyl-tRNA from the ribosomal A-site to the P-site, enabling the ribosome to advance along the mRNA by one codon and facilitating accurate polypeptide chain elongation.^[2] eEF2 interacts dynamically with the 80S ribosome, undergoing conformational changes that ensure precise decoding and translocation while preventing errors such as frameshifting.^[3] Structurally, eEF2 is organized into five domains (I-V), with domain I containing the GTP-binding G domain, domain II, domain III serving as a flexible linker, and domains IV-V facilitating tRNA and ribosomal interactions; these domains exhibit relative mobility critical for its catalytic function.^[4] The protein is post-translationally modified, including diphthamide formation at histidine 715^[5] (essential for function but targeted by bacterial toxins like diphtheria toxin for ADP-ribosylation and inhibition) and trimethylation at lysine 525, which modulates activity.^[2] Regulation of eEF2 occurs primarily through phosphorylation at threonine 56 by eukaryotic elongation factor 2 kinase (eEF2K), which inactivates it under stress conditions to conserve energy by slowing global protein synthesis.^[1] eEF2 is ubiquitously expressed across tissues, with highest levels in the ovary and thyroid, and plays roles beyond translation, including interactions with viral proteins (e.g., HIV-1 Gag and Vpr) that hijack host machinery.^[1] Mutations in EEF2, such as the P596H variant, are associated with spinocerebellar ataxia type 26 (SCA26), an autosomal dominant neurodegenerative disorder characterized by impaired cerebellar function due to disrupted translocation and increased ribosomal frameshifting.^[2] Dysregulation of eEF2 signaling is implicated in cancer progression, where its hyperactivity promotes tumor growth, positioning it as a potential therapeutic target for inhibitors that modulate translation rates. As of 2025, ongoing research explores eEF2K inhibitors and eEF2-targeted therapies, such as siRNA nanoparticles, for cancers including triple-negative breast cancer.^[6][7]

Gene and Expression

Genomic Organization

The EEF2 gene is located on the short arm of human chromosome 19 at cytogenetic band 19p13.3, spanning genomic coordinates 3,976,044 to 3,985,479 (approximately 9.4 kb) on the reverse strand according to the GRCh38.p14 assembly.^[8] The gene comprises 15 exons, all of which are coding in the canonical transcript ENST00000309311.7, producing a mature mRNA of 3,158 bp that encodes the 858-amino-acid eEF2 protein; the exons are interspersed with 14 introns, contributing to the overall genomic span. Notable pathogenic variants include the P596H mutation associated with spinocerebellar ataxia type 26 (SCA26).^[9][2] The promoter region of EEF2 lies upstream of exon 1 and includes regulatory elements such as transcription factor binding sites identified through ENCODE data, with the gene classified as a housekeeping gene featuring a CpG island that supports constitutive expression.^[10] EEF2 exhibits strong evolutionary conservation across eukaryotes, reflecting its essential role in translation; orthologs are present in fungi, plants, and animals, with human eEF2 sharing over 99% sequence identity with other mammals. The gene also shows homology to archaeal aEF2, underscoring its ancient origin from the last universal common ancestor.^[11][12] Common polymorphisms in EEF2 are typically located in intronic or synonymous regions and exert neutral effects on protein structure and function.

Tissue Expression and Regulation

The EEF2 gene exhibits ubiquitous expression across human tissues, reflecting its essential role in protein synthesis. According to RNA-seq data from the GTEx consortium, median TPM values are notably higher in skeletal muscle (~4,000 TPM), various brain regions (~2,000–3,000 TPM), and liver (~2,000–3,000 TPM) compared to other tissues such as ovary and thyroid (~2,000 TPM); note that older datasets (e.g., NCBI RPKM) suggest relatively higher expression in ovary and thyroid, highlighting dataset-specific variations.^[13][1] This pattern underscores EEF2's heightened demand in metabolically active tissues requiring robust translational capacity.^[14] Transcriptional regulation of EEF2 involves binding sites for key transcription factors in its promoter region, including Sp1, which facilitates basal expression, along with c-Myb, C/EBPalpha, p53, STAT5A, TBP, and YY1.^[10] These factors enable responsiveness to cellular cues, such as nutrient availability and stress signals, where EEF2 transcription adjusts to support adaptive protein synthesis under varying physiological demands.^[10] Post-transcriptional control of EEF2 mRNA includes regulation by microRNAs that influence stability and translation; for instance, miR-143-5p directly targets the EEF2 3' UTR, repressing its expression in contexts like intervertebral disc degeneration.^[15] Additionally, alternative splicing generates multiple isoforms, with Ensembl annotating 27 transcripts, though most are minor variants that may fine-tune tissue-specific functions without altering core activity.^[16] During development, EEF2 expression is upregulated in embryogenesis (e.g., Carnegie stages CS13–CS20) and neuronal differentiation, particularly in neural progenitor and stem cells, supporting the intense protein synthesis needs of proliferating and differentiating tissues.^[10]

Protein Structure

Primary Sequence and Domains

The human EEF2 protein is composed of 858 amino acids, resulting in a molecular weight of approximately 95 kDa.^[5] This primary sequence is highly conserved across eukaryotes, reflecting its essential role in translation elongation. The protein exhibits a theoretical isoelectric point of 6.16.^[17] EEF2's domain architecture is characteristic of translational GTPases, consisting of five major domains (I-V) that facilitate nucleotide binding and ribosomal interaction. Domains I and II form the N-terminal GTP-binding G domain (~residues 1–480, including the G' subdomain ~220–330), domain III (~residues 480–620) serves as a flexible linker connecting the GTPase and effector regions, domain IV (~residues 620–700) adopts a structure mimicking the anticodon loop of tRNA to engage the ribosomal A-site, and domain V (~residues 700–858) provides additional stability to the overall fold.^[18] This modular arrangement enables precise coordination during translocation. Key sequence features include the five conserved GTP-binding motifs (G1–G5) located primarily in the G domain. The G1 motif, also known as the P-loop (residues 31–38: GKSTL TDS), binds the phosphate groups of GTP, while G2 (switch I, residues ~50–60) and G3 (switch II, residues ~80–90) undergo conformational changes upon nucleotide binding to activate catalysis. The G4 and G5 motifs (residues ~120 and ~150–160, respectively) recognize the guanine base. Additionally, conserved residues such as Thr56 in switch I serve as a critical phosphorylation site that regulates activity, and His715 in domain IV is post-translationally modified to diphthamide, essential for fidelity.^[5]

Conformational Dynamics

EEF2, a GTPase involved in ribosomal translocation, exhibits significant conformational flexibility between its GTP-bound and GDP-bound states, primarily within its G domain. In the GTP-bound form, the switch I and switch II regions adopt a closed, active conformation that positions catalytic residues for hydrolysis, while upon GTP hydrolysis to GDP, these switches undergo disordering and repositioning, leading to a more open G domain structure. This transition involves a relative movement of approximately 6 Å in the switch regions, facilitating the uncoupling of EEF2 from the ribosome and enabling tRNA-mRNA translocation.^[19] Cryo-electron microscopy (cryo-EM) structures have elucidated these domain rearrangements during translocation. For instance, in late translocation intermediates, the GTP-bound EEF2 displays domain IV protruding about 7 Å deeper into the A-site to stabilize the codon-anticodon interaction, with subsequent hydrolysis triggering a ratchet-like motion where domains III, IV, and V rotate by 5–9° relative to the G domain (domains I, II, and G'). Key structures include PDB entries 6GZ3 and 6GZ5, which capture these states on the eukaryotic 80S ribosome, revealing how EEF2's rigidity in the GTP state gives way to flexibility post-hydrolysis for efficient subunit movements. Recent high-resolution cryo-EM studies (up to 1.97 Å resolution, as of 2025) have further detailed the fidelity mechanisms during elongation.^[3][19][20] Allosteric communication between the GTPase (G) domain and the tRNA-mimic domains (III–V) coordinates these dynamics, with signals from nucleotide hydrolysis propagating through hinge regions to induce global rearrangements. Specifically, the hinge in domain IV allows for flexible pivoting, enabling a 6 Å shift at its tip toward ribosomal helix 44 upon hydrolysis, which disrupts interactions with the decoding center and promotes translocation. This inter-domain crosstalk ensures synchronized tRNA movement without back-sliding.^[19][21] Magnesium ions play a critical role in stabilizing the active GTP-bound conformation of EEF2 by coordinating the γ-phosphate of GTP and key residues in switch I, such as an invariant aspartate, thereby maintaining the closed G domain architecture essential for ribosomal engagement. Depletion of Mg²⁺ disrupts this coordination, impairing hydrolysis and translocation efficiency.^[22][23]

Function in Translation

Role in Elongation Cycle

EEF2 plays a central role in the elongation phase of eukaryotic translation by catalyzing the translocation step, which advances the mRNA-tRNA complex within the ribosome following peptide bond formation. After the incoming aminoacyl-tRNA, delivered by eEF1A, is accommodated in the A site and undergoes peptidyl transfer with the peptidyl-tRNA in the P site, the ribosome enters a pre-translocation state with deacylated tRNA in the P site and peptidyl-tRNA in the A site. EEF2, bound to GTP, associates with this complex to facilitate the coordinated movement of the tRNAs and mRNA: the deacylated tRNA shifts to the E site, the peptidyl-tRNA to the P site, and the mRNA advances by three nucleotides to expose the next codon in the A site. This process ensures the continuous synthesis of the polypeptide chain.^[3][24] The GTP hydrolysis cycle of EEF2 is integral to translocation efficiency and directionality. In its GTP-bound form, EEF2 binds to the ribosomal stalk and stabilizes the hybrid positioning of tRNAs (A/P and P/E states), priming the ribosome for movement. GTP hydrolysis, triggered by interactions with ribosomal components such as the sarcin-ricin loop, induces conformational changes in EEF2—particularly in its G domain and domain IV—that provide the mechanical force for translocation while uncoupling EEF2 from the mRNA-tRNA module to prevent backward slipping. The resulting EEF2-GDP complex then dissociates from the post-translocation ribosome, allowing the cycle to repeat with a new GTP-bound EEF2 molecule. This hydrolysis step not only accelerates translocation but also ensures its irreversibility under physiological conditions.^[3] EEF2 contributes to translational fidelity by maintaining precise codon-anticodon pairing during translocation, thereby minimizing errors such as ribosomal frameshifting. Its domain IV mimics the tRNA anticodon stem-loop, interacting with the decoding center to lock the mRNA-tRNA duplex in register and prevent slippage of the mRNA relative to the tRNAs. Modifications like diphthamide on histidine 715 of EEF2 further stabilize these interactions, reducing the incidence of -1 frameshifting; defects in this modification lead to increased translational errors. In vitro studies demonstrate that EEF2 enhances translocation fidelity by discriminating against mismatched codon-anticodon pairs during movement.^[3][24] Kinetic analyses reveal that EEF2 dramatically accelerates translocation, with modeled rate constants around 10–35 s⁻¹ based on analogous prokaryotic systems adapted for eukaryotic modeling.^[25]

Interactions with Ribosomal Components

EEF2 binds to the 80S ribosome in a 1:1 stoichiometry during the elongation phase of translation, with its GTP-bound form facilitating translocation of the tRNAs and mRNA. The factor engages both the 40S and 60S subunits through multiple contact points, ensuring stable association until GTP hydrolysis triggers dissociation post-translocation. Structural studies, primarily from yeast, reveal that domain I and V of EEF2 anchor primarily to the 60S subunit, while domain IV extends into the A site of the 40S subunit, mimicking the anticodon arm of A-site tRNA to stabilize the codon-anticodon interaction during movement. This binding configuration positions EEF2 to catalyze ribosomal subunit ratcheting and head swivel, with dissociation kinetics accelerated by GTP hydrolysis, allowing rapid cycling for subsequent elongation rounds. These features are conserved in human eEF2.^[3][26] Key interactions occur with specific rRNA helices on both subunits. On the 40S subunit, EEF2 contacts helices h5, h15, h33, h34, and h44 of the 18S rRNA to support decoding center stability. The 60S subunit interactions involve helices H34, H43, H44, and H69 of the 28S rRNA, where contacts facilitate GTPase activation at the sarcin-ricin loop and GTPase-associated center. Additionally, EEF2 interfaces with ribosomal proteins, including uS12 on the 40S shoulder, which stimulates its GTPase activity, and eS30, which enhances domain IV stabilization through co-evolved structural adaptations. These contacts, particularly with H43 and H44, position EEF2 proximal to the peptidyl transferase center and intersubunit bridges, enabling coordinated ribosomal dynamics. Specific residue interactions are conserved but numbered differently in human versus yeast; for example, in yeast, residues in domain IV engage h44, and others interact with H44 and H43.^{[27][26][3][28]} Interface residues in domain IV, including conserved arginines, contribute to tRNA mimicry by forming electrostatic interactions that replicate A-site tRNA contacts with the decoding center, preventing backward slippage of tRNAs. The diphthamide modification at His715 in domain IV protrudes into a cleft formed by mRNA, P-site tRNA, and rRNA, further stabilizing the translocating complex and ensuring translocation fidelity. EEF2 cooperates with eEF1A during elongation, where eEF1A's delivery of aminoacyl-tRNA to the A site sets up the pre-translocation state, and their sequential actions maintain proofreading accuracy by coupling decoding fidelity to translocation efficiency; ribosomal components like uS12 act as GTPase-activating elements for EEF2, analogous to bacterial systems involving ribosome recycling factor (RRF) for post-termination GTPase stimulation. These interactions collectively ensure precise ribosomal progression without excessive kinetic delays.^[3][26][28]

Regulation Mechanisms

Phosphorylation by eEF2K

Eukaryotic elongation factor 2 kinase (eEF2K) is a calcium/calmodulin-dependent serine/threonine kinase that specifically phosphorylates eukaryotic elongation factor 2 (EEF2) at threonine 56 (Thr56) within its G1 motif.^[29][30] This phosphorylation event serves as the primary inhibitory modification of EEF2, reducing its affinity for the ribosome and thereby impairing its GTPase activity during the translocation step of translation elongation.^[30][31] The kinetic parameters of eEF2K toward EEF2 as a substrate indicate a Km of approximately 1 μM, reflecting efficient recognition and modification under physiological conditions.^[32] Phosphorylation by eEF2K halts translational elongation in response to cellular stress, such as nutrient deprivation, conserving energy by limiting protein synthesis.^[31] This inhibition is particularly critical during energy stress, where eEF2K activation slows ribosome transit, allowing cells to prioritize survival over growth.^[30] Under such conditions, eEF2K is triggered by hypoxia, which inhibits proline hydroxylation on eEF2K itself, thereby activating the kinase, and by energy depletion via the AMP-activated protein kinase (AMPK) pathway, where AMPK phosphorylates eEF2K at serine 398 to enhance its activity.^[33][31] These mechanisms ensure rapid downregulation of elongation when ATP levels drop or oxygen is limited. Dephosphorylation of EEF2 at Thr56, which restores its full activity, is primarily mediated by protein phosphatase 2A (PP2A).^[34] PP2A efficiently reverses the inhibitory effects of eEF2K, enabling reactivation of translation once stress subsides, and its activity remains consistent across physiological contexts like ageing.^[35] This dynamic balance between phosphorylation and dephosphorylation fine-tunes EEF2 function in response to fluctuating cellular demands.

ADP-Ribosylation and Other Modifications

One prominent post-translational modification of eukaryotic elongation factor 2 (eEF2) is ADP-ribosylation, primarily mediated by bacterial toxins such as diphtheria toxin (DT). This irreversible modification occurs at the diphthamide residue on histidine 715 (His715) in human eEF2, where an ADP-ribose moiety is transferred from NAD⁺ to the modified histidine, thereby inactivating eEF2 and halting protein synthesis.^[36][37] The diphthamide modification itself is a conserved post-translational event essential for eEF2 function, but its ADP-ribosylation by DT or related exotoxins (e.g., Pseudomonas exotoxin A) specifically blocks the GTP-dependent ribosomal translocation step, leading to cell death in intoxicated cells.^[24][38] The functional consequences of ADP-ribosylation include impaired binding of eEF2 to pre-translocational ribosomes and reduced GTPase activity, preventing the movement of peptidyl-tRNA from the A-site to the P-site on the ribosome.^[39][40] This modification has minimal impact on eEF2's affinity for GTP or GDP but disrupts its conformational dynamics necessary for translocation, contrasting with reversible phosphorylation by eEF2 kinase that temporarily inactivates eEF2 under stress.^[41][38] Beyond ADP-ribosylation, eEF2 undergoes other post-translational modifications that regulate its stability and localization. Sumoylation targets lysine residues on eEF2, enhancing its protein stability and conferring anti-apoptotic effects, particularly in lung adenocarcinoma cells where sumoylated eEF2 correlates with drug resistance and survival.^[42][43] This modification also facilitates proteolytic cleavage of eEF2 and promotes nuclear translocation of the resulting C-terminal fragment, enabling non-canonical roles in apoptosis regulation, such as during ischemia/reperfusion injury in cardiomyocytes.^[44][45] Sumoylation often competes with ubiquitination at overlapping lysine sites, modulating eEF2's fate between stabilization and degradation.^[46] Ubiquitination of eEF2 marks it for proteasomal degradation, a process accelerated under proteotoxic stress to maintain cellular proteostasis by clearing excess or damaged elongation factors.^[43] This modification reduces eEF2 levels, indirectly slowing translation elongation and alleviating the buildup of misfolded proteins during conditions like heat shock or oxidative stress.^[47] Lysine acetylation has also been identified on eEF2 in proteomic studies of stress responses, potentially competing with ubiquitination to enhance stability, though specific acetyltransferases like p300 remain to be fully characterized in this context.^[48][49] eEF2 is also subject to trimethylation at lysine 525 (Lys525) by the lysine methyltransferase EEF2KMT (also known as FAM86A), a modification that occurs in domain III and is essential for efficient translation elongation by stabilizing eEF2's interaction with the ribosome.^[2][50] This trimethylation enhances global protein synthesis rates and has been linked to tumor progression in cancers such as lung adenocarcinoma, where its dysregulation affects cellular growth.^[51] Collectively, these modifications fine-tune eEF2's affinity for GTP and ribosomal components, ensuring adaptive control of translation under diverse physiological and pathological conditions.

Clinical Significance

Neurodevelopmental Disorders

De novo heterozygous variants in the EEF2 gene are associated with a rare neurodevelopmental disorder characterized by developmental delay, intellectual disability, autism spectrum disorder features, macrocephaly, and benign external hydrocephalus, often presenting in infancy.^[52] These variants disrupt the function of eukaryotic elongation factor 2 (eEF2), a critical component of the translation elongation cycle, leading to impaired protein synthesis in neuronal cells. For instance, de novo heterozygous missense variants such as p.V28M have been identified in affected individuals through whole-exome sequencing, resulting in reduced eEF2 activity and translational inefficiency.^[53] The pathophysiology involves defective mRNA translation in developing neurons, which impairs synaptic plasticity and connectivity, contributing to neurodevelopmental deficits. This is evidenced by animal models, where complete Eef2 knockout in mice leads to embryonic or perinatal lethality due to the essential role of eEF2 in global protein synthesis, while heterozygous models exhibit reduced protein synthesis in prefrontal cortex excitatory neurons, synaptic transmission abnormalities, and behavioral impairments such as social novelty deficits.^[54] In affected individuals, these variants impair overall translational fidelity, contributing to the observed neurological phenotypes.^[55] The disorder typically arises from de novo heterozygous missense or nonsense variants. Reported features include motor and speech delays, hypotonia, autism, macrocephaly, and occasional dysmorphic features such as prominent forehead and thin upper lip. The disorder is extremely rare, with fewer than 10 cases reported in the literature as of 2025.^[56] Diagnosis relies on whole-exome or whole-genome sequencing to detect damaging variants, confirming the genetic etiology in symptomatic children.

Implications in Cancer and Other Pathologies

EEF2 has been implicated in cancer progression through its overexpression in various solid tumors, including breast, lung, pancreatic, and gastrointestinal cancers, where it enhances protein synthesis to support rapid cell proliferation.^[57] This overexpression facilitates the selective translation of oncogenes such as c-Myc; eEF2K-mediated phosphorylation of eEF2, which reduces its activity, promotes c-Myc expression to support tumor growth in breast cancer cells, while inhibiting eEF2K decreases c-Myc levels.^[58] In hypoxic tumor microenvironments, activation of eEF2K sustains cancer cell viability by modulating translation elongation, allowing adaptation to nutrient stress and resistance to apoptosis.^[4] In breast cancer, TCGA data indicate EEF2 is often downregulated compared to normal tissue, with higher expression associated with improved survival outcomes.^[59] Beyond cancer, EEF2 contributes to neurodegeneration, such as in amyotrophic lateral sclerosis (ALS), where dysregulation of translation elongation factors like EEF2 influences stress granule formation, potentially leading to pathological protein aggregation in motor neurons.^[60] In cardiovascular pathologies, ischemia induces phosphorylation of EEF2, inhibiting translation to conserve energy during myocardial oxygen deprivation and exacerbating reperfusion injury through subsequent SUMOylation of phosphorylated EEF2.^[61] Viruses exploit EEF2 modifications to achieve host shutoff and favor viral replication; for example, alphaviruses like Venezuelan equine encephalitis virus induce eEF2 phosphorylation via nonstructural protein 2-mediated cAMP-PKA-eEF2K signaling, thereby suppressing host translation while permitting cap-independent viral protein synthesis.^[62]