The nuclear actin‐related protein Act3p/Arp4p is involved in the dynamics of chromatin‐modulating complexes

Abstract

## This work is dedicated to the memory of Dr Shigeki Mizuno, an outstanding scientist, an excellent teacher (to M.H., N.S. and Y.O.) and a dear friend (to U.W.). AbstractChromatin remodelling and histone-modifying complexes govern the modulation of chromatin structure. While components of these complexes are diverse, nuclear actinrelated proteins (Arps) have been repeatedly found in these complexes from yeast to mammals. In most cases, Arps are required for functioning of the complexes, but the molecular mechanisms of nuclear Arps have as yet been largely unknown. The Arps and actin, sharing a common ancestor, are supposed to be highly similar in the three-dimensional structure of their core regions, including the ATP-binding pocket. The Arp Act3p/Arp4p of Saccharomyces cerevisiae exists within the nucleus, partly as a component of several high molecular mass complexes, including the NuA4 histone acetyltransferase (HAT) complex, and partly as uncomplexed molecules. We observed that mutations in the putative ATP-binding pocket of Act3p/Arp4p increased its concentration in the high molecular mass complexes and, conversely, that an excess of ATP or ATPγ S led to the release of wild-type Act3p/Arp4p from the complexes. These results suggest a requirement of ATP binding by Act3p/Arp4p for its dissociation from the complexes. In accordance, a mutation in the putative ATP binding site of Act3p/Arp4p inhibited the conversion of the NuA4 complex into the smaller piccoloNuA4, which does not contain Act3p/Arp4p and exhibits HAT activity distinct from that of NuA4. Although the in vitro binding activity of ATP by recombinant Act3p/Arp4p was found to be rather weak, our observations, taken together, suggest that the ATP-binding pocket of Act3p/Arp4p is involved in the function of chromatin modulating complexes by regulating their dynamics.

Interactions between the nucleosome histone core and Arp8 in the INO80 chromatin remodeling complex

“…Nucleotide binding has been demonstrated directly for Arp1-Arp4 (5,(29)(30)(31)(32). We used a fluorescent ATP analog, 2′,3′-O-(2,4,6-trinitrophenylcyclo-hexadienylidene)-ATP (TNP-ATP), to explore the ability of yArp8 to bind nucleotides.…”

Actin-related proteins localized in the nucleus: from discovery to novel roles in nuclear organization

“…61 Gel-filtration analysis showed that only a proportion of Arp4 and Arp6 is found in the chromatin-remodeling complexes. 46,73,74 Consistently, yeast arp6Δ cells show a different pattern of gene expression to swr1Δ cells. 74 In addition, by genetic screenings, Arp5 and Arp6, but not Ino80 and Swr1, were identified as proteins involved in vacuolar protein sorting 75 and Arp5 was shown to be involved in nuclear architecture.…”

Nuclear actin-related proteins take shape

“…It has been shown that a portion of nuclear Arp4 occurs in a monomeric pool separate from the high molecular weight fraction that is usually associated with nuclear complexes. 29 Moreover, a role of Arp8 in chromosome alignment state are tricky and not well established. Furthermore, the roles of Arp4 and Arp8 in interacting with histones need to be separated from their potential roles in actin metabolism.…”