Free glycans derived from O-mannosylated glycoproteins suggest the presence of an O-glycoprotein degradation pathway in yeast

In eukaryotic cells, unconjugated oligosaccharides structurally related to N-glycans (FNGs) are generated either from misfolded N-glycoproteins destined for endoplasmic reticulum (ER)-associated degradation (ERAD), or from lipid-linked oligosaccharides, donor substrates for N-glycosylation of proteins. The mechanism responsible for the generation of FNGs is now well understood, but whether there are other type of free glycans remain unclarified. Here, we report on the accumulation of O-mannosylated free glycans in budding yeast that were cultured in media containing mannose as a carbon source. A structural analysis of these glycans revealed that their structures are identical to those of O-mannosyl glycans that are attached to glycoproteins. Deletion of the cyc8 gene, encoding a general transcription repressor, resulted in the accumulation of excessive amounts of the free O-glycans, concomitant with a severe growth defect, a reduction in the level of cellular O-mannosylated proteins, and compromised cell wall integrity. Our findings provide evidence for a regulated pathway for O-glycoprotein degradation in yeast and offer critical insights into the catabolic mechanisms that control the fate of O-glycosylated proteins. Overall design: RNA-seq