The Sick With Rat8 (SWR1) complex, a member of the Inositol requiring 80 (INO80) family of remodelers, mediates deposition of H2A.
Z into nucleosomes. The SWR1 complex is unique in swapping H2A-H2B dimers for H2A.Z-H2B dimers (Qin, Zhao et al. 2014). SWR1-C is an evolutionarily conserved multi-protein complex first described in budding Yeast, three components of SWR1-C is essential for its function in yeast (Xu and Leichty 2018) The SWR1 complex (SWR1c) is an evolutionarily conserved Swi2/Snf2-related ATPase-containing chromatin remodelling complex that catalyzes the replacement of H2A by the histone variant H2A.Z in nucleosomes (Berriri, Gangappa et al. 2016). The ATPase, Swr1, provides the catalytic activity for the complex, and works in association with accessory proteins, Arp6 and Swc6.
The orthologue of Swr1 in Arabidopsis is PHOTOPERIOD INDEPENDENT EARLY FLOWERING1 (PIE1); the orthologues of Arp6 and Swc6 are, respectively, ACTIN-RELATED PROTEIN6 (ARP6) and SERRATED LEAVES AND EARLY FLOWERING (SEF) (Wu, Alami et al. 2005). As in yeast, these proteins associate with each other, and loss-of function mutations in any of these proteins causes a significant decrease in H2A.Z (Wu, Alami et al. 2005, March-Diaz, Garcia-Dominguez et al. 2007, Xu and Leichty 2018).
Several of the non-catalytic subunits including ACTIN-RELATED PROTEIN 6 (ARP6) and SWR1 COMPLEX 6 (SWC6) have been shown to be essential for histone replacement. ARP6, SWC6, and SWC2 act as a sub-complex, where the proteins are mutually essential for each other’s association and function within the complex. The components and function of SWR1c have been shown to be conserved in Arabidopsis (Wu, Alami et al. 2005, Berriri, Gangappa et al. 2016).
Unlike the relatively simple two- to four-subunit composition of many ISWI complexes, the S. cerevisiae SWR1 complex contains up to fourteen distinct components, eight of which—Swr1, Swc2 (also called Vps72), Swc3, Swc5 (also called Aor1), Swc6 (also called Vps71), Swc7, Arp6, Yaf9 and Bdf1—are encoded by genes that are nonessential for cell viability. Other subunits of the complex—Act1 (actin), Arp4, Swc4 (also called God1), Rvb1 and Rvb2—are essential. Some of these have functions apart from the SWR1 complex or are shared components of the INO80 chromatin-remodeling complex and the NuA4 histone acetyltransferase complex, providing a basis for the mutant phenotype (Wu, Alami et al. 2005)