Intersection of ChIP and FLIP, genomic methods to study the dynamics of the cohesin proteins

The evolutionarily conserved cohesin proteins Smc1, Smc3, Rad21 (Mcd1), and Scc3 function in the cohesin complex that provides the basis for chromosome cohesion and is involved in gene regulation. Understanding how these proteins link together the genome requires the use of whole-genome approaches to study the molecular mechanisms of these essential proteins. While chromatin immunoprecipitation followed by DNA microarray (ChIP-chip) studies have provided a snapshot in time of where these proteins associate with various genomes, the cohesin proteins are dynamic in their localization and interactions on chromatin. Study of the dynamic nature of these proteins requires approaches such as live cell imaging. We present evidence from fluorescence loss in photobleaching (FLIP) experiments in budding yeast that the decay constant of each cohesin subunit is approximately 60-90 s in interphase. The decay constant on chromatin increases from G(1) to S phase to metaphase, consistent with the interaction with chromatin becoming more stable once chromosomes are cohered. A small population of Smc3 at a position consistent with centromeric location has a longer decay constant than bulk Smc3. The characterization of the interaction of cohesin with chromatin, in terms of both its position and its dynamics, may be key to understanding how this protein complex contributes to chromosome segregation and gene regulation.