Quality control machinery of the endoplasmic reticulum in health and disease

Newly synthesized polypeptides must reach their native fold to accomplish the wide array of biochemical activities essential for life. The cellular machinery involved in the production of properly folded proteins and in the destruction of malfolded polypeptides is broadly referred to as protein quality control. As a major site of protein synthesis, the endoplasmic reticulum (ER) hosts a large array of pro-folding components to assist in client folding. Even so, a significant fraction of proteins fail to fold and are triaged through the ER-associated degradation (ERAD) pathway, a process involving the retrotranslocation of terminally misfolded substrates to the cytoplasm for proteasomal degradation. Although these mechanisms are ubiquitous, constant, and essential for homeostasis of the organelle and cell, little is known about the cross-talk and interactions between pro-folding components and pro-disposal factors. To explore this, we examined in detail the complex formed between the molecular chaperone GRP94 and the ERAD lectin OS-9. These quality control factors had been shown to associate, yet the significance and function of this interaction was not clear. Contrary to expectation, we discovered that GRP94 and OS-9 do not associate for the coordinate disposal of misfolded substrates. Instead, OS-9 preferentially sequesters non-native molecules of GRP94 marked by hyper-glycosylation. These species of GRP94 have altered conformations, are less active, and are degraded by OS-9 in an ERAD-independent, lysosomal-like mechanism. This work demonstrates a novel mode of protein regulation, whereby glycosylation of cryptic acceptor sites dictates the function and fate of an ER molecular chaperone. Whereas the regulation of GRP94 by OS-9 occurs constitutively, we also examined the functional overlap of folding and degradation pathways during disease pathology. We discovered a set of novel ERAD substrates implicated in the development of Tay Sachs disease, a lysosomal storage disorder characterized by loss of function in the enzyme β-hexosaminidase A (HexA). The folding of HexA, as well as its recognition by ERAD machinery, was investigated in detail to establish new avenues for disease intervention. Proof of principle studies to manipulate the endoplasmic reticulum offer hints towards future therapeutic routes for Tay Sachs disease