More direct evidence of a role

More direct evidence of a role for Hat1 in the acetylation of newly synthesized histone H4 has come from a number of recent studies that have shown that Hat1 is responsible for at least a portion of the acetylation that occurs on histone H4 lysines 5 and 12 in the pool of cytosolic histones. In chicken DT40 cells in which the HAT1 gene has been deleted the level of histone H4 lysine 5 and 12 acetylation in bulk chromatin does not change but there is a significant, but not complete, loss of these modifications on soluble histone H4 [59]. A similar result was obtained in S. cerevisiae where treatment of the cells with hydroxyurea leads to Mesoridazine arrest in cells and an accumulation of soluble histones. In the absence of HAT1, there is a large decrease in histone H4 lysine 5 and 12 acetylation on these accumulated cytosolic histones [60]. The siRNA knockdown of Hat1 in mammalian cells has a more subtle effect on cytosolic histone H4 lysine 12 acetylation. The relatively minor effect on lysine 12 acetylation in these experiments may be due to the incomplete repression of Hat1 expression observed with the siRNA knockdown [61]. While definitive proof of Hat1s' involvement in the acetylation of newly synthesized histone H4 will require the use of pulse-labeling techniques in cells genetically deleted for the HAT1 gene, the circumstantial evidence now strongly implicates Hat1 in this process. In addition, the accumulated evidence also strongly supports the idea that Hat1 is not the sole enzyme capable of acetylating newly synthesized histone H4 on lysines 5 and 12. Identifying other HATs that are capable of modifying the pool of new histones will significantly advance our efforts to understand the function of this conserved modification pattern.
A conserved core structure for type B histone acetyltransferases Hat1 does not appear to function in isolation. However, in stark contrast to the large, multi-subunit complexes associated with type A HATs, Hat1 has been has been found to be a component of complexes that have remarkably simple subunit compositions [62]. In yeast, the core Hat1p complex appears to consist of Hat1p and Hat2p. This complex was originally purified from cytoplasmic extracts [50]. Similar complexes were subsequently isolated from a variety of other organisms (human, Xenopus, chicken and corn) where the Hat2p subunit is one of the homologous proteins Rbap46 or Rbap48 (Rbbp7/Rbbp4) [63], [64], [65]. Hat2p and related proteins are WD repeat proteins that are components of several types of complexes that are involved in histone metabolism such as CAF-1, HDAC complexes, NURF and PRC2 [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79]. These proteins have been shown to possess histone chaperone activity and, therefore, are thought to mediate the interactions of these varied complexes with histones [65], [80], [81], [82], [83]. The basic subunit structure of the core Hat1 complex, a HAT associated with a histone chaperone, may be a conserved property of type B histone acetyltransferases. Rtt109p shares several important characteristics with Hat1p, which suggest that it be considered as the second member of the type B histone acetyltransferase family. This enzyme is responsible for the acetylation of newly synthesized histone H3 lysine 56 and contributes to the acetylation of several lysine residues in the H3 NH2-terminal tail. In addition, Rtt109p is specific for non-nucleosomal substrates. Interestingly, this enzyme is also found as part of a two-subunit complex with Vps75p, a NAP1-family histone chaperone. As seen for the association of Hat2p with Hat1p, the binding of Vps75p to Rtt109p has a dramatic effect on the enzyme's catalytic activity [21], [22], [23], [24], [84], [85], [86], [87], [88], [89]. Hence, while there does not appear to be any structural similarity between the components of the Hat1 and Rtt109 complexes, the functional significance of the association of type B HATs with a partner histone chaperone has been conserved.