HSPA5 Gene encoding Hsp70 chaperone BiP in the endoplasmic reticulum

Abstract

The HSPA5 gene encodes the binding immunoglobulin protein (BiP), an Hsp70 family chaperone localized in the ER lumen. As a highly conserved molecular chaperone, BiP assists in a wide range of folding processes via its two structural domains, a nucleotide-binding domain (NBD) and substrate-binding domain (SBD). BiP is also an essential component of the translocation machinery for protein import into the ER, a regulator for Ca²⁺ homeostasis in the ER, as well as a facilitator of ER-associated protein degradation (ERAD) via retrograde transportation of aberrant proteins across the ER membrane. When unfolded/misfolded proteins in the ER overwhelm the capacity of protein folding machinery, BiP can initiate the unfolded protein response (UPR), decrease unfolded/misfolded protein load, induce autophagy, and crosstalk with apoptosis machinery to assist in the cell survival decision. Post-translational modifications (PTMs) of BiP have been shown to regulate BiP's activity, turnover, and availability upon different extrinsic or intrinsic stimuli. As a master regulator of ER function, BiP is associated with cancer, cardiovascular disease, neurodegenerative disease, and immunological diseases. BiP has been targeted in cancer therapies and shows promise for application in other relevant diseases.

Keywords: Apoptosis; Calcium homeostasis; Drug target; ER-associated protein degradation (ERAD); Post-translational modification; Unfolded protein response (UPR).

Figure 1. Half-life of BiP in different species and tissue types

BiP’s half-life is compared across three species (yeast, mouse, and bank vole), five tissue types (heart, liver, brain, kidney, and skeletal muscle), and six mouse strains (A/J, BALB/cJ, C57BL/6J, CE/J, DBA/2J, and FVB/NJ) (Lau et al., 2016). (A) This set of box- and-whisker plots illustrates how BiP’s half-life may fluctuate greatly depending on species and tissue type. While BiP’s half-life remains consistently longer than only 30% proteins in the proteomes of both rodent species, the half-life of yeast BiP is longer than 85% of proteins in the yeast proteome. (B) BiP’s half-life can vary in different species, from a few hours in yeast up to 15 days in bank vole. In addition to yeast, mouse, and bank vole, the half-life of BiP in maize, bean, and human tumor cells are shown. (C) BiP’s half-life also differs between tissue types within the same species. Notably, liver BiP has a shorter half-life compared to other tissue types in both mouse and bank vole. (D) BiP’s half-life can deviate significantly (*) in different strains of mouse cardiac tissue, such as between A/J and BALB/cJ.

Figure 2. Protein structure of BiP

The upper panel represents the primary structure of BiP, including the nucleotide binding domain (NBD) and substrate binding domain (SBD). In addition to the NBD and SBD, BiP has an N-terminal signal sequence, which is an 18-amino acid long signal peptide for ER targeting and translocation, and a C-terminal ER retrieval sequence, lys-asp-glu-leu (KDEL), which prevents BiP from being secreted from the ER. The lower panel shows the structures of the NBD (PDB: 3IUC) and SBD (PDB: 5e85) of human BiP. The magenta stick represents the ADP that binds to the NBD of BiP.

Figure 3. Allosteric ATPase cycle of BiP

When the nucleotide binding domain (NBD, blue) binds to ATP, the substrate-binding domain (SBD, green) of BiP has low binding affinity to the substrate (purple). When the NBD is bound to ADP, the SBD has high substrate binding affinity. The allosteric ATPase cycle is known to allow BiP to act as a foldase and transport proteins into the ER.

Figure 4. Conservation of Primary Structure Within Hsp70 Family Members

(A) The primary structure of BiP (HSPA5) is compared with that of the other seven members of the Hsp70 family (HSPA1A, HSPA1B, HSPA9, HSPA8, HSPA2, HSPA6, and HSPA1L) using NCBI COBALT sequence alignment tool (https://www.ncbi.nlm.nih.gov/tools/cobalt/re_cobalt.cgi). Numbers at the ends of each row indicate the position of the residue, with a maximum of 80 residues per row. Red regions indicate identical amino acid (aa) matches; blue regions indicate less conserved aa matches; and black regions indicate non-conserved aa. (B) For each Hsp70 member (column 1), the number of identical matches with BiP over total length (column 2), as well as the resulting percentage (column 3) are summarized. The high degree of sequence similarity across all eight Hsp70 family members accounts for two shared functional domains: the nucleotide-binding domain (NBD) and substrate-binding domain (SBD).

Figure 5. Clinical significance of BiP

BiP is associated with six major disease classifications: cardiovascular disease, especially heart failure; neurodegenerative disease, including Alzheimer’s and prion disease; autoimmune disease, particularly rheumatoid arthritis; metabolic disease, including diabetes mellitus and obesity; infectious disease, which includes prokaryotic pathogens and eukaryotic parasites employing BiP homologs; and cancer and viral disease, both which rely the host’s BiP expression. The biological mechanisms underlying BiP’s involvement in each disease group—unfolded protein response (UPR), apoptosis, ER- associated degradation (ERAD), Ca²⁺ homeostasis, and other signaling pathways—are illustrated. Treatments for each disease group can be classified as BiP inhibitors, which compromise the survival of infected and cancerous cells, and BiP inducers, which counteract excessive protein misfolding and cell death.