Novel roles for O-linked glycans in protein folding.

N-Glycosylation has long been linked to protein folding and quality control in the endoplasmic reticulum (ER). Recent work has shown that O-linked glycosylation and the corresponding glycosyltransferases also participate in this important function. Notably, Protein O-fucosyltransferase 1 (Ofut1/Pofut1), a soluble, ER localized enzyme that fucosylates Epidermal Growth Factor-like (EGF) repeats, functions as a chaperone involved in the proper localization of the Notch receptor in certain contexts. Pofut2, a related enzyme that modifies Thrombospondin type I repeats (TSRs), has also been hypothesized to play a role in the folding and quality control of TSR-containing proteins. Both enzymes only modify fully folded substrates suggesting that they are able to distinguish between folded and unfolded structures. Pofuts have known physiological relevance and are conserved across metazoans. Though consensus sequences for O-fucosylation have been established and structures of both Pofuts have been studied, the mechanism of how they participate in protein folding is not known. This article discusses past and recent advances made in novel roles for these protein O-glycosyltransferases.