Characterization of the components of the putative mammalian sister chromatid cohesion complex

Establishing and maintaining proper sister chromatid cohesion throughout the cell cycle are essential for maintaining genome integrity. To understand how sister chromatid cohesion occurs in mammals, we have cloned and characterized mouse orthologs of proteins known to be involved in sister chromatid cohesion in other organisms. The cDNAs for the mouse orthologs of SMC1S.c. and SMC3S.c. , mSMCB and mSMCD respectively, were cloned, and the corresponding transcripts and proteins were characterized. mSMCB and mSMCD are transcribed at similar levels in adult mouse tissues except in testis, which has an excess of mSMCD transcripts. The mSMCB and mSMCD proteins, as well as the PW29 protein, a mouse homolog of Mcd1pS.c./Rad21S.p., form a complex similar to cohesin in X. laevis. mSMCB, mSMCD and PW29 protein levels show no significant cell-cycle dependence. The bulk of the mSMCB, mSMCD and PW29 proteins undergo redistribution from the chromosome vicinity to the cytoplasm during prometaphase and back to the chromatin in telophase. This pattern of intracellular localization suggests a complex role for this group of SMC proteins in chromosome dynamics. The PW29 protein and PCNA, which have both been implicated in sister chromatid cohesion, do not colocalize, indicating that these proteins may not function in the same cohesion pathway. Overexpression of a PW29-GFP fusion protein in mouse fibroblasts leads to inhibition of proliferation, implicating this protein and its complex with SMC proteins in the control of mitotic cycle progression.

A direct link between sister chromatid cohesion and chromosome condensation revealed through the analysis of MCD1 in S. cerevisiae

The S. cerevisiae MCD1 (mitotic chromosome determinant) gene was identified in genetic screens for genes important for chromosome structure. MCD1 is essential for viability and homologs are found from yeast to humans. Analysis of the mcd1 mutant and cell cycle-dependent expression pattern of Mcd1p suggest that this protein functions in chromosome morphogenesis from S phase through mitosis. The mcd1 mutant is defective in sister chromatid cohesion and chromosome condensation. The physical association between Mcd1p and Smc1p, one of the SMC family of chromosomal proteins, further suggests that Mcd1p functions directly on chromosomes. These data implicate Mcd1p as a nexus between cohesion and condensation. We present a model for mitotic chromosome structure that incorporates this previously unsuspected link.

Mitotic chromosome condensation

In this chapter, we review the structure and composition of interphase and mitotic chromosomes. We discuss how these observations support the model that mitotic condensation is a deterministic process leading to the invariant folding of a given chromosome. The structural studies have also placed constraints on the mechanism of condensation and defined several activities needed to mediate condensation. In the context of these activities and structural information, we present our current understanding of the role of cis sites, histones, topoisomerase II, and SMC proteins in condensation. We conclude by using our current knowledge of mitotic condensation to address the differences in chromosome condensation observed from bacteria to humans and to explore the relevance of this process to other processes such as gene expression.