Binding immunoglobulin protein

3IUC, 3LDL, 3LDN, 3LDO, 3LDP, 5E84, 5E86, 5E85, 5F1X, 5EX5, 5EVZ, 5F2R, 5F0X, 5EY4, 5EXW330914828ENSG00000044574ENSMUSG00000026864P11021P20029NM_005347NM_001163434NM_022310NP_005338NP_001156906NP_071705Binding immunoglobulin protein (BiP) also known as (GRP-78) or heat shock 70 kDa protein 5 (HSPA5) or (Byun1) is a protein that in humans is encoded by the HSPA5 gene. Binding immunoglobulin protein (BiP) also known as (GRP-78) or heat shock 70 kDa protein 5 (HSPA5) or (Byun1) is a protein that in humans is encoded by the HSPA5 gene. BiP is a HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum (ER) that binds newly synthesized proteins as they are translocated into the ER, and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery and plays a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BiP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER. BiP contains two functional domains: a nucleotide-binding domain (NBD) and a substrate-binding domain (SBD). The NBD binds and hydrolyzes ATP, and the SBD binds polypeptides. The NBD consists of two large globular subdomains (I and II), each further divided into two small subdomains (A and B). The subdomains are separated by a cleft where the nucleotide, one Mg2+, and two K+ ions bind and connect all four domains (IA, IB, IIA, IIB). The SBD is divided into two subdomains: SBDβ and SBDα. SBDβ serves as a binding pocket for client proteins or peptide and SBDα serves as a helical lid to cover the binding pocket. An inter-domain linker connects NBD and SBD, favoring the irformation of an NBD–SBD interface. The activity of BiP is regulated by its allosteric ATPase cycle: when ATP is bound to the NBD, the SBDα lid is open, which leads to the conformation of SBD with low affinity to substrate. Upon ATP hydrolysis, ADP is bound to the NBD and the lid closes on the bound substrate. This creates a low off rate for high-affinity substrate binding and protects the bound substrate from premature folding or aggregation. Exchange of ADP for ATP results in the opening of the SBDα lid and subsequent release of the substrate, which then is free to fold. The ATPase cycle can be synergistically enhanced by protein disulfide isomerase (PDI), and its cochaperones. When K12 cells are starved of glucose, the synthesis of several proteins, called glucose-regulated proteins (GRPs), is markedly increased. GRP78 (HSPA5), also referred to as 'immunoglobulin heavy chain-binding protein' (BiP), is a member of the heat-shock protein-70 (HSP70) family and involved in the folding and assembly of proteins in the ER. The level of BiP is strongly correlated with the amount of secretory proteins (e.g. IgG) within the ER. Substrate release and binding by BiP facilitates diverse functions in the ER such as folding and assembly of newly synthesized proteins, binding to misfolded proteins to prevent protein aggregation, translocation of secretory proteins, and initiation of the UPR. BiP can actively fold its substrates (acting as a foldase) or simply bind and restrict a substrate from folding or aggregating (acting as a holdase). Intact ATPase activity and peptide binding activity are required to act as a foldase: temperature-sensitive mutants of BiP with defective ATPase activity (called class I mutations) and mutants of BiP with defective peptide binding activity (called class II mutations) both fail to fold carboxypeptidase Y (CPY) at non-permissive temperature. As an ER molecular chaperone, BiP is also required to import polypeptide into the ER lumen or ER membrane in an ATP-dependent manner. ATPase mutants of BiP were found to cause a block in translocation of a number of proteins (invertase, carboxypeptidase Y, a-factor) into the lumen of the ER.