Structure and molecular assembly of a MuvB transcription factor complex with the nucleosome

The core cell cycle machinery genes are transcriptionally regulated by the MuvB family of protein complexes in a cell cycle specific manner. During cell cycle exit in quiescence or senescence, the DREAM complex, which is the repressive form of MuvB, directs transcriptional repression of cell cycle genes; conversely during cell proliferation, the complex of MuvB with the transcription factors (TFs) B-MYB and FOXM1 activate mitotic genes during the G2 phase of the cell cycle. The mechanisms of transcriptional regulation of these complexes are still poorly characterised. Here we combine biochemical analysis and in vitro reconstitution, with structural analysis by cryo-electron microscopy (cryo-EM) and cross-linking mass spectrometry (XL-MS), to functionally examine these complexes. Our data suggests that MuvB is a chromatin regulator whereby a core region binds the nucleosome and remodels it, thereby exposing nucleosomal DNA. This remodelling activity is supported by B-MYB which directly binds the remodelled DNA. Given the remodelling activity on the nucleosome, we propose that the MuvB complex with B-MYB (MMB) function as a pioneer transcription factor complex. Our data rationalises prior biochemical and cellular studies and provides a molecular framework of interactions on a protein complex, which is key for cell cycle regulation.