Scc2/Nipbl hops between chromosomal cohesin rings after loading.

James Rhodes,Davide Mazza,Stephan Uphoff,Kim Nasmyth

doi:10.7554/elife.30000

Abstract

The cohesin complex mediates DNA-DNA interactions both between (sister chromatid cohesion) and within chromosomes (DNA looping). It has been suggested that intra-chromosome loops are generated by extrusion of DNAs through the lumen of cohesin's ring. Scc2 (Nipbl) stimulates cohesin's ABC-like ATPase and is essential for loading cohesin onto chromosomes. However, it is possible that the stimulation of cohesin's ATPase by Scc2 also has a post-loading function, for example driving loop extrusion. Using fluorescence recovery after photobleaching (FRAP) and single-molecule tracking in human cells, we show that Scc2 binds dynamically to chromatin, principally through an association with cohesin. Scc2's movement within chromatin is consistent with a 'stop-and-go' or 'hopping' motion. We suggest that a low diffusion coefficient, a low stoichiometry relative to cohesin, and a high affinity for chromosomal cohesin enables Scc2 to move rapidly from one chromosomal cohesin complex to another, performing a function distinct from loading.

Highlights

The organisation of chromosomes during interphase has an important role in the regulation of gene expression
Recent advances in mapping DNA interactions have demonstrated that the human genome is organised into a series of sub-megabase, self-interacting regions called topologically associating domains (TADs) whose boundaries correspond to binding sites for the CCCTC-binding factor (CTCF) (Nora et al, 2012)
Using fluorescence recovery after photobleaching (FRAP) and single-molecule imaging, we show that Scc2 binds dynamically to chromatin, principally through an association with cohesin

Summary

Introduction

The organisation of chromosomes during interphase has an important role in the regulation of gene expression. There is mounting evidence that the mechanism by which TADs and enhancer-promoter interactions are formed involves cohesin (Kagey et al, 2010; Rollins et al, 1999; Wendt et al, 2008) This Smc/kleisin complex holds sister chromatids together from their replication until chromosome segregation in mitosis (Guacci et al, 1997; Michaelis et al, 1997). The core cohesin complex is a ring-shaped heterotrimer of Smc, Smc and Scc (Rad21) subunits (Toth et al, 1999) Dimerization via their hinge domains creates V-shaped Smc1/Smc heterodimers whose apical head domains come together to form a composite ABC-like ATPase (Haering et al, 2002). It has been suggested that cohesin associates with chromatin by entrapping DNA within the ring’s lumen while sister chromatid cohesion is mediated by co-entrapment of sister DNAs (Haering et al, 2008)

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