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 α-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 Man5GlcNAc2 by N-acetylglucosaminyltransferase I, an α-mannosidase (EC 3.2.1.114) removes one α1,3-linked and one α1,6-linked mannose residue. In this study, we have identified the relevant α-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 α-mannosidase and a Co(II)-activated α-mannosidase. The analysis of the N-glycan structures of an aman-2 mutant strain demonstrates that the absence of α-mannosidase II activity results in a shift to structures not seen in wild-type worms (e.g. N-glycans with the composition Hex5-7HexNAc2-3Fuc2Me) and an accumulation of hybrid oligosaccharides. Paucimannosidic glycans are almost absent from aman-2 worms, indicative also of a general lack of α-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.