SMCHD1 compacts DNA directly in an ATP-regulated manner

Structural maintenance of chromosome (SMC) complexes play crucial roles in genome organization by DNA loop extrusion. SMCHD1 is a non-canonical SMC protein important for X-inactivation, imprinting and the silencing of specific autosomal genes. While it is known to be a repressor, little is known about its structure or mechanism of action. Here, we show that the SMCHD1 homodimer is flexible and dynamic in solution. This flexibility is conferred by its yet uncharacterized linker domain, which can alter its length by dynamically switching between compact and extended conformations. Interestingly, we observed that SMCHD1 can directly bridge and compact DNA, independently of other proteins, forming large protein-DNA clusters. SMCHD1 contains a GHKL ATPase domain and SMC hinge domain, however each domain alone is insufficient for DNA compaction. DNA compaction rate decreases when the linker domain is removed. The coiled-coil domain does not affect compaction rate but facilitates interaction with a partner protein LRIF1. Surprisingly, DNA compaction by SMCHD1 does not require ATP and paradoxically, compaction rate is reduced with the addition of ATP. Similarly, SMCHD1 forms clusters with reconstituted nucleosome arrays in the absence of ATP, and the addition of ATP results in a reduction in cluster sizes. Our data provides biophysical and mechanistic insights into the role of SMCHD1 in gene silencing and genome organization.