Structural basis of complex assembly and nucleosome recognition by the chromatin remodeling ncBAF complex.

Abstract

BRG1/BRM-associated factor (BAF)-family chromatin remodelers regulate transcription and genome organization by repositioning nucleosomes. The human non-canonical BAF (ncBAF) complex is uniquely defined by the presence of BRD9 and GLTSCR1/GLTSCR1L, as well as the absence of the ARID1/2 scaffold and the canonical nucleosome-binding module found in cBAF and PBAF. How ncBAF assembles and engages nucleosomes remains elusive. Here, we present a cryo-EM structure of ncBAF bound to a nucleosome, integrated with biochemical assays and crosslinking mass spectrometry analyses. The ncBAF complex adopts a three-module architecture comprising an ATPase motor module, a repositioned actin-related protein (ARP) module, and a highly flexible Base module. The ncBAF-specific subunits BRD9 and GLTSCR1L are largely dispensable for complex assembly and nucleosome remodeling. Instead, BCL7A directly engages the H2A-H2B acidic patch and the H2A N-terminal tail, providing a structural substitute for SMARCB1 in stimulating remodeling activity. These findings reveal how ncBAF compensates for the loss of the canonical nucleosome-binding module through modular reorganization, providing a structural framework for understanding ncBAF-mediated chromatin regulation and its roles in development and disease.