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

Although Man II appears to have the domain structure common to the other Golgi glycosyltransferases no direct sequence homology was found with any of the glycosyltransferases. The previously described similarity between rat Man II and rabbit GnT 1 (45) was found to be marginally significant (4 SDs above random) . When the murine and human Man II equivalents of this sequence were compared to either the rabbit or human (15) GnT I sequences the similarities were even less significant (2 .1 and 1 .7 SDs above random for the murine and human Man II sequences, respectively) . Comparison of the Man II polypeptide to a translated form of the GenBank database (GenPept, version 64 .3) revealed that a 215 amino acid region of Man II bears a statistically significant similarity to a corresponding region in the rat ER a-mannosidase (2) and the vacuolar ci-mannosidase from Saccharomyces cerevisiae (59) (6 .0 and 9 .7 SDs above random, Fig . 4) . The rat ER a-mannosidase and the yeast vacuolar a-mannosidase share extensive homology across their entire length (2), whereas Man II shares a more limited similarity within a region of the soluble catalytic domain of the enzyme . All three enzymes recognize a-mannoside linkages and cleave the synthetic substrates p-nitrophenyl a-Dmannoside and 4-methylumbelliferyl a-D-mannoside, but differ in their specificity toward natural high mannose oligosaccharides, inhibition by alkaloid inhibitors, antibody crossreactivity, and subcellular-_Iocalization (2, 29, 59, 60) . The sequence similarity and the common activity toward synthetic substrates between the three enzymes would, however, suggest that this region may represent a portion of the active site involved in a-mannoside recognition .