Peptidase M18 family, aminopeptidase I (vacuolar aminopeptidase I; polypeptidase; Leucine aminopeptidase IV; LAPIV; aminopeptidase III; aminopeptidase yscI; EC 3.4.11.22) subfamily. Aminopeptidase I is widely distributed in bacteria and eukaryotes, but only the yeast enzyme has been characterized to date. It is a vacuolar enzyme, synthesized as a cytosolic proform, and proteolytically matured upon arrival in the vacuole. The pro-aminopeptidase I (proAPI) does not enter the vacuole via the secretory pathway. In non-starved cells, it uses the cytoplasm to vacuole targeting (cvt) pathway and in cells starved for nitrogen, it is targeted to the vacuole via autophagy. Yeast aminopeptidase I is active only in its dodecameric form with broad substrate specificity, acting on all aminoacyl and peptidyl derivatives that contain a free alpha-amino group; this is in contrast to the highly selective M18 mammalian aspartyl aminopeptidase. N-terminal leucine and most other hydrophobic amino acid residues are the best substrates while glycine and charged amino acid residues in P1 position are cleaved much more slowly. This enzyme is strongly and specifically activated by zinc (Zn2+) and chloride (Cl-) ions.
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