The C‐mannosyltransferase

Abstract

C‐mannosylation of tryptophan residues occurs on proteins containing the consensus sequence WXXW, particularly thrombospondin repeat containing proteins and type I cytokine receptors. Mutagenesis of the WXXW motif in different proteins had already established important roles of this motif, suggesting that protein C‐mannosylation is as vital as N‐or O‐glycosylation. However, the genetic base of the enzyme catalyzing C‐mannosylation was so far not known.We identified C. elegans DPY‐19 as a protein with low sequence identity to the enzyme responsible for N‐glycosylation (oligosaccharyltransferase, OST). Homology is mainly based on identical overall membrane topology. Few amino acids are conserved, but three of these have been suggested to be involved in lipid linked donor‐substrate recognition of OST.We demonstrated that DPY‐19 is a C‐mannosyltransferase. Interestingly, the gene encoding DPY‐19 has long been known from a mutant presenting a defect in neuroblast migration. We showed that the dpy‐19 mutant is devoid of C‐mannosyltransferase activity. Moreover, by recombinant expression of DPY‐19 and mass spectrometric analysis, we could show C‐mannosylation of target proteins such as UNC‐5 and MIG‐21. Remarkably, dpy‐19 and mig‐21 mutants present identical phenotypes in neuroblast migration. Thus, our work showing C‐mannosylation of MIG‐21 by DPY‐19 provides a functional link between both mutants.