The Role of Endoplasmic Reticulum Chaperones in Protein Folding and Quality Control

Abstract

Molecular chaperones assist the folding of nascent chains in the cell. Chaperones also aid in quality control decisions as persistent chaperone binding can help to sort terminal misfolded proteins for degradation. There are two major molecular chaperone families in the endoplasmic reticulum (ER) that assist proteins in reaching their native structure and evaluating the fidelity of the maturation process. The ER Hsp70 chaperone, BiP, supports adenine nucleotide-regulated binding to non-native proteins that possess exposed hydrophobic regions. In contrast, the carbohydrate-dependent chaperone system involving the membrane protein calnexin and its soluble paralogue calreticulin recognize a specific glycoform of an exposed hydrophilic protein modification for which the composition is controlled by a series of glycosidases and transferases. Here, we compare and contrast the properties, mechanisms of action and functions of these different chaperones systems that work in parallel, as well as together, to assist a large variety of substrates that traverse the eukaryotic secretory pathway.

Keywords: Endoplasmic reticulum; Molecular chaperones; Quality control.

Figure 1.. An overview of the BiP substrate binding cycle.

In the ATP-bound state, BiP is in the open conformation (1), with both the SBD (purple) and NBD (green) domains docked together. Substrate may be bound directly or transferred to BiP via a co-factor. Here, ERdj6 brings substrate (red) bound in the TPR domain (orange) to pass to BiP (2), then stimulates the ATPase activity of BiP via the J-domain (grey) (PDB: 2Y4K). Upon stimulation, BiP hydrolyzes ATP to ADP and the SBD clamps onto the substrate (3), undocking from the NBD for more dynamic movement (PDB: 5E85). Nucleotide exchange is facilitated by a NEF (Sil1) (4), exchanging ADP for ATP, which causes BiP to unclamp from the substrate and occupy the ATP- bound open conformation once more (5) (PDB: 3QML). BiP sequestering can be achieved through modification, specifically AMPylation via FICD. FICD AMPylates BiP as a monomer and de-AMPylates BiP as a dimer (5) (AMP shown on the structure in red) (PDB: 6I7K, SO4P, 6I7G, respectively).

Figure 2.. The calnexin and calreticulin cycle.

Nascent glycoproteins possess N-linked glycans with a Glc₃ Man₉ GlcNAc₂ structure. The terminal glucoses are sequentially processed by α-glucosidase I (PDB 4J5T) and α-glucosidase II (PDB 5F0E; Catalytic GRH31 domain, green). In the monoglucosylated state, the substrate is engaged by calnexin (CNX) or calreticulin (CRT) (PDB 1JHN; lectin domain, yellow; P-domain, teal). Upon release from CNX/CRT, the terminal glucose may then be trimmed by α-glucosidase II. Folding to a native state promotes release from the cycle, while non-native substrates are reglucosylated by UGGT1/2 (PDB 5MZO; Folding sensor domain, blue; catalytic domain, red). Reglucosylation promotes re-engagement by CNX/CRT.