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

a single G1cNAc by G1cNAc transferase I (GnT 1),' before the final hydrolytic steps in the pathway are accomplished by a-mannosidase II (Man 11), catalyzing the removal of a1,3and a1,6-mannosyl residues (50) . The trimming and elongation phases ofthe pathway overlap at the GnT I/Man II steps, with each reaction being obligatory for further processing steps. GnT I is essential for processing to hybrid or complex type structures (34) , while the absence of Man II activity, either by inhibition with the alkaloid, swainsonine (52) , or in the human autosomal genetic disease hereditary erythroblastic multinuclearity associated with positive acidified serum (HEMPAS), characterized by the reduced expression of Man 11 (12) , results in the accumulation of Asn-linked hybrid oligosaccharides in lieu of the standard array of complex type structures . The cleavage of glycoprotein processing in-termediates by Man II also confers resistance to cleavage by endoglycosidase H, a commonly used marker for transit through the Golgi complex (42) . A number of the mammalian Golgi glycosyltransferases havebeen cloned recently (1, 7, 17, 21, 25, 32, 45, 46, 53, 56) allowing a comparison of the polypeptide structures and the ability to examine the regulation of transferase expression in relation to terminal oligosaccharide processing events. Among these enzymes a common domain motifhas been described (37) of a type II transmembrane structure with a small NHz-terminal cytoplasmic tail, a single transmembrane domain, a variable stem region, and a large 000Hterminal catalytic domain . Among the processing hydrolases, only an a-mannosidase presumably responsible for the removal of a single a1,2-mannosyl linkage in the ER has been fully cloned (2) . This enzyme bears no resemblance to the glycosyltransferase domain structure, but it does share extensive homology to the yeast vacuolar a-mannosidase. Both enzymes contain no classical signal sequence or membrane-spanning domain. Characterization of the primary structures and molecular aspects of the early processing steps therefore await the cloning and characterization of the remainder of the processing a-glucosidases and a-mannosidases .