The major cytoplasmic histone acetyltransferase in yeast: Links to chromatin replication and histone metabolism

We have isolated the predominant cytoplasmic histone acetyltransferase activity from Saccharomyces cerevisiae. This enzyme acetylates the lysine at residue 12 of free histone H4 but does not modify histone H4 when packaged in chromatin. The activity contains two proteins, Hat1p and Hat2p. Hat1p is the catalytic subunit of the histone acetyltransferase and has an intrinsic substrate specificity that modifies lysine in the recognition sequence GXG (K) under bar XG. The specificity of the enzyme in the yeast cytoplasm is restricted relative to recombinant Hat1p suggesting that it is negatively regulated in vivo. Hat2p, which is required for high affinity binding of the acetyltransferase to histone H4, is highly related to Rbap48, which is a subunit of the chromatin assembly factor, CAF-1, and copurifies with the human histone deacetylase HD1. We propose that the Hat2p/Rbap48 family serve as escorts of histone metabolism enzymes to facilitate their interaction with histone H4.