Involvement of actin-related proteins in ATP-dependent chromatin remodeling

Actin-related proteins (Arps) and conventional actin are enigmatic components of many chromatin-remodeling enzyme complexes. The yeast INO80 ATP-dependent chromatin-remodeling complex contains stoichiometric amounts of Arp4, Arp5, Arp8, and actin. Here we have revealed functions of Arp5 and Arp8 by analysis of mutants. arp5Delta and arp8Delta mutants display an ino80Delta phenotype. Purification of INO80 complexes from arp5Delta and arp8Delta cells shows that protein complexes remain intact but are compromised for INO80 ATPase activity, DNA binding, and nucleosome mobilization. The INO80 (arp8Delta) complex is strikingly deficient, not only for the Arp8 subunit, but also for Arp4 and actin, suggesting an ordered assembly of Arps. Binding of Arp8 to the INO80 complex requires an N-terminal region of Ino80 adjacent to the conserved ATPase domain. GST-Arp8 binds preferentially to histones H3 and H4 in vitro, suggesting a histone chaperone function. These findings show direct involvement of Arps in the chromatin-remodeling process.