Authors
L Kafkova2; E W Debler3; J C Fisk2; K Jain1; S G Clarke1; L K Read2; 1 Department of Chemistry and Biochemistry and The Molecular Biology Institute, University of California, Los Angeles, CA, United States; 2 Department of Microbiology and Immunology, Witebsky Center for Microbial Pathogenesis and Immunology, SUNY Buffalo School of medicine, Buffalo, NY, United States; 3 Howard Hughes Medical Institute, The Rockefeller University, New York, NY, United States Discussion
Arginine methylation is posttranslational modification catalyzed by protein arginine methyltransferases (PRMTs). In Trypanosoma brucei, over ten percent of the proteome bears arginine methylmarks. These include proteins in wide-ranging processes such as RNA processing, DNA repair, metabolism, and protein trafficking. The majority of asymmetrically dimethylated arginine is catalyzed in vivo by the T. brucei homolog of human PRMT1, previously termed TbPRMT1. Here, we show that TbPRMT1 functions as a heteromeric enzyme-prozyme pair. In vivo and in vitro studies demonstrate that active TbPRMT1 is a heterotetramer comprised of two subunits: TbPRMT1ENZ (previously TbPRMT1) and TbPRMT1PRO, an apparently inactive paralog. Heterotetrameric TbPRMT1 catalyzes robust production of monomethylarginine and asymmetric dimethylarginine. Mutational analysis definitively demonstrates that TbPRMT1ENZ is the sole AdoMet binding and catalytic subunit. Conserved catalytic residues in TbPRMT1PRO are completely dispensable for complex function, but TbPRMT1PRO is essential for allosteric activation of TbPRMT1ENZ. These results expand the prozyme paradigm in T. brucei, and present a novel mode of PRMT function that likely promotes unique types of PRMT regulation. 
