The protective and destructive roles played by molecular chaperones during ERAD (endoplasmic-reticulum-associated degradation)

Abstract

Over one-third of all newly synthesized polypeptides in eukaryotes interact with or insert into the membrane or the lumenal space of the ER (endoplasmic reticulum), an event that is essential for the subsequent folding, post-translational modification, assembly and targeting of these proteins. Consequently, the ER houses a large number of factors that catalyse protein maturation, but, in the event that maturation is aborted or inefficient, the resulting aberrant proteins may be selected for ERAD (ER-associated degradation). Many of the factors that augment protein biogenesis in the ER and that mediate ERAD substrate selection are molecular chaperones, some of which are heat- and/or stress-inducible and are thus known as Hsps (heat-shock proteins). But, regardless of whether they are constitutively expressed or are inducible, it has been assumed that all molecular chaperones function identically. As presented in this review, this assumption may be false. Instead, a growing body of evidence suggests that a chaperone might be involved in either folding or degrading a given substrate that transits through the ER. A deeper appreciation of this fact is critical because (i) the destruction of some ERAD substrates results in specific diseases, and (ii) altered ERAD efficiency might predispose individuals to metabolic disorders. Moreover, a growing number of chaperone-modulating drugs are being developed to treat maladies that arise from the synthesis of a unique mutant protein; therefore it is critical to understand how altering the activity of a single chaperone will affect the quality control of other nascent proteins that enter the ER.

Figure 1. The Hsp70 cycle

The ATPase domain of Hsp70/Hsc70 is indicated as being bound to either ATP or ADP. ATP hydrolysis is catalysed by a J-domain-containing Hsp40, which may interact with the Hsp70 on its own or with a bound polypeptide substrate (shown in orange). ATP hydrolysis induces a conformational change in the nucleotide- and peptide-binding sites, which results in lid closure and polypeptide trapping. ADP release, which may be triggered by an NEF (nucleotide-exchange factor), and ATP re-binding are required for polypeptide release.

Figure 2. Models for calnexin function in the ER

Calnexin (Cnx) and calreticulin (not shown) bind most avidly to monoglucosylated oligosaccharyl side chains, which result from trimming of the triply glucosylated species by glucosidases. Calnexin binding either may stabilize the bound substrate (A) or the binding may be competed for by other pro-degradative lectins that facilitate ERAD (B), such as EDEM or Yos9. The polypeptide is shown as a thin blue line, N-acetylglucosamine is shown as a yellow square, mannose is shown as a blue circle, and glucose is shown as an orange triangle.

Figure 3. A model for chaperone-based diversity during ER substrate degradation compared with folding

A wild-type polypeptide substrate (Substrate 1) is shown as a thin blue line. This polypeptide interacts with a unique chaperone composite (depicted here as an orange circle) that facilitates folding. In contrast, mutant (A or B) forms of the polypeptide result in conformers that favour interactions with other chaperone composites, of which only one may be sufficient to trigger ERAD (mutant A). This conformation may be found in other aberrant proteins (Substrate 2), and, assuming that the same chaperone composite binds to this mutated substrate, ERAD also occurs. Consequently, specific conformations, even in different proteins, may have identical chaperone requirements for ERAD. In contrast, a given substrate, and certainly different substrates, depending on the nature of the mutation, may exhibit different chaperone requirements for ERAD or may even escape ERAD.