The endoplasmic reticulum-associated degradation pathways of budding yeast

Abstract

Protein misfolding is a common cellular event that can produce intrinsically harmful products. To reduce the risk, quality control mechanisms are deployed to detect and eliminate misfolded, aggregated, and unassembled proteins. In the secretory pathway, it is mainly the endoplasmic reticulum-associated degradation (ERAD) pathways that perform this role. Here, specialized factors are organized to monitor and process the folded states of nascent polypeptides. Despite the complex structures, topologies, and posttranslational modifications of client molecules, the ER mechanisms are the best understood among all protein quality-control systems. This is the result of convergent and sometimes serendipitous discoveries by researchers from diverse fields. Although major advances in ER quality control and ERAD came from all model organisms, this review will focus on the discoveries culminating from the simple budding yeast.

Figure 1.

Schematic representation of Hrd1 and Doa10 complexes. Individual subunits of each complex are depicted with their known partners. (Top) The Doa10 complex monitors the folding state of cytosolic domains of membrane proteins (ERAD-C). (Bottom) The Hrd1 complex recognizes lesions of luminal domains of membrane and soluble proteins (ERAD-L) and of lesions within transmembrane domains (ERAD-M).

Figure 2.

Targeting glycoproteins for degradation. The core GlcNAc₂-Man₉-Glc₃ glycan is rapidly added to the side-chain nitrogen of Asn (N) residues part of the consensus sequence Asn-X-Ser/Thr (N-X-S/T). This posttranslational modification occurs as soon as polypeptides enter the endoplasmic reticulum through the translocon. Subsequently, the three glucoses of branch A are trimmed consecutively by the glucosidase I (Gls1) and glucosidase II (Gls2) to generate GlcNAc₂-Man₉. Mannosidase I (Mns1) cleaves the α1,2-linked mannose of branch B to produce GlcNAc₂-Man₈. At this stage, folded and glycosylated proteins may leave the ER. However, glycosylated proteins failing to fold are recognized by Htm1/Pdi1, which cleaves the α1,2-linked mannose of branch C to yield the terminal α1,6-linked mannose residue as the Yos9 ligand (red circle).

Figure 3.

Retrotranslocation and degradation of ERAD substrates. (A) Proteins targeted for degradation by ERAD complex are retrotranslocated through a “retrotranlocon” (Hrd1, Doa10, and Sec61 are proposed to serve this role but only the E3s are shown for simplicity) with the mechanical force provided by the Cdc48-Npl4-Ufd1 AAA-ATPase complex. Polypeptides are subsequently ubiquitinated as depicted. (B) In the cytosol, Png1 removes N-linked glycans from substrates. Ufd2 can lengthen ubiquitin chains for some substrates. Polyubiquitinated species are recognized by Rad23p and passed to the proteasome for degradation.