A novel pathway of import of alpha-mannosidase, a marker enzyme of vacuolar membrane, in Saccharomyces cerevisiae

We have investigated the vacuolar delivery of alpha-mannosidase, a marker enzyme of the vacuolar membrane in the yeast Saccharomyces cerevisiae, and found that the enzyme has several unique characteristics in its biosynthesis and vacuolar delivery. alpha-Mannosidase has no typical signal sequence (Yoshihisa, T., and Anraku, Y. (1989) Biochem. Biophys. Res. Commun. 163, 908-915) but is located on the inner surface of the vacuolar membrane. The enzyme is synthesized as a 107-kDa polypeptide and converted to a 73-kDa polypeptide. Although the conversion depends on a vacuolar processing protease, proteinase A, it is much slower (t1/2 = 10 h) than the proteinase A-dependent processing of other vacuolar proteins. None of Asn-X-Thr/Ser sites on the 107-kDa alpha-mannosidase or on two alpha-mannosidase-invertase fusion proteins that are localized inside the vacuole receives N-linked oligosaccharide, whereas those sites on a carboxypeptidase Y-alpha-mannosidase fusion protein are N-glycosylated. The newly synthesized alpha-mannosidase is normally delivered to the vacuole and converted to the 73-kDa polypeptide even when the secretory pathway is blocked by a subset of sec mutations. These characteristics are different from those of other vacuolar proteins targeted to the vacuole via the secretory pathway. We conclude that alpha-mannosidase is delivered to the vacuole in a novel pathway separate from the secretory pathway.