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Condensin
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Condensin
Condensins are large protein complexes that play a central role in chromosome condensation and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in Xenopus egg extracts.
Many eukaryotic cells possess two different types of condensin complexes, known as condensin I and condensin II, each of which is composed of five subunits (Figure 2). Condensins I and II share the same pair of core subunits, SMC2 and SMC4, both belonging to a large family of chromosomal ATPases, known as SMC proteins (SMC stands for Structural Maintenance of Chromosomes). Each of the complexes contains a distinct set of non-SMC regulatory subunits (a kleisin subunit and a pair of HEAT repeat subunits). Both complexes are large, having a total molecular mass of 650-700 kDa.
The core subunits condensins (SMC2 and SMC4) are conserved among all eukaryotic species that have been studied to date. The non-SMC subunits unique to condensin I are also conserved among eukaryotes, but the occurrence of the non-SMC subunits unique to condensin II is highly variable among species.
The following table summarizes the names of SMC complex subunits in representative eukaryotic model organisms.
Condensin is one of the three major SMC protein complexes found in eukaryotes. The other two are: cohesin, which is involved in sister chromatid cohesion and interphase chromosome organization; and the SMC5/6 complex, which functions in DNA repair and chromosome segregation.
SMC-ScpAB: Condensin-like protein complexes also exist in prokaryotes, where they contribute to the organization and segregation of chromosomes (nucleoids). The best-studied example is the SMC–ScpAB complex (Figure 3, left), which is considered the evolutionary ancestor of the eukaryotic condensin complexes. Compared to its eukaryotic counterparts, SMC–ScpAB has a simpler architecture. For instance, while eukaryotic condensins contain an SMC heterodimer, prokaryotic SMC proteins form a homodimer. Among the regulatory subunits, ScpA belongs to the kleisin family, suggesting that the basic SMC–kleisin trimeric structure is conserved across prokaryotes and eukaryotes. By contrast, ScpB is classified as a member of the kite (Kleisin Interacting Tandem Elements) family, which is structurally distinct from the HEAT-repeat subunits found in eukaryotic condensins.
MukBEF: While most bacteria and archaea possess the SMC–ScpAB complex, a subset of gammaproteobacteria, including Escherichia coli, instead have a distinct SMC complex known as MukBEF. MukBEF forms a "dimer-of-dimers" through dimerization mediated by the kleisin subunit MukF (Figure 3, center). The third subunit, MukE, belongs to the kite family. Although sequence similarity between the subunits of MukBEF and those of SMC–ScpAB is low, their overall molecular architecture observed by electron microscopy and phenotypic defects in mutants suggest that the two are functional homologs. As such, they are often collectively referred to as prokaryotic condensins.
MksBEF/Wadjet: More recently, a third type of bacterial SMC complex (called MksBEF), structurally similar to MukBEF, has been reported. Pseudomonas aeruginosa have both SMC–ScpAB and MksBEF, which contribute to chromosome organization and segregation through distinct mechanisms. In contrast, in Corynebacterium glutamicum, SMC–ScpAB is responsible for chromosome architecture and segregation, whereas MksBEF, together with the nuclease subunit MksG, is specialized for plasmid defense. The MksBEFG complex is orthologous to the JetABCD complex in Bacillus cereus and the EptABCD complex in Mycobacterium smegmatis. These complexes, which serve a common function in plasmid defense, are collectively referred to as the Wadjet complexes (Figure 3, right).
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Condensin
Condensins are large protein complexes that play a central role in chromosome condensation and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in Xenopus egg extracts.
Many eukaryotic cells possess two different types of condensin complexes, known as condensin I and condensin II, each of which is composed of five subunits (Figure 2). Condensins I and II share the same pair of core subunits, SMC2 and SMC4, both belonging to a large family of chromosomal ATPases, known as SMC proteins (SMC stands for Structural Maintenance of Chromosomes). Each of the complexes contains a distinct set of non-SMC regulatory subunits (a kleisin subunit and a pair of HEAT repeat subunits). Both complexes are large, having a total molecular mass of 650-700 kDa.
The core subunits condensins (SMC2 and SMC4) are conserved among all eukaryotic species that have been studied to date. The non-SMC subunits unique to condensin I are also conserved among eukaryotes, but the occurrence of the non-SMC subunits unique to condensin II is highly variable among species.
The following table summarizes the names of SMC complex subunits in representative eukaryotic model organisms.
Condensin is one of the three major SMC protein complexes found in eukaryotes. The other two are: cohesin, which is involved in sister chromatid cohesion and interphase chromosome organization; and the SMC5/6 complex, which functions in DNA repair and chromosome segregation.
SMC-ScpAB: Condensin-like protein complexes also exist in prokaryotes, where they contribute to the organization and segregation of chromosomes (nucleoids). The best-studied example is the SMC–ScpAB complex (Figure 3, left), which is considered the evolutionary ancestor of the eukaryotic condensin complexes. Compared to its eukaryotic counterparts, SMC–ScpAB has a simpler architecture. For instance, while eukaryotic condensins contain an SMC heterodimer, prokaryotic SMC proteins form a homodimer. Among the regulatory subunits, ScpA belongs to the kleisin family, suggesting that the basic SMC–kleisin trimeric structure is conserved across prokaryotes and eukaryotes. By contrast, ScpB is classified as a member of the kite (Kleisin Interacting Tandem Elements) family, which is structurally distinct from the HEAT-repeat subunits found in eukaryotic condensins.
MukBEF: While most bacteria and archaea possess the SMC–ScpAB complex, a subset of gammaproteobacteria, including Escherichia coli, instead have a distinct SMC complex known as MukBEF. MukBEF forms a "dimer-of-dimers" through dimerization mediated by the kleisin subunit MukF (Figure 3, center). The third subunit, MukE, belongs to the kite family. Although sequence similarity between the subunits of MukBEF and those of SMC–ScpAB is low, their overall molecular architecture observed by electron microscopy and phenotypic defects in mutants suggest that the two are functional homologs. As such, they are often collectively referred to as prokaryotic condensins.
MksBEF/Wadjet: More recently, a third type of bacterial SMC complex (called MksBEF), structurally similar to MukBEF, has been reported. Pseudomonas aeruginosa have both SMC–ScpAB and MksBEF, which contribute to chromosome organization and segregation through distinct mechanisms. In contrast, in Corynebacterium glutamicum, SMC–ScpAB is responsible for chromosome architecture and segregation, whereas MksBEF, together with the nuclease subunit MksG, is specialized for plasmid defense. The MksBEFG complex is orthologous to the JetABCD complex in Bacillus cereus and the EptABCD complex in Mycobacterium smegmatis. These complexes, which serve a common function in plasmid defense, are collectively referred to as the Wadjet complexes (Figure 3, right).
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