Methyl-CpG-binding domain 9 (MBD9) is required for H2A.Z incorporation into chromatin at a subset of H2A.Z-enriched regions in the Arabidopsis genome

Abstract

The SWR1 chromatin remodeling complex, which deposits the histone variant H2A.Z into nucleosomes, has been characterized in yeast and animals but had not been purified from plants. We used the conserved SWR1 subunit ACTIN RELATED PROTEIN 6 (ARP6) as bait in tandem affinity purification experiments to isolate associated proteins from Arabidopsis thaliana. We identified all 11 subunits found in yeast SWR1 and the homologous mammalian SRCAP complexes, demonstrating that this complex is conserved in plants. We also identified several additional proteins not previously associated with SWR1, including Methyl-CpG-BINDING DOMAIN 9 (MBD9). Since mbd9 mutant plants were phenotypically similar to arp6 mutants, we further explored a potential role for MBD9 in H2A.Z deposition. We found that MBD9 is required for proper H2A.Z incorporation at thousands of discrete sites, which represent a subset of the regions normally enriched with H2A.Z. Genetic analyses showed that arp6;mbd9 double mutants have far more severe phenotypes than either single mutant. In conjunction with the finding that MBD9 does not appear to be a core subunit of the Arabidopsis SWR1 complex, this suggests that MBD9 also has important roles beyond H2A.Z deposition. Our data establish the SWR1 complex as being conserved across eukaryotes and also provide new insights into the mechanisms that target H2A.Z to chromatin.