Extracellular Secretion of Overexpressed Glycosylphosphatidylinositol-Linked Cell Wall Protein Utr2/Crh2p as a Novel Protein Quality Control Mechanism in Saccharomyces cerevisiae

Eukaryotic cells employ a variety of mechanisms to maintain protein quality control and homeostasis. Here we provide evidence that one such mechanism in Saccharomyces cerevisiae involves the regulated release of excess or misfolded proteins to the extracellular space. The overexpression of an epitope-tagged allele of the glycosylphosphatidylinositol (GPI)-linked cell wall protein Utr2/Crh2p (Utr2/Crh2-green fluorescent protein [GFP] or -hemagglutinin [HA]) causes endoplasmic reticulum (ER) stress and the secretion of Crh2-GFP/HA into the extracellular space. Secretion is dependent on two GPI-linked aspartyl proteases (Yps1p/2p) and components of the unfolded protein response (Ire1p and Hac1p) but is independent of ER-associated degradation (ERAD) components such as Hrd1p and Doa10p. Supporting the idea that this process represents a mechanism for protein quality control, the level of Crh2-HA is increased in strains lacking Bst1p, a protein required for the proteasomal degradation of GPI-linked proteins. Furthermore, secretion is dependent on Sec18p, indicating that it requires ER-to-Golgi trafficking, and accordingly, Crh2-HA accumulates in the ER in ire1 Delta and bst1 Delta mutants by cycloheximide chase experiments. Since a fraction of Utr2/Crh2-GFP properly localizes to the cell wall in an ire1 Delta mutant, extracellular secretion appears to occur through a pathway that is distinct from the normal GPI protein-trafficking pathway. Taken together, these data support a model in which the unfolded protein response (UPR)/yapsin-mediated extracellular release of overexpressed Utr2/Crh2-HA or -GFP is an alternative pathway for the removal of excess or misfolded secretory proteins functioning in parallel with proteasome-mediated degradation in S. cerevisiae. This model provides an explanation for the deleterious effects of Yps1/2p on the industrial production of some recombinant proteins in S. cerevisiae.