Functions of Protein <i>C</i>-Mannosylation in Physiology and Pathology

Protein C-mannosylation is a unique type of protein glycosylation in which a single α-mannose is attached to the indole C2 of tryptophan (Trp) through a C–C bond. The Trp-x-x-Trp (WxxW) sequences, whose first Trp residue may be C-mannosylated, constitute the consensus motifs of this rare glycosylation modification. Dpy-19 was recognized as a gene encoding C-mannosyltransferase in Caenorhabditis elegans. DPY19L1 and DPY19L3 were later confirmed as mammalian C-mannosyltransferases. The consensus motif can be found in the thrombospondin type 1 repeat and cytokine receptor type I families as well as in many other proteins, and recent studies suggest critical roles of C-mannosylation in the folding, sorting, and/or secretion of the substrate proteins. We successfully synthesized C-mannosylated Trp-containing Trp-Ser-Pro-Trp (WSPW) peptides. As a result of using these peptides in our investigations, we proposed that C-mannosylation may have biological functions in addition to contributing to the folding and stability of the substrate proteins. In this mini-review, we discuss the biological roles of C-mannosylation in physiology and pathology as based on our recent findings.