Quality control: ER-associated degradation: protein quality control and beyond

Abstract

Even with the assistance of many cellular factors, a significant fraction of newly synthesized proteins ends up misfolded. Cells evolved protein quality control systems to ensure that these potentially toxic species are detected and eliminated. The best characterized of these pathways, the ER-associated protein degradation (ERAD), monitors the folding of membrane and secretory proteins whose biogenesis takes place in the endoplasmic reticulum (ER). There is also increasing evidence that ERAD controls other ER-related functions through regulated degradation of certain folded ER proteins, further highlighting the role of ERAD in cellular homeostasis.

Figure 1.

The different steps and branches in ERAD. (A) The events defining the ERAD of a generic luminal substrate with a misfolded domain (red star). Substrate recognition, retrotranslocation, and ubiquitination are coordinated by a membrane-embedded E3 ligase complex. Ubiquitin is depicted as small circles. (B) The E3 ligase complexes involved in ERAD in S. cerevisiae and their substrate specificities. ER proteins with a misfolded domain in the cytoplasm (ERAD-C substrates) are degraded via the Doa10 complex. Proteins with luminal (ERAD-L) or intramembrane (ERAD-M) misfolded domains are degraded via the Hrd1 complex. Misfolded domains on proteins are indicated by a red star. The Cdc48 cofactors Npl4 and Ufd1 are depicted as N and U, respectively.

Figure 2.

Mechanisms of substrate recognition in ERAD. (A) Recognition of misfolded luminal glycoproteins in yeast. Newly synthesized glycoproteins are bound by lectins and other chaperones which facilitate their folding. If properly folded, the proteins leave the ER. Prolonged residency in the ER, indicative of a persistent misfolded domain (red star), leads to Htm1-dependent exposure of an α-1,6–linked mannose residue (red bar). Together, the misfolded domain and the terminal α-1,6 mannose form the degradation signal recognized by Hrd3/Yos9. (B) Recognition of native MHC I heavy chain by the cytomegalovirus -encoded US2 adaptor in infected cells. US2 binds to folded MHC I in the ER membrane and delivers it to an E3 ligase complex containing the E3 Trc8 and the signal-peptide peptidase SPP resulting in MHC I degradation by ERAD. (C) Sterol-dependent recognition of HMGR by Insigs in mammalian cells. Under low sterol levels, HMGR is a stable protein at the ER membrane. High sterol levels, in particular the accumulation of 24,25-dihydrolanosterol (four-ringed structure in gray), cause Insig to bind to HMGR and to deliver it to an E3 ligase complex that promotes HMGR degradation by ERAD. The p97 cofactors Npl4 and Ufd1 are depicted as N and U, respectively. Ubiquitin is depicted as small circles.

Figure 3.

A working model for Hrd1-mediated retrotranslocation of a luminal misfolded glycoprotein. Upon recognition (not depicted), the misfolded protein (gray) is transferred to Hrd1. The binding can occur either to Hrd1 monomers or to Usa1-mediated Hrd1 dimers (1). Substrate-bound Hrd1 dimer self-ubiquitinates (2), which leads to the recruitment of the Cdc48 ATPase complex. ATP hydrolysis by Cdc48 induces a conformational change in Hrd1 dimer that facilitates the early stages of substrate retrotranslocation (3). Once exposed to the cytoplasm, the substrate is ubiquitinated by Hrd1 and recognized by the Cdc48 complex (4), which uses its ATPase activity to complete substrate retrotranslocation (5). After retrotranslocation, the ubiquitinated substrate is released in the cytosol for degradation by the proteasome (6). The Cdc48 cofactors Npl4 and Ufd1 are depicted as N and U, respectively. Ubiquitin is depicted as small circles.