Remodeling an exclusive type III Protein Arginine Methyltransferase

The methylation of arginine side chain guanidino groups is an important post-translational modification found on both cytoplasmic and nuclear proteins. Proteins that are arginine methylated are involved in a number of different cellular processes, including transcriptional regulation, RNA metabolism, and DNA damage repair. These proteins are also important in human diseases, especially in cardiovascular disease and cancer. In cells, arginine methylation is carried out by the family of Protein Arginine Methyltransferases (PRMTs). The PRMTs catalyze three types of arginine methylation products: Monomethylarginine by type I, type II, and type III PRMTs, asymmetric dimethylarginine by type I PRMTs, and symmetric dimethylarginine by type II PRMTs. Each type of methylated arginine can have a distinct biological function; therefore, there is great interest in understanding how product specificity is achieved within the PRMT family. Despite the great amount of information available regarding PRMTs, the biochemistry of these enzymes is not completely understood. Interestingly, most PRMTs have the ability to form two different methylation products. Protein arginine methyltransferase 7 is unique within the family as it is the only exclusive type III methyltransferase known to date, meaning that it can only monomethylate substrates. So far the structural and mechanistic basis for the inability of PRMT7 to dimethylate peptide substrates is unknown. Our hypothesis is that steric hindrances in the active site prevent the PRMT7 enzymes from adding a second methyl group, thus, restricting TbPRMT7 to monomethylation. To test this hypothesis, we initiated mutational studies on Trypanosomal PRMT7, followed by methylation and product analysis assays. As a result we have been able to remodel a type III methyltransferase, into a mixed type I, II and III; that is, an enzyme that can now perform dimethylation. By remodeling the PRMT7 enzyme have a better understanding for how product specificity is regulated by PRMTs