AMINO-TERMINAL PROTEIN PROCESSING IN SACCHAROMYCES-CEREVISIAE IS AN ESSENTIAL FUNCTION THAT REQUIRES 2 DISTINCT METHIONINE AMINOPEPTIDASES

We previously characterized a methionine aminopeptidase (EC 3.4.11.18; Met-AP1; also called peptidase M) in Saccharomyces cerevisiae, which differs from its prokaryotic homologues in that it (i) contains an N-t erminal zinc-finger domain and (ii) does not produce lethality when di srupted, although it does slow growth dramatically; it is encoded by a gene called MAP1. Here we describe a second methionine aminopeptidase (Met-AP2) in S. cerevisiae, encoded by MAP2, which was cloned as a su ppressor of the slow-growth phenotype of the map1 null strain. The DNA sequence of MAP2 encodes a protein of 421 amino acids that shows 22% identity with the sequence of yeast Met-AP1. Surprisingly, comparison with sequences in the GenBank data base showed that the product of MAP 2 has even greater homology (55% identity) with rat p(67), which was c haracterized as an initiation factor 2-associated protein but not yet shown to have Met-AP activity. Transformants of map1 null cells expres sing MAP2 in a high-copy-number plasmid contained 3-to 12-fold increas es in Met-AP activity on different peptide substrates. The epitope-tag ged suppressor gene product was purified by immunoaffinity chromatogra phy and shown to contain Met-AP activity; To evaluate the physiologica l significance of Met-AP2, the MAP2 gene was deleted from wild-type an d map1 null yeast strains. The map2 null strain, like the map1 null st rain, is viable but with a slower growth rate. The map1, map2 double-n ull strains are nonviable. Thus, removal of N-terminal methionine is a n essential function in yeast, as in prokaryotes, but yeast require tw o methionine aminopeptidases to provide the essential function which c an only be partially provided by Met-AP1 or Met-AP2 alone.