This superfamily represents a conserved C-terminal region found in bacterial segregation and condensation protein A (ScpA) as well as in eukaryotic cohesins of the Rad21 and Scc1 families. ScpA participates in chromosomal partition during cell division. It may act via the formation of a condensin-like complex containing Smc and ScpB that pull DNA away from mid-cell into both cell halves.
Structural maintenance of chromosomes protein 1A (SMC1A) is a homologue of the yeast Smc1 protein, which is a component of the cohesin complex required for sister chromatid cohesion []. In human, it is part of the core cohesion complex composed of SMC1A, SMC3, RAD21 and STAG proteins []. These proteins form a ring structure that encircles sister chromatids to mediate sister chromatid cohesion []. SMC1A binds to SMC3 through its hinge domain []. Besides sister chromatid cohesion function, SMC1A-SMC3 heterodimer can also found in the RC-1 complex, a mammalian protein complex that promotes repair of DNA gaps and deletions through recombination [, ]. This entry also includes Smc1 homologue from Caenorhabditis elegans, SMCL-1. Unlike canonical SMC proteins, SMCL-1 lacks hinge and coil domains, and its ATPase domain lacks conserved amino acids required for ATP hydrolysis []. Mutations in SMC1A gene cause Cornelia de Lange syndrome 2 (CDLS2), which is a form of Cornelia de Lange syndrome, a clinically heterogeneous developmental disorder associated with malformations affecting multiple systems [, ].
This entry represents the C-terminal domain of Shugoshin (Sgo1) kinetochore-attachment proteins. Shugoshin has a conserved coiled-coil N-terminal domain and a highly conserved C-terminal basic region (). Shugoshin is a crucial target of Bub1 kinase that plays a central role in chromosome cohesion during mitosis and meiosis divisions by preventing premature dissociation of cohesin complex from centromeres after prophase, when most of cohesin complex dissociates from chromosomes arms [, ]. Shugoshin is thought to act by protecting Rec8 and Rad21 at the centromeres from separase degradation during anaphase I (during meiosis) so that sister chromatids remain tethered []. Shugoshin also acts as a spindle checkpoint component required for sensing tension between sister chromatids during mitosis, its degradation when they separate preventing cell cycle arrest and chromosome loss in anaphase, a time when sister chromatids are no longer under tension. Human shugoshin is diffusible and mediates kinetochore-driven formation of kinetochore-microtubules during bipolar spindle assembly []. Further, the primary role of shugoshin is to ensure bipolar attachment of kinetochores, and its role in protecting cohesion has co-developed to facilitate this process [].
This entry represents the N-terminal domain of Shugoshin (Sgo1) kinetochore-attachment proteins. Shugoshin has this conserved coiled-coil N-terminal domain and a highly conserved C-terminal basic region ().Shugoshin is a crucial target of Bub1 kinase that plays a central role in chromosome cohesion during mitosis and meiosis divisions by preventing premature dissociation of cohesin complex from centromeres after prophase, when most of cohesin complex dissociates from chromosomes arms [, ]. Shugoshin is thought to act by protecting Rec8 and Rad21 at the centromeres from separase degradation during anaphase I (during meiosis) so that sister chromatids remain tethered []. Shugoshin also acts as a spindle checkpoint component required for sensing tension between sister chromatids during mitosis, its degradation when they separate preventing cell cycle arrest and chromosome loss in anaphase, a time when sister chromatids are no longer under tension. Human shugoshin is diffusible and mediates kinetochore-driven formation of kinetochore-microtubules during bipolar spindle assembly []. Further, the primary role of shugoshin is to ensure bipolar attachment of kinetochores, and its role in protecting cohesion has co-developed to facilitate this process [].
This group of cysteine peptidases belong to MEROPS peptidase family C50 (separase family, clan CD). The active site residues for members of this family and family C14 occur in the same order in the sequence: H,C.The separases are caspase-like proteases, which plays a central role in the chromosome segregation. In yeast they cleave the rad21 subunit of the cohesin complex at the onset of anaphase. During most of the cell cycle, separase is inactivated by the securin/cut2 protein, which probably covers its active site. A cysteine peptidase is a proteolytic enzyme that hydrolyses a peptide bond using the thiol group of a cysteine residue as a nucleophile. Hydrolysis involves usually a catalytic triad consisting of the thiol group of the cysteine, the imidazolium ring of a histidine, and a third residue, usually asparagine or aspartic acid, to orientate and activate the imidazolium ring. In only one family of cysteine peptidases, is the role of the general base assigned to a residue other than a histidine: in peptidases from family C89 (acid ceramidase) an arginine is the general base. Cysteine peptidases can be grouped into fourteen different clans, with members of each clan possessing a tertiary fold unique to the clan. Four clans of cysteine peptidases share structural similarities with serine and threonine peptidases and asparagine lyases. From sequence similarities, cysteine peptidases can be clustered into over 80 different families []. Clans CF, CM, CN, CO, CP and PD contain only one family.Cysteine peptidases are often active at acidic pH and are therefore confined to acidic environments, such as the animal lysosome or plant vacuole. Cysteine peptidases can be endopeptidases, aminopeptidases, carboxypeptidases, dipeptidyl-peptidases or omega-peptidases. They are inhibited by thiol chelators such as iodoacetate, iodoacetic acid, N-ethylmaleimide or p-chloromercuribenzoate.Clan CA includes proteins with a papain-like fold. There is a catalytic triad which occurs in the order: Cys/His/Asn (or Asp). A fourth residue, usually Gln, is important for stabilising the acyl intermediate that forms during catalysis, and this precedes the active site Cys. The fold consists of two subdomains with the active site between them. One subdomain consists of a bundle of helices, with the catalytic Cys at the end of one of them, and the other subdomain is a β-barrel with the active site His and Asn (or Asp). There are over thirty families in the clan, and tertiary structures have been solved for members of most of these. Peptidases in clan CA are usually sensitive to the small molecule inhibitor E64, which is ineffective against peptidases from other clans of cysteine peptidases [].Clan CD includes proteins with a caspase-like fold. Proteins in the clan have an α/β/α sandwich structure. There is a catalytic dyad which occurs in the order His/Cys. The active site His occurs in a His-Gly motif and the active site Cys occurs in an Ala-Cys motif; both motifs are preceded by a block of hydrophobic residues []. Specificity is predominantly directed towards residues that occupy the S1 binding pocket, so that caspases cleave aspartyl bonds, legumains cleave asparaginyl bonds, and gingipains cleave lysyl or arginyl bonds.Clan CE includes proteins with an adenain-like fold. The fold consists of two subdomains with the active site between them. One domain is a bundle of helices, and the other a β-barrell. The subdomains are in the opposite order to those found in peptidases from clan CA, and this is reflected in the order of active site residues: His/Asn/Gln/Cys. This has prompted speculation that proteins in clans CA and CE are related, and that members of one clan are derived from a circular permutation of the structure of the other.Clan CL includes proteins with a sortase B-like fold. Peptidases in the clan hydrolyse and transfer bacterial cell wall peptides. The fold shows a closed β-barrel decorated with helices with the active site at one end of the barrel []. The active site consists of a His/Cys catalytic dyad.Cysteine peptidases with a chymotrypsin-like fold are included in clan PA, which also includes serine peptidases. Cysteine peptidases that are N-terminal nucleophile hydrolases are included in clan PB. Cysteine peptidases with a tertiary structure similar to that of the serine-type aspartyl dipeptidase are included in clan PC. Cysteine peptidases with an intein-like fold are included in clan PD, which also includes asparagine lyases.
