Purification and characterization of cellular proteins associated with histone H4 tails.

The histone H4 N-terminal tail has long been regarded as a major regulator in chromatin structure and function. Although the underlying mechanism has not been unraveled, an emerging body of evidence supports that H4 tail and its post-translational modification function as a recruitment motif for key factors required for proper regulation of chromatin transcription. To investigate these aspects, we have generated HeLa cell lines that constitutively express ectopic H4 tail domain for biochemical purification of proteins associated with H4 tail. We found that expressed H4 tails stably associate with sets of transcription regulatory factors and histone methyltransferases distinct from those that associate with histone H3 tails. Importantly, point mutations of four major lysine substrates to block cellular acetylation of ectopic H4 tail significantly inhibited the association of histone methyltransferases and sets of transcription-activating factors, supporting a major role of acetylation on recruitmentbased action of H4 tail during transcription. Further, our transcription analysis revealed that the proteins associated with wild-type/acetylated H4 tail, but not with mutant/unacetylated H4 tail, can enhance p300-dependent chromatin transcription. Taken together, these findings demonstrate novel roles for H4 tail and its acetylation in mediating recruitment of multiple regulatory factors that can change chromatin states for transcription regulation.