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Journal of Biochemistry Advance Access originally published online on January 7, 2008
Journal of Biochemistry 2008 143(4):555-567; doi:10.1093/jb/mvm249
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© 2008 The Japanese Biochemical Society.

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

Hiroto Hirayama1,2, Morihisa Fujita1, Takehiko Yoko-o1 and Yoshifumi Jigami1,2,*

1Research Institute for Cell Engineering, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8566; and 2Graduate School of Life and Environmental Science, University of Tsukuba, Tsukuba, Ibaraki 305-8752, Japan

*To whom correspondence should be addressed. Tel: +81 29 861 6160, Fax: +81 29 861 6161, E-mail: jigami.yoshi{at}aist.go.jp

Received November 22, 2007; Accepted December 26, 2007


  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.

Key Words: ERAD, Gas1*p, O-mannosylation, PMT, post-ER degradation

Abbreviations: CHX, cycloheximide; CPY*, mutant carboxypeptidase Y; ER, endoplasmic reticulum; ERAD, endoplasmic reticulum-associated degradation; GPI, glycosylphosphatidylinositol; HA, haemagglutinin; mutant p{alpha}F, mutant pro-{alpha}-factor; KHN, Kar2p signal sequence fused to simian virus 5 haemagglutinin neuraminidase; KHNt, HA-tagged yeast Kar2p signal sequence fusion to simian virus 5 haemagglutinin neuraminidase; PNGase F, peptide N-glycanase F; {Delta}pro, pro-region-deleted derivative of Rhizopus niveus aspartic proteinase-I; sol-Gas1*p, non-GPI-anchored soluble version of Gas1*p


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