Condensin goes with the family but not with the flow

Abstract

Condensin and cohesin are loaded onto yeast chromosomes by a common mechanism at RNA polymerase III transcribed genes. Whereas cohesin translocates from these loading sites to mediate cohesion at secondary locations, condensin remains, bringing distant sites together into clusters. Structural maintenance of chromosome proteins, or SMCs for short, are components of a variety of complexes that are central to the organization, utilization and segregation of chromosomes [1]. SMCs are unusually large proteins that fold on themselves to form long coiled coils with an ATPase head at one end. A dimerization motif at the other end allows the proteins to form SMC pairs, which in turn associate with additional structural and regulatory factors. The Smc1 and Smc3 dimer forms the core of the complex known as cohesin, which mediates sister-chromatid cohesion by directly binding sister chromatids together until the onset of anaphase. The Smc2 and Smc4 dimer forms the core of condensin, a protein complex that facilitates DNA chromosome condensation in preparation for mitotic segregation. An additional pair of proteins, Smc5 and Smc6, forms the core of a less well understood complex with important roles in several critical processes including DNA damage checkpoint response and repair. Figure 1a shows schematic representations of SMC complexes and their subunits.