Isolation, characterization, and expression of cDNAs encoding murine alpha-mannosidase II, a Golgi enzyme that controls conversion of high mannose to complex N-glycans

Man II occupies a central position in the Asn-linked oligosaccharide processing pathway acting as the committed step in the synthesis ofcomplex type structures (29) . The enzyme has long been considered a marker for the Golgi complex by cell fractionation (14) and immunocytochemistry (35) , as well as being the biochemical marker responsible for the conversion of N-linked glycans to structures which are resistant to cleavage by endoglycosidase H (50) . Intact Man II has been purified and characterized from rat liver (29) and shown to be a disulfide-linked homodimer (30) with a catalytic domain oriented toward the Golgi lumen. Protease protection experiments in intact Golgi membranes were unable to detect any cytoplasmically oriented polypeptide (28) . The release of a soluble form of the enzyme following chymotrypsin digestion ofsolubilized Golgi membranes allowed us to purify and compare the intact and soluble forms of the enzyme demonstrating that the soluble catalytic domain retains all of the catalytic characteristics of the intact enzyme. The cloning of the full-length murine Man II open reading frame has allowed us to demonstrate that Man II shares several of the biochemical features of the Golgi glycosyltransferases (37) , namely a type II transmembrane orientation with an NH2-terminal membrane anchor domain and susceptibility to proteolytic release of a soluble, catalytically active form of the enzyme.