A deletion in the Golgi α-mannosidase II gene of Caenorhabditis elegans results in unexpected non-wild-type N-glycan structures*

The processing of N-linked oligosaccharides by alpha-mannosidases in the endoplasmic reticulum and Golgi is a process conserved in plants and animals. After the transfer of a GlcNAc residue to Asn- bound Man(5)GlcNAc(2) by N-acetylglucosaminyl-transferase I, an alpha-mannosidase (EC 3.2.1.114) removes one alpha 1,3- linked and one alpha 1,6- linked mannose residue. In this study, we have identified the relevant alpha-mannosidase II gene (aman- 2; F58H1.1) from Caenorhabditis elegans and have detected its activity in both native and recombinant forms. For comparative studies, the two other cDNAs encoding class II mannosidases aman-1 (F55D10.1) and aman- 3 (F48C1.1) were cloned; the corresponding enzymes are, respectively, a putative lysosomal alpha-mannosidase and a Co(II)-activated alpha-mannosidase. The analysis of the N-glycan structures of an aman- 2 mutant strain demonstrates that the absence of alpha-mannosidase II activity results in a shift to structures not seen in wild-type worms (e. g. N-glycans with the composition Hex(5-7)HexNAc(2-3)Fuc(2)Me) and an accumulation of hybrid oligosaccharides. Paucimannosidic glycans are almost absent from aman-2 worms, indicative also of a general lack of alpha-mannosidase III activity. We hypothesize that there is a tremendous flexibility in the glycosylation pathway of C. elegans that does not impinge, under standard laboratory conditions, on the viability of worms with glycotypes very unlike the wild-type pattern.