Elucidation of the functional role of the Q- and I-motif in the human chromatin remodeling enzyme BRG1

Helen Hoffmeister et al., (2019) JBC; PMID: 30647132

The Snf2 proteins, comprising 53 different enzymesin humans, belong to the SF2 family. Many Snf2enzymes possess chromatin remodeling activity,requiring a functional ATPase domain consisting ofconserved motifs named Q and I-VII. These motifsform two recA-like domains, creating an ATP-binding pocket. Little is known about the functionof the conserved motifs in chromatin remodelingenzymes. Here, we characterized the function of theQ- and I (Walker I)-motif in hBRG1 (SMARCA4).The motifs are in close proximity to the bound ATP,suggesting a role in nucleotide binding and/orhydrolysis. Unexpectedly, when substituting theconserved residues Q758 (Q-motif) or K785 (I-motif) of both motifs, all variants still bound ATPand exhibited basal ATPase activity similar to thatof wild type BRG1 (wtBRG1). However, allmutants lost the nucleosome-dependent stimulationof the ATPase domain. Their chromatin remodelingrates were impaired accordingly, but nucleosomebinding was retained and still comparable to that ofwtBRG1. Interestingly, a cancer-relevantsubstitution, L754F (Q-motif) displayed defects similar to the Q-758-variant(s), arguing for acomparable loss of function. Since we excluded amutual interference of A TP and nucleosomebinding, we postulate that both motifs stimulate theA TPase and chromatin remodeling activities uponbinding of BRG1 to nucleosomes, probably viaallosteric mechanisms. Furthermore, mutations ofboth motifs similarly affect the enzymaticfunctionality of BRG1 in vitro and in living cells.Of note, in BRG1-deficient H1299 cells,exogenously expressed wtBRG1, but not BRG1Q758A and BRG1 K785R, exhibited a tumorsuppressor like function.