We present a comprehensive analysis of the human methyltransferasome. Primary sequences, predicted secondary structures, and solved crystal structures of known methyltransferases were analyzed by hidden Markov models, Fisher-based statistical matrices, and fold recognition prediction-based threading algorithms to create a model, or profile, of each methyltransferase superfamily. These profiles were used to scan the human proteome database and detect novel methyltransferases. 208 proteins in the human genome are now identified as known or putative methyltransferases, including 38 proteins that were not annotated previously. To date, 30% of these proteins have been linked to disease states. Possible substrates of methylation for all of the SET domain and SPOUT methyltransferases as well as 100 of the 131 seven--strand methyltransferases were surmised from sequence similarity clusters based on alignments of the substrate-specific domains. Molecular & Cellular Proteomics 10: 10.1074/mcp.M110.000976, 1-12, 2011.A significant percentage of proteins across all organisms are enzymes that catalyze the transfer of a methyl group from the cofactor S-adenosylmethionine to a substrate (1-5). In yeast, these proteins make up about 1.2% of all gene products (6, 7). The ability of methyltransferases to use a variety of different substrates, including RNA, DNA, lipids, small molecules, and proteins, is responsible for their diverse roles in different biological pathways (1-3). Methyltransferases have been shown to be essential in epigenetic control, lipid biosynthesis, protein repair, hormone inactivation, and tissue differentiation (8 -14). The identification of new enzymes may allow the delineation of additional pathways and modes of regulation as well as increase our understanding of S-adenosylmethionine metabolism.Although there are hundreds of known substrates for these reactions, methyltransferases are found in a small number of distinct structural arrangements that are used to classify them into superfamilies (2,3,5,6,15). Proteins in each superfamily also share conserved amino acid sequences. The seven--strand superfamily (also referred to as "Class I" methyltransferases) is the most abundant. These proteins catalyze a wide array of substrates and feature a Rossmann-like structural core (2,3,5,6,15). The SPOUT methyltransferase superfamily contains a distinctive knot structure and methylates RNA substrates (16). SET domain methyltransferases catalyze the methylation of protein lysine residues with histones and ribosomal proteins as major targets (17)(18)(19). Smaller superfamilies with at least one three-dimensional structure available include the precorrin-like methyltransferases (20), the radical SAM 1 methyltransferases (21, 22), the MetH activation domain (23), the Tyw3 protein involved in wybutosine synthesis (24), and the homocysteine methyltransferases (25-27). Lastly, an integral membrane methyltransferase family has been defined by sequence alone where no three-dimensional structure is yet available (28,29).Ad...