THE P55 SUBUNIT OF DROSOPHILA CHROMATIN ASSEMBLY FACTOR-1 IS HOMOLOGOUS TO A HISTONE DEACETYLASE-ASSOCIATED PROTEIN

To gain a better understanding of DNA replication-coupled chromatin as sembly, we have isolated the cDNA encoding the smallest (apparent mole cular mass, 55 kDa: termed p55) subunit of Drosophila melanogaster chr omatin assembly factor 1 (dCAF-1), a multisubunit protein that is requ ired for the assembly of nucleosomes onto newly replicated DNA in vitr o. The p55 polypeptide comprises seven WD repeat motifs and is homolog ous to the mammalian RbAp48 protein, which is associated with the HD1 histone deacetylase. dCAF-1 was immunopurified by using affinity-purif ied antibodies against p55; the resulting dCAF-1 preparation possessed the four putative subunits of dCAF-1 (p180, p105, p75, and p55) and w as active for DNA replication-coupled chromatin assembly. Moreover, dC AF-1 activity was specifically depleted with antibodies against p55. T hus, p55 is an integral component of dCAF-1. p55 is localized to the n ucleus and is present throughout Drosophila development. Consistent wi th the homology between p55 and the HD1-associated RbAp48 protein, his tone deacetylase activity was observed to coimmunoprecipitate specific ally with p55 from a Drosophila nuclear extract. Furthermore, a fracti on of the p55 protein becomes associated with the newly assembled chro matin following DNA replication. These findings collectively suggest t hat p55 may function as a link between DNA replication-coupled chromat in assembly and histone modification.