| Histone acetylation plays an important role in the assembly and modulation of chromatin structure. Two main types of H&barbelow;istone A&barbelow;cetylT&barbelow;ransferase (HAT), HAT-A and HAT-B, are responsible for the acetylation of histories. The HAT-A enzymes catalyze the acetylation of nucleosomal core histories and are located in the nucleus. The HAT-B enzymes are primarily cytoplasmic and specific for the acetylation of free histories. The only HAT-B identified to date is comprised of two subunits, Hat1p and Hat1p. Hat1p is the catalytic subunit and Hat1p is required for the high affinity binding of Hat1p to histone H4 and for full catalytic activity. While the Hat1p/Hat2p complex was originally isolated from cytoplasmic extracts, evidence suggested that it is present in the nucleus as well. To characterize the nuclear form of this enzyme, we tagged Hat1p and Hat1p with a protein A-TEV-calmodulin binding protein (CBP) tag. By using the tandem affinity purification (TAP) method combined with other chromatographic techniques, we were able to identify several proteins that associate with Hat1p and Hatlp in the yeast nucleus. These proteins include the uncharacterized open reading frame YLL022C (named Hif1p), histone H3, and histone H4. The functional significance of the association of Hif1p with the Hat1p/Hat2p complex is confirmed by the observation that hif1 Delta and hat1Delta strains display similar defects in telomeric silencing and DNA double strand break repair. Functional analysis revealed that Hif1p is a novel histone chaperone that selectively interacts with histories H3 and H4. Hif1p is also a chromatin assembly factor, promoting the deposition of histories in the presence of a yeast cytosolic extract. In vivo, the nuclear Hat1p/Hat2p/Hif1p complex is bound to acetylated histone H4, as well as histone H3. The association of Hif1p with acetylated H4 requires Hat1p and Hat2p to provide a direct link between type B histone acetyltransferases and chromatin assembly. In addition, the histone H4 associated with the nuclear Hat1p/Hat2p/Hif1p complex contains novel post-translational modifications in the core domain. One site of core domain acetylation, lysine 91, lies at the interface between the H3/H4 tetramer and H2A/H2B dimers, and might be critical for the chromatin assembly. |
