Structure and function of histone methyltransferases

Histones are the major protein constituent of chromatin in the eukaryotic nucleus. These proteins undergo a host of different post-translational modifications, including phosphorylation, acetylation, and methylation, which have profound effects on the remodeling of chromatin. Histone modifications can function either individually or combinatorially to govern such processes as transcription, replication, DNA repair, and apoptosis. Recent studies have focused on histone arginine and lysine methylation and the roles of these modifications in transcriptional regulation and the establishment of heterochromatin. Concomitantly, several families of histone methyltransferases (HMTs) have been identified that catalyze the methylation of specific arginines or lysines in histones H3 and H4. Not surprisingly, many of these methyltransferase genes had been previously identified as important genetic regulators in organisms such as yeast and Drosophila, which underscores the importance of histone methylation in transcriptional control and chromatin remodeling. Structures of several representatives of these HMT families have recently been determined, yielding insight into their catalytic mechanism and histone substrate specificity. The focus of this review is to briefly summarize the roles of histone methylation in chromatin remodeling and to discuss the structures, substrate specificities, and mechanisms of the different classes of HMTs.