Summary
Newly synthesized proteins of the secretory pathway are quality-controlled inside the endoplasmic reticulum (ER) and, if not properly folded, are retained. An exception are glycosylphosphatidylinositol-anchored proteins (GPI-APs) which can leave the ER even when misfolded and are routed to the vacuole/lysosome for degradation by largely unknown mechanisms linked to post-ER quality control. Using yeast as model organism, we show that Gas1*, an ER-exported misfolded GPI-AP, is diverted from the secretory pathway to endosomes for transport to the vacuole. However, Gas1* is not sorted into endosomal intraluminal vesicles but internalizes directly from the vacuolar membrane. There, the vacuolar protease Pep4, but not any other known vacuolar protease, is required for Gas1* internalization. Our data reveal novel and unexpected mechanisms for invaginations from the vacuolar membrane.
Highlights
ER-exited misfolded GPI-anchored proteins are routed to the vacuole via endosomes but do not internalize into intraluminal vesicles
Internalization occurs directly from the vacuolar membrane into intravacuolar mobile structures
Internalization from the vacuolar membrane depends on the proteolytic activity of the vacuolar protease Pep4
Footnotes
Abbreviations: GPI anchor: glycosylphophatidylinositol anchor; ESCRT: endosomal sorting complexes required for transport; MVBs: multi vesicular bodies; ILVs: intraluminal vesicles; ERAD: endoplasmic reticulum-associated protein degradation; TMD: transmembrane domain; PVC: prevacuolar compartment; SNARE: soluble NSF attachment protein receptor; Cvt vesicle: cytoplasm to vacuole targeting vesicle