Cellular physiology depends on the alteration of protein structures by covalent modification reactions. Using a combination of bioinformatic, genetic, biochemical, and mass spectrometric approaches, it has been possible to probe ribosomal proteins from the yeast Saccharomyces cerevisiae for posttranslationally methylated amino acid residues and for the enzymes that catalyze these modifications. These efforts have resulted in the identification and characterization of the first protein histidine methyltransferase, the first N-terminal protein methyltransferase, two unusual types of protein arginine methyltransferases, and a new type of cysteine methylation. Two of these enzymes may modify their substrates during ribosomal assembly because the final methylated histidine and arginine residues are buried deep within the ribosome with contacts only with RNA. Two of these modifications occur broadly in eukaryotes, including humans, while the others demonstrate a more limited phylogenetic range. Analysis of strains where the methyltransferase genes are deleted has given insight into the physiological roles of these modifications. These reactions described here add diversity to the modifications that generate the typical methylated lysine and arginine residues previously described in histones and other proteins.
Regulation of biological function by protein methylation reactionsIt is more and more apparent just how much of biological function is dependent upon posttranslational modifications (1). The human genome encodes some 900 enzymes catalyzing just protein phosphorylation or ubiquitination (2,3). However, it is now clear that methyl groups can stand beside phosphate groups and ubiquitin as major players in controlling the physiological functions of proteins. We are beginning to understand how the much greater diversity of protein methylation reactions can give rise to a greater diversity of function (1,4-8). We are also learning the importance of crosstalk between protein and DNA methylation reactions (9) and among protein methylation, phosphorylation, acetylation, and ubiquitination reactions (10-12). Finally, we are discovering how alterations in protein methylation pathways can lead to human pathology, particularly in cancer (13-15).The collection of over sixty human protein arginine and lysine methyltransferases that leave histone "marks" recognized by reader proteins to guide gene expression are perhaps the poster children for the importance of protein methyltransferases (16-17). However, recent work has emphasized the importance of methylating non-histone substrates at these residues, particularly ribosomal proteins, translation factors, and transcription factors in a wide variety of organisms (7,8,15,18,19). Furthermore, the methylation of lysine and arginine residues represents just the tip of the iceberg in protein methylation -modifications have also been established at histidine, modified histidine (diphthamide), glutamate, glutamine, asparagine, Lisoaspartate, D-aspartate, cysteine, isoprenylcysteine, me...