MOD_CMANNOS

C-Mannosylation is a type of protein glycosylation that involves covalent attachment of an alpha-mannopyranosyl residue to the indole C2 carbon atom of tryptophan via a C-C link (Hofsteenge,1994; de Beer,1995). The exact recognition sequence was determined by site-directed mutagenesis of individual amino acids and was found to be WXXW, where the first tryptophan residue becomes C-mannosylated. The significance of the amino acids in both X positions was not studied as the shortest peptides used consisted of only four amino acids forming a recognition sequence, WAKW (Hartmann,2000). The search for the pattern, restricted to the mammalian proteins that cross the endoplasmic reticulum (ER) membrane, yielded 336 proteins. Some of the proteins found in the database search have already been examined for the presence of C-mannosylation. In total, 49 C-mannosylated tryptophan residues were found in 11 proteins. The precursor in the biosynthesis of (C2-Man)-Trp is dolichylphosphate mannose (Dol-P-Man) precursor in the biosynthetic pathway of C-mannosyltryptophan (Doucey,1998). The whole biosynthetic pathway, from GDPMan through Dol-P-Man to the C-mannosylated peptide was reconstructed in vitro. The activity was found in Caenorhabditis elegans, Amphibia, Aves, Mammalia, but not in Escherichia coli, Insecta and Saccharomyces cerevisiae (Doucey,1998; Krieg,1997). C-mannosyltransferase activity can be found in most of the parts of the mammalian organism (Doucey,1998).