Authors

Discussion

Arginine methylation is a key post-translational modification that can alter the structure, dynamics and interaction profiles of proteins. Protein arginine methyltransferases (PRMTs) catalyse the transfer of a methyl group from a S-adenosylmethionine molecule onto the arginine side chain guanidino group. Mammalian PRMTs are classified into subtypes – PRMT1, 2, 3, 4, 6 and 8 catalyse asymmetric dimethylation (ADMA); PRMT5 and 9 catalyse symmetric dimethylation (SDMA); and PRMT7 catalyses monomethylation (MMA). Kinetoplastids possess five homologues: PRMT1, 3, 5, 6 and 7. T. brucei PRMT3 has been shown to be a pro-enzyme (prozyme) which lacks key conserved motifs including in the catalytic double E loop. T. brucei PRMT1 is only active in complex with the PRMT3 prozyme. In Leishmania, however, PRMT3 retains the conserved double E loop, which raises questions about its role in this organism. Here we use recombinant protein samples to investigate L. braziliensis (Lbr) PRMT1 and 3 in vitro. Activity assays show that methylation of a substrate peptide only occurs when PRMT1 and 3 are both presence. Analytical size exclusion chromatography (SEC) and SEC-MALLS show that LbrPRMT1 and 3 form a heterotetrameric complex in solution. Mutation of double E loop residues revealed that LbrPRMT1 is the active component of the complex. Previous work suggested LbrPRMT3 could interact with and modulate the activity of other LbrPRMTs, however methyltransferase assays showed that LbrPRMT3 had no effect on the activities of LbrPRMT5 and 7 in vitro. Moreover, LbrPRMT3 could not methylate a peptide substrate previously monomethylated with PRMT7. Our data suggests that LbrPRMT1 and 3 form a similar complex to T. brucei PRMT1-3. However, the retention of the conserved double E loop in Leishmania PRMT3 enzymes suggests an as of yet undiscovered functional difference between the two trypanosmatids.