NUCLEOSOMAL DNA REGULATES THE CORE-HISTONE-BINDING SUBUNIT OF THE HUMAN HAT1 ACETYLTRANSFERASE

Background: In eukaryotic cells, newly synthesized histone H4 is acety lated at lysines 5 and 12, a transient modification erased by deacetyl ases shortly after deposition of histones into chromosomes. Genetic st udies in Saccharomyces cerevisiae revealed that acetylation of newly s ynthesized histones H3 and H4 is likely to be important for maintainin g cell viability; the precise biochemical function of this acetylation is not known, however. The identification of enzymes mediating site-s pecific acetylation of H4 at Lys5 and Lys12 may help explain the funct ion of the acetylation of newly synthesized histones. Results: A cDNA encoding the catalytic subunit of the human Hall acetyltransferase was cloned and, using specific antibodies, the Hat1 holoenzyme was purifi ed from human 293 cells. The human enzyme acetylates soluble but not n ucleosomal H4 at Lys5 and Lys12 and acetylates histone H2A at Lys5. Un expectedly, we found Hall in the nucleus of S-phase cells. Like its ye ast counterpart, the human holoenzyme consists of two subunits: a cata lytic subunit, Hall, and a subunit that binds core histones, p46, whic h greatly stimulates the acetyltransferase activity of Hall. Both p46 and the highly related p48 polypeptide (the small subunit of human chr omatin assembly factor 1; CAF-1) bind directly to helix 1 of histone H 4, a region that is not accessible when H4 is in chromatin. Conclusion s: We suggest that p46 and p48 are core-histone-binding subunits that target chromatin assembly factors, chromatin remodeling factors, histo ne acetyltransferases and histone deacetylases to their histone substr ates in a manner that is regulated by nucleosomal DNA.