Destabilization of vitelline membrane outer layer protein 1 homolog (VMO1) by C‐mannosylation

C‐mannosylation is a rare type of protein glycosylation whereby a single mannose is added to the first tryptophan in the consensus sequence Trp‐Xaa‐Xaa‐Trp/Cys (in which Xaa represents any amino acid). Its consensus sequence is mainly found in proteins containing a thrombospondin type‐1 repeat (TSR1) domain and in type I cytokine receptors. In these proteins, C‐mannosylation affects protein secretion, intracellular localization, and protein stability; however, the role of C‐mannosylation in proteins that are not type I cytokine receptors and/or do not contain a TSR1 domain is less well explored. In this study, we focused on human vitelline membrane outer layer protein 1 homolog (VMO1). VMO1, which possesses two putative C‐mannosylation sites, is a 21‐kDa secreted protein that does not contain a TSR1 domain and is not a type I cytokine receptor. Mass spectrometry analyses revealed that VMO1 is C‐mannosylated at Trp(105) but not at Trp(44). Although C‐mannosylation does not affect the extracellular secretion of VMO1, it destabilizes the intracellular VMO1. In addition, a structural comparison between VMO1 and C‐mannosylated VMO1 showed that the modification of the mannose changes the conformation of three loops in VMO1. Taken together, our results demonstrate the first example of C‐mannosylation for protein destabilization of VMO1.