Cohesin: building loops, but not compartments

Abstract

DNA is subjected to major cellular events, such as transcription, replication and DNA repair. To control these processes, the architecture of the DNA is tightly regulated. Recent work, including two studies in this issue of The EMBO Journal, provides compelling evidence that cohesin structures chromosomes through the processive enlargement of loops. While cohesin promotes chromosomal looping, it rather counteracts nuclear compartmentalization.

Figure 1. Cohesin‐dependent loop formation

(A) Depletion of the core cohesin subunit SCC1 results in loss of loops. Cohesin is depicted as a green ring. Loss of the SCC2/SCC4 complex results in fewer and shorter loops, while removal of WAPL or PDS5 results in longer loops. CTCF (triangles) binds to CTCF sites (red for forward‐oriented and blue for reverse‐oriented sites) and acts as a boundary element that halts cohesin. (B) Cohesin‐dependent loops negatively affect compartmentalization. Compartments are less well defined if there is an increase in looping. Conversely, a decrease in looping results in a clearer distinction between compartments (light grey is A compartment, and dark grey is B compartment). The A compartment generally contains actively transcribed genes and lies in the inner nucleus, while the B compartment is more repressed and lies in the nuclear periphery.