EUKARYOTIC METHIONYL AMINOPEPTIDASES - 2 CLASSES OF COBALT-DEPENDENT ENZYMES

Using partial amino acid sequence data derived from porcine methionyl aminopeptidase (MetAP; methionine aminopeptidase, peptidase M; EC 3.4. 11.18), a full-length clone of the homologous human enzyme has been ob tained. The cDNA sequence contains 2569 nt with a single open reading frame corresponding to a protein of 478 amino acids. The C-terminal po rtion representing the catalytic domain shows limited identity with Me tAP sequences from various prokaryotes and yeast, while the N terminus is rich in charged amino acids, including extended strings of basic a nd acidic residues. These highly polar stretches likely result in the spuriously high observed molecular mass (67 kDa). This cDNA sequence i s highly similar to a rat protein, termed p67, which was identified as an inhibitor of phosphorylation of initiation factor eIF2 alpha and w as previously predicted to be a metallopeptidase based on limited sequ ence homology. Model building established that human MetAP (p67) could be readily accommodated into the Escherichia coli MetAP structure and that the Co2+ ligands were fully preserved. However, human MetAP was found to be much more similar to a yeast open reading frame that diffe red markedly from the previously reported yeast MetAP. A similar parti al sequence from Methanothermus fervidus suggests that this p67-like s equence is also found in prokaryotes. These findings suggest that ther e are two cobalt-dependent MetAP families, presently composed of the p rokaryote and yeast sequences (and represented by the E. coli structur e) (type I), on the one hand, and by human MetAP, the yeast open readi ng Frame, and the partial prokaryotic sequence (type II), on the other .