Rad21 is the core subunit of the cohesin complex involved in directing genome organization

Abstract The ring-shaped cohesin complex is an important factor regulating genome structure. It is thought to mediate the formation of chromatin loops and topologically associating domains (TADs) by loop extrusion. However, the regulation of association between cohesin and chromatin is poorly understood. In this study, we directly visualized cohesin loading after up-regulation of cohesin subunit Rad21 by identifying the formation of vermicelli-like structures via live cell super-resolution imaging. We also reveal that cohesin loading can be promoted by Rad21-loader interactions and accumulated contacts were shown at TAD corners while inter-TAD interactions increased after vermicelli formation, indicating that Rad21 is an important determinant of chromatin structure. Moreover, we find that cohesin saddle on topologically associating domains by FISH assay, which is consistent with the CTCF/cohesin-anchored chromatin loop model. Importantly, expression of Rad21 is strictly controlled, and aberrant expression of Rad21 leads to the formation of Rad21 “beads” in the nucleus. In summary, our observations provided important new biological insights into the mechanism of cohesin loading and its functions.