Analysis of the yeast arginine methyltransferase Hmt1p/Rmt1p and its in vivo function - Cofactor binding and substrate interactions

Many eukaryotic RNA-binding proteins are modified by methylation of arginin e residues. The yeast Saccharomyces cerevisiae contains one major arginine methyltransferase, Hmt1p/Rmt1p, which is not essential for normal cell grow th, However, cells missing HIMT1 and also bearing mutations in the mRNA-bin ding proteins Np13p or Cbp80p can no longer survive, providing genetic back grounds in which to study Hmt1p function. We now demonstrate that the catal ytically active form of Hmt1p is required for its activity in vivo. Amino a cid changes in the putative Kmt1p S-adenosyl-L-methionine-binding site were generated and shown to be unable to catalyze methylation of Np13p in vitro and in vivo or to restore growth to strains that require HMT1. In addition these mutations affect nucleocytoplasmic transport of Np13p. A cold-sensit ive mutant of Hmt1p was generated and showed reduced methylation of Np13p, but not of other substrates, at 14 degrees C. These results define new aspe cts of Hmt1 and reveal the importance of its activity in vivo.