ABSTRACT 3D chromatin organization plays a critical role in regulating gene expression, DNA replication, recombination, and repair. While initially discovered for its role in sister chromatid cohesion, emerging evidence suggests that the cohesin complex (SMC1, SMC3, RAD21, and SA1/SA2), facilitated by NIPBL, mediates topologically associating domains (TADs) and chromatin loops through DNA loop extrusion. However, information on how conformational changes of cohesin-NIPBL drive its loading onto DNA, initiation, and growth of DNA loops is still lacking. Using high-speed AFM (HS-AFM) imaging, we show that cohesin-NIPBL captures DNA through arm extension, followed by transfer of DNA to its globular domain and DNA loop initiation independent of ATPase hydrolysis. Additional shorter protrusions (feet) from cohesin-NIPBL transiently bind to DNA, facilitating its loading onto DNA. Furthermore, HS-AFM imaging reveals distinct forward and reverse DNA loop extrusion steps by cohesin-NIPBL. These results provide critical missing links in our understanding of DNA binding and loop extrusion by cohesin-NIPBL.

DNA capture and loop extrusion dynamics by cohesin-NIPBL