| Research Abstract |
Amino-terminal processing of proteins is a very common event. In eukaryotes, translation of proteins is initiated with methionine (Met^i), and more than half of the methionine residue jis cleaved. Subsequently, 60-80% of the intracellular proteins are amino-terminally blocked by acetylation. In this project, I have tried to elucidate the enzyme(s) system related to such amino-terminal processing and further to estimate its biological functions. As the first trial, I have found at least three enzymes are engaged in the event in yeast, Saccharomyces cerevisiae. One is a methionine aminopeptidase (MAP), which specifically removes Met^i from certain proteins before the completion of nascent polypeptide chains, and the others are two N-acetyltransfeases with diffrent specificity, Nat1 and Nat2. Nat1 acts on acetylation for proteins having glycine, alanine, serine, threonine, proline and valine, whereas Nat2 acts only on amino-terminal acetylation for Met-Asp/-Glu/-Asn sequence. Further studies have clarified some molecular properties for each enzymes. For the biological functions of amino-terminal processing, it is suggested that the event tends to convert amino-terminal residues to stabilizing residues in N-end rule presented on the ubiquitin dependent protein degradation sysytem in cell.
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