The role of actin-related protein 6 (ARP6) in the epigenetic regulation of Arabidopsis development
Deal, Roger Bancroft
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The actin-related proteins (ARPs) are an evolutionarily conserved group of proteins that sharebetween 17-60% amino acid identity with cytoskeletal actins. Phylogenetic analysis of ARPsfrom a wide range of organisms indicates that several distinct subclasses of these proteins aregenerally shared among all eukaryotes. The ARPs that are most closely related to actin are foundin the cytoplasm and have been implicated in a number of cytoskeletal functions, while the moredivergent ARPs are nuclear proteins, most of which are known to be stable components ofvarious chromatin modifying complexes. In Saccharomyces cerevisiae, ARP6 is a component ofthe SWR1 chromatin remodeling complex, which catalyzes the replacement of histone H2A withthe H2A.Z variant at specific chromosomal loci. In this study we sought to examine the role ofArabidopsis ARP6 in plant growth and development, and to elucidate its underlying molecularfunctions. We found that Arabidopsis ARP6 was localized to the nucleus and was expressed inall vegetative tissues as well a subset of reproductive tissues. Null mutations in ARP6 resulted ina multitude of defects including altered development of the leaf, inflorescence, and flower, aswell as reduced female fertility and early flowering in both long- and short-day photoperiods. The early flowering of arp6 mutants was associated with reduced expression of the central floralrepressor gene FLOWERING LOCUS C (FLC), and the FLC-like genes MADS AFFECTINGFLOWERING 4 (MAF4) and MAF5. Furthermore, we found that mutations in PHOTOPERIODINDEPENDENT EARLY FLOWERING 1 (PIE1), a homolog of the yeast SWR1 ATPase subunit,produced developmental phenotypes similar to those of arp6 plants, and both mutants showedmisregulation of a common set of genes. Using H2A.Z-specific antibodies we demonstrated thatARP6 and PIE1 were both required for the deposition of H2A.Z at the FLC, MAF4, and MAF5loci, supporting the existence of a SWR1-like complex in plants and indicating that H2A.Z isnormally required to promote high-level expression of these genes. Collectively, these resultsindicate that ARP6 controls several fundamental aspects of Arabidopsis development bypromoting the deposition of H2A.Z into chromatin and thereby regulating the expression ofmultiple developmentally important genes.