Formation of histone proteins and histone methyl modifications.

Abstract

Histones are linked to many diverse nuclear events due to their structural association with DNA. The relevance of histones to nuclear events is also due to modifications, such as lysine methylation, found at specific residues. Non-histone proteins bind histone modifications and mediate the aforementioned processes. A crucial connection between the direct structural role of histone proteins and the indirect recruitment role of histone modifications is that the canonical histones are translated during DNA replication as generally unmodified proteins. Thus, the question arises of how and when histones acquire their modifications following replication-dependent synthesis and pertains to nearly all histone modifications as well as the processes linked to those modifications. To interrogate the formation of specific histone methylation marks in reference to the formation of histone proteins, in vivo pulse labeling procedures with isotopically heavy methionine and lysine were developed and applied to unsynchronized and synchronized HeLa cells. Notable among the results is that trimethylation of lysine 9 and lysine 27 on histone H3 requires nearly the full duration of the HeLa cell cycle to be complete, which has important implications for how both marks can stably maintain silent transcriptional states in an epigenetic manner. The discussions of these results will hopefully inspire future investigations into the compelling properties of histone modifications and histone proteins.