a-D-Mannosidases of Rat Liver Golgi Membranes MANNOSIDASE I1 IS THE GlcNAcMAN5-CLEAVING ENZYME IN GLYCOPROTEIN BIOSYNTHESIS AND MANNOSIDASES IA AND IB ARE THE ENZYMES CONVERTING MAN9 PRECURSORS TO MAN5 INTERMEDIATES*

Current evidence indicates that the trimming of mannosy1 residues from intermediates in the biosynthesis of the N-linked oligosaccharides of glycoproteins occurs in the Golgi complex. We now present evidence that mannosidase 11 (Tulsiani, D. R. P., Opheim, D. J., and Touster, 0. (1977) J. Bwl Chem 252,3227-3233) is the Golgi enzyme that converts GlcNAcMans species to GlcNAcMans species in completing the mannosyl trimming process required in the biosynthesis of complex type glycoproteins. GlcNAc([3H]Man)6GlcNAc-mannosidase andp-nitrophenyl a-D-mannosidase activities copurify throughout the preparative procedure and show the same properties. In addition to mannosidase IA (Tabas, I., and Kornfeld, S. (1979) J. Biol. Chem 254,11665-11663), a second a-1,2-mannosidase (mannosidase IB) can be prepared from Golgi membranes which is effective in converting ManeGlcNAc to Man6GlcNAc. The two a-l,2-mannosidases are very similar in catalytic properties, but they are also distinguishable by several criteria. Although these two enzymes have not been extensively purified, several lines of evidence lead to the tentative conclusion that they are distinct enzymes. They appear to be present in comparable activities in the Golgi membranes and together account for the a-1,2-mannosidase activity of these membranes. The particular role of each a-l,2-mannosidase remains to be determined.