Endoplasmic reticulum associated degradation (ERAD) overflow pathways.
Accumulation of misfolded proteins in the endoplasmic reticulum (ER) causes numerous human pathologies. Biochemical evidence suggests that soluble misfolded proteins are retrotranslocated out of the ER, via the endoplasmic reticulum associated degradation (ERAD) pathway, for proteosome-mediated cytoplasmic degradation. Excess, misfolded- or insoluble proteins, are suggested to cause induction of “overflow” degradation pathways. For soluble proteins, overflow to vacuole-mediated destruction is suggested to occur via two Golgi-to-vacuole (Gvt) routes, the alkaline phosphatase (ALP), direct route, or, a carboxypeptidase Y- (CPY-), prevacuolar compartmentvacuole, indirect route, though only the CPY route is thought to degrade soluble proteins. Insoluble aggregate-containing structures are suggested to be degraded by engulfment by membranes of unknown origin and trafficking to the vacuole for destruction, via an autophagic pathway. To confirm biochemical evidence, wild-type (BY4742), autophagosome- (W303/ATG14), CPY- and autophagy pathway- (W303/VPS30), and proteosome (WCG/2) mutants of S. cerevisiae yeasts were transformed with a high expression pYES plasmid and mutant (Z) human alpha-1- proteinase inhibitor (A1PiZ), giving rise to the derivatives cells BY4742/Z, W303/ATG14/Z, W303/VPS30/Z and WCG/2/Z, respectively. Electron microscopy using gold labeling for A1PiZ, markers for the ER, the ERAD ER channel protein, Sec61, or the chaperone, binding protein (BiP), ALP for the ALP pathway, and CPY for the CPY pathway, was used. Overexpression of A1PiZ seems to result in targeting to the vacuole via a prevacuolar, CPY-like compartment (PVC, 200-500 nm), though CPY and A1PiZ appears not to colocalise, unconvincingly confirming collaborative biochemical data. Large amounts of A1PiZ localise in the cytosol, possibly indicating a largely proteasome-mediated degradation. ER-resident A1PiZ targeting to the vacuole seems also to occur by the budding of the ER and peripheral plasma membrane or ER membrane only. This occurs in all cells, but especially in ATG14 gene (ΔATG14) mutants, possibly indicating autophagosome-mediated degradation independence, in the latter mutants. The ATG14 mutation gave rise to crescent-shaped, initiating membranelike (IM-like) structures of approximately Cvt vesicle-diameter, possibly indicating that ΔATG14 blocks autophagosome- (500-1000 nm) and Cvt vesicle (100-200 nm) enclosure, after core IM formation.