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Actin' in the nucleus

Actin and actin-related proteins (Arps) are involved in numerous processes in the cytosol. However, more recently it has become apparent that actin and Arps are also found in the nucleus where they carry out distinct functions. Two papers published in this issue now provide new insight into the role of actin, Arp4 and Arp6 in the nucleus. On page 3739, Masahiko Harata and colleagues describe a role for Arp6 in the spatial organisation of chromatin. Using chicken DT40 cells in which the ARP6 gene has been conditionally knocked out, they show that Arp6 is required for the radial distribution of macrochromosomes to the periphery of the nucleus and of microchromosomes to the nuclear centre. This effect is mediated by changes in the deposition of H2A.Z and results in transcriptional misregulation. On page 3870, Masatoshi Fujita and co-workers illustrate how β-actin and human Arp4 (BAF53) affect chromatin structure and function. The researchers show that β-actin and Arp4 form a heterocomplex in the nucleus and that depleting human cell lines of Arp4 impairs the integrity of the Brg1 chromatin remodeling complex. Mutations in Arp4 that impair β-actin binding reduce its ability to bind to the Brg1 complex and the Tip60 histone acetyltransferase complex, which indicates that the formation of the heterodimer is important for Arp4 function.