NIPBL and STAG1 enable loop extrusion by providing differential DNA-cohesin affinity
DNA loop extrusion by cohesin has emerged as a critical pathway for chromosome organization. In vitro single-molecule experiments indicate that loop extrusion requires the assembly of an heteropentameric complex consisting of the SMC1/SMC3 heterodimer, STAG1, NIPBL and the kleisin SCC1. The multimeric nature of this complex and regulatoin of its dynamic DNA interactions that are modulated by ATP binding-hydrolysis cycles make it challenging to reveal the molecular mechanism of loop extrusion. Here, we use mass photometry to quantify the key interactions responsible for cohesin assembly, DNA binding, and their modulation by ATP binding and hydrolysis. We find that STAG1 binds tightly to the trimeric complex formed by the SMC1/SMC3 heterodimer and SCC1, creating a DNA binding site whose strength is modulated by the presence of ATP. NIPBL binds strongly to both DNA and the STAG1-tetramer, enabling assembly of the holoenzyme in the absence of DNA or ATP. Taken together, these results suggest that NIPBL acts as a DNA anchor, while a dynamic STAG1-Trimer binding site drives DNA loop formation.