COHESIN COMPLEX

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The cohesin complex holds the sister chromatids together after replication until metaphase and anaphase. It is composed of four core protein subunits in vertebrates, Rad21, SMC1, SMC3, SCC3 (SA1, SA2). The mass of cohesin complex is about 600 kDa. Sister chromatids are pulled apart when one cohesin subunit - SCC1/Rad21 - is cleaved by an endopeptidase called separase. Mutation of cohesin components may lead to delayed, premature and/or total inhibition of sister chromatid separation and result in aneuploidy. The N- and C-termini of both the SMC1 and SMC3 components are ABC-like ATPases , while the central region is a hinge domain, forming antiparallel intramolecular coiled coils. SMC1 and SMC3 also form a dimer via the hinge domain. The SMC1-SMC3 dimer looks like an open loop by metal-shadowing. The length of the SMC1-SMC3 heterodimer arm is about 60 nm. A cohesin ring model derived from studies on budding yeast seems reasonable (Gruber et al., 2003 Cell 112, 765-777). It suggests that SCC1/Rad21 C- and N-terminus bind to the ATPase heads of SMC1 and SMC3 heterodimer, respectively, to form a triangular ring. SCC3 then binds to reinforce the ring. Sister chromatids are surrounded by the ring (Uhlmann, 2004, Exp.Cell Res. 296, 80-85). However, according to the EM data, the maximum diameter of the ring is less than 35nm if the triangular ring can become a circle, which is not big enough to hold two 30 nm sister chromatids. We have studied the protein-protein interactions among the cohesin subunits in human cell lines. Our results indicate that cohesin complex is not a simple ring. To confirm our biochemical results, we are visualizing the complex by cryoEM. Our goal is to solve the 3-D structure of cohesin complex and to reveal how cohesin complex interact with DN