O-Mannosylation is Required for Degradation of the Endoplasmic Reticulum-associated Degradation Substrate Gas1*p via the Ubiquitin/Proteasome Pathway in Saccharomyces cerevisiae

Abstract

In Saccharomyces cerevisiae, protein O-mannosylation, which is executed by protein O-mannosyltransferases, is essential for a variety of biological processes as well as for conferring solubility to misfolded proteins. To determine if O-mannosylation plays an essential role in endoplasmic reticulum-associated degradation (ERAD) of misfolded proteins, we used a model misfolded protein, Gas1*p. The O-mannose content of Gas1*p, which is transferred by protein O-mannosyltransferases, was higher than that of Gas1p. Both Pmt1p and Pmt2p, which do not transfer O-mannose to correctly folded Gas1p, participated in the O-mannosylation of Gas1*p. Furthermore, in a pmt1 Delta pmt2 Delta double-mutant background, degradation of Gas1*p is altered from a primarily proteasome dependent to a vacuolar protease-dependent pathway. This process is in a manner dependent on a Golgi-to-endosome sorting function of the VPS30 complex II. Collectively, our data suggest that O-mannosylation plays an important role for proteasome-dependent degradation of Gas1*p via the ERAD pathway and when O-mannosylation is insufficient, Gas1*p is degraded in the vacuole. Thus, we propose that O-mannosylation by Pmt1p and Pmt2p might be a key step in the targeting of some misfolded proteins for degradation via the proteasome-dependent ERAD pathway.

Secretory Protein Biogenesis and Traffic in the Early Secretory Pathway

“…O-linked mannosyl modification of secretory proteins in the ER is essential in yeast (Gentzsch and Tanner 1996) and required for cell wall integrity as well as normal morphogenesis (Strahl-Bolsinger et al 1999). The role of O-linked glycosylation in ER quality control processes remains unclear although investigators have reported influences of specific pmt mutations on turnover rates of misfolded glycoproteins (Harty et al 2001;Vashist et al 2001;Hirayama et al 2008;Goder and Melero 2011) and the PMT genes are upregulated by activation of the UPR (Travers et al 2000).…”

TheO-Mannosyltransferase PMT4 Is Essential for Normal Appressorium Formation and Penetration inUstilago maydis

“…Over the last decade, many roles have been described for fungal O-mannosylation, including in cell wall integrity, cell morphology, and the stability, sorting, and localization of proteins Hirayama et al, 2008;Lommel and Strahl, 2009). The initial O-mannosyltransferase reaction is performed by protein mannose transferases (PMTs), which are integral endoplasmic reticulum membrane proteins that add the first mannose to Ser or Thr residues of target proteins.…”

Protein O-mannosyltransferases participate in ER protein quality control

“…The fate of misfolded proteins that have been Omannosylated is controversial. Whereas some of them seem increasingly protected from degradation, others are reported to be degraded by the cytosolic proteasome to which they are targeted by an unknown mechanism (Harty et al, 2001;Hirayama et al, 2008). Interestingly, like most ER chaperones or members of the ERAD machineries, PMTs are upregulated during ER stress by the unfolded protein response (UPR) (Travers et al, 2000).…”