The recognition and retrotranslocation of misfolded proteins from the endoplasmic reticulum

Abstract

Secretory and membrane proteins that fail to fold in the endoplasmic reticulum (ER) are retained and may be sorted for ER-associated degradation (ERAD). During ERAD, ER-associated components such as molecular chaperones and lectins recognize folding intermediates and specific oligosaccharyl modifications on ERAD substrates. Substrates selected for ERAD are then targeted for ubiquitin- and proteasome-mediated degradation. Because the catalytic steps of the ubiquitin-proteasome system reside in the cytoplasm, soluble ERAD substrates that reside in the ER lumen must be retrotranslocated back to the cytoplasm prior to degradation. In contrast, it has been less clear how polytopic, integral membrane substrates are delivered to enzymes required for ubiquitin conjugation and to the proteasome. In this review, we discuss recent studies addressing how ERAD substrates are recognized, ubiquitinated and delivered to the proteasome and then survey current views of how soluble and integral membrane substrates may be retrotranslocated.

Figure 1. A current working model of the ERAD pathway in yeast

ERAD-L substrates are recognized by Hsp70/Hsp40 chaperones (Kar2, Jem1 and Scj1), PDI, putative lectins (Yos9 and Htm1/Mnl1) and the lumenal domain of Hrd3. These substrates are then retrotranslocated to the cytoplasm. In contrast, ERAD-M substrates may be directly recognized by the Hrd1 E3 ligase. ERAD-C substrates are recognized by cytoplasmic Hsp70/Hsp40 chaperones (Ssa1, Ydj1 and Hlj1) and by the Doa10 E3 ligase. The ubiquitinated substrates are delivered to the proteasome core for degradation by a series of escort factors, including the Cdc48 complex and the 19S proteasome cap. Steps following substrate ubiquitination are not yet clear for ERAD substrates. However, the polyubiquitin chain may be further remodeled by Ufd2 and by deubiquitinating enzymes (Rpn1, and/or DUBs such as Otu1). N-glycans may be cleaved by Png1, and some substrates are escorted to the proteasome by Rad23 and Dsk2. Membrane-associated E2–E3 enzymes, ERAD-L-specific components and retrotranslocation components are colored in yellow, blue and gray, respectively. The red star depicts a mutation that leads to protein misfolding, and the proteasome image was adopted from Voges et al. (126).

Figure 2. Models for membrane substrate delivery to the proteasome

Polytopic membrane proteins may be processively dislocated and degraded from either the N- or the C-terminus by the proteasome (i), as was proposed for the FtsH proteolytic system in bacteria (127). Alternatively, polytopic membrane proteins may be extracted to the cytoplasm and then degraded by the proteasome (iii). In this case, the solubility of hydrophobic transmembrane segments may be maintained by the Cdc48 complex, the 19S cap of the proteasome and/or other components including chaperones and proteasome-escorting factors. Because the proteasome has an endoproteolytic activity (128), cytoplasmic loop(s) of substrates may first be ‘clipped’ by the proteasome and then dislocated or extracted (ii or iv). Not shown in these models is the potential role of a retrotranslocon in substrate degradation. The proteasome image was adopted from Voges et al. (126).