Protein quality control in the secretory pathway

Abstract

Protein folding is inherently error prone, especially in the endoplasmic reticulum (ER). Even with an elaborate network of molecular chaperones and protein folding facilitators, misfolding can occur quite frequently. To maintain protein homeostasis, eukaryotes have evolved a series of protein quality-control checkpoints. When secretory pathway quality-control pathways fail, stress response pathways, such as the unfolded protein response (UPR), are induced. In addition, the ER, which is the initial hub of protein biogenesis in the secretory pathway, triages misfolded proteins by delivering substrates to the proteasome or to the lysosome/vacuole through ER-associated degradation (ERAD) or ER-phagy. Some misfolded proteins escape the ER and are instead selected for Golgi quality control. These substrates are targeted for degradation after retrieval to the ER or delivery to the lysosome/vacuole. Here, we discuss how these guardian pathways function, how their activities intersect upon induction of the UPR, and how decisions are made to dispose of misfolded proteins in the secretory pathway.

Figure 1.

The three branches of the ERAD pathway in yeast. Lower panels in A, B, and C define the different steps during ERAD-L, ERAD-M, and ERAD-C, respectively. (A) ERAD-L substrates containing lumenal folding lesions (red star) and N-linked glycans (gray diamond) are recognized and processed by an enzyme cascade to generate an ERAD-targeting glycan (yellow diamond). Chaperones (e.g., Kar2) and lectins (e.g., Yos9) capture the substrate for transfer to the Hrd1 complex for retrotranslocation-coupled ubiquitination (purple triangle). (B) ERAD-M substrates containing a membrane-embedded folding lesion (red star) are recognized and ubiquitinated by the Hrd1 complex. (C) ERAD-C substrates containing cytosolic folding lesions are instead recognized by cytosolic chaperones (e.g., Ydj1/Hsp40 and Ssa1/Hsp70), which bridge the Doa10 ubiquitin ligase to an ERAD substrate. The three ERAD branches converge at a Cdc48-complex–dependent retrotranslocation step (D, top). The Cdc48 complex also contains Ufd1/Npl4, which interacts with ubiquitin (purple triangle). Following retrotranslocation, substrates are escorted to the 26S proteasome for degradation with the help of the Ras23 and Dsk2 shuttling factors.

Figure 2.

ER-phagy in yeast and mammals. ER-phagy receptors reside in distinct ER subdomains (yeast, left; mammals, right). ER-phagy receptors concentrate cargo in an ER subdomain via interaction with Atg8 (yeast) or LC3/GABARAP (mammal) in the growing autophagic membrane, the phagophore. ER-phagy substrates are next enclosed in a double-membrane-bound autophagosome, which fuses with the lysosome/vacuole. In yeast, the UPR (black lightning bolt) can give rise to ER whorls, which are nonselectively delivered to the vacuole. In mammals, Sec62 helps reestablish ER homeostasis after UPR induction.

Figure 3.

The GQC pathway in yeast. Misfolded proteins targeted for GQC are transported to the cis-Golgi via COPII vesicles. Proteasome-targeted GQC: ER chaperones (e.g., Kar2 [in pink]) and retrieval receptors (in blue) in the cis-Golgi bind and retrieve misfolded proteins to the ER in COPI vesicles. These substrates are eliminated by ERAD-L. Lysosome/vacuole–targeted GQC: misfolded proteins that migrate to the trans-Golgi are sorted by the ESCRT/MVB pathway via Vps10 (green) or a ubiquitin ligase (e.g., Tul1). Following sorting, misfolded proteins are delivered to the vacuole.

Figure 4.

The fates of misfolded proteins in the ER. Most misfolded proteins in the ER are eliminated by ERAD via the ubiquitin-proteasome system. Some misfolded proteins exit the ER and are sorted to the lysosome/vacuole for degradation. Other misfolded proteins are transported to the lysosome/vacuole for degradation via ER-phagy. The three QC machineries act coordinately to safeguard ER proteostasis and are induced by the UPR (defined by red text).