Binding immunoglobulin protein

Template:PBB Binding immunoglobulin protein (BiP) also known as 78 kDa glucose-regulated protein (GRP-78) or heat shock 70 kDa protein 5 (HSPA5) 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, as well as playing 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.

Structure

BiP contains two functional domains: a nucleotide-binding domain (NBD) and a substrate-binding domain (SBD). NBD binds and hydrolyzes ATP; the substrates for SBD are extended polypeptides.

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). 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 formation of an NBD–SBD interface.

Mechanism

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.

BiP and PDI act synergistically in the in vitro folding of the denatured and reduced Fab fragment. When the nucleotide-binding domain of GRP78 interacts with ATP, protein disulfide isomerase (PDI) can then work to promote disulfide reduction, rearrangement, and re-oxidation until the correct protein conformation is achieved. ADP/ATP exchange ends the interaction of GRP78 with the protein and thus PDI's work is halted, as well. Once the correct protein structure is achieved, it is no longer a candidate for GRP78 binding.

Function

When Chinese hamster 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), a member of the heat-shock protein-70 (HSP70) family, is involved in the folding and assembly of proteins in the endoplasmic reticulum (ER). The level of GRP78 is strongly correlated with the amount of secretory proteins (e.g. IgG) within the ER. Because so many ER proteins interact transiently with GRP78, it is presumed that it may play a key role in assisting protein transport through the cell.

Interactions

Binding immunoglobulin protein has been shown to interact with thyroglobulin and SIL1.

Conservation of BiP and BiP's Cysteines

BiP is highly conserved among eukaryotes including mammals (Table 1), and it is also widely-expression among all tissue types. In the human BiP, there are two highly conserved cysteines. These cysteines have been shown to undergo post-translational modifications in both yeast and mammalian cells. In yeast cells, the N-terminus cysteine has been shown to be sulfenylated and glutathionylated upon oxidative stress. Both modifications enhance BiP's ability to prevent protein aggregation. In mice cells, the conserved cysteine pair forms disulfide bond upon activation of GPx7 (NPGPx). Disulfide bond enhances BiP's binding to denatured proteins.

Table 1. Conservation of BiP in mammalian cells
	Species common name	Species scientific name	Conservation of BiP	Conservation of BiP's cysteine	Cysteine number
Primates	Human	Homo sapiens	Yes	Yes	2
	Macaque	Macaca fuscata	Yes	Yes	2
	Vervet	Chlorocebus sabaeus	Predicted*	Yes	2
	Marmoset	Callithrix jacchus	Yes	Yes	2
Rodents	Mouse	Mus musculus	Yes	Yes	2
	Rat	Rattus norvegicus	Yes	Yes	3
	Guinea pig	Cavia porcellus	Predicted	Yes	3
	Naked mole rat	Heterocephalus glaber	Yes	Yes	3
	Rabbit	Oryctolagus cuniculus	Predicted	Yes	2
	Tree shrew	Tupaia chinensis	Yes	Yes	2
Ungulates	Cow	Bos taurus	Yes	Yes	2
	Minke whale	Balaenoptera acutorostrata scammoni	Yes	Yes	2
	Pig	Sus scrofa	Predicted	Yes	2
Carnivores	Dog	Canis familiaris	Predicted	Yes	2
	Cat	Hammondia hammondi	Yes	Yes	3
	Ferret	Mustela putorius furo	Predicted	Yes	2
Marsupials	Opossum	Monodelphis domestica	Predicted	Yes	2
	Tasmanian Devil	Sarcophilus harrisii	Predicted	Yes	2
*Predicted: Predicted sequence according to NCBI protein

Immunological Properties

Like many stress and heat shock proteins, BiP/GRP78 has potent immunological activity when released from the internal environment of the cell into the extracelluar space. Specifically, it feeds anti-inflammatory and pro-resolutory signals into immune networks, thus helping to resolve inflammation.

The mechanisms underlying BiP's immunological activity are incompletely understood. However, it has been shown that it binds to a receptor on the surface of monocytes and induces anti-inflammatory cytokine secretion dominated by IL-10, IL-1Ra, and soluble TNFR. Furthermore, it downregulates critical molecules involved in T-lymphocyte activation such as HLA-DR and CD86. It also modulates the differentiation pathway of monocytes into dendritic cells, causing them to develop tolerogenic characteristics, which, in turn, can facilitate the development of regulatory T-lymphocytes.

The potent immunomodulatory activities of BiP/GRP78 have also been demonstrated in animal models of autoimmune disease including collagen-induced arthritis, a murine disease that resembles human rheumatoid arthritis. Prophylactic or therapeutic parenteral delivery of BiP has been shown to ameliorate clinical and histological signs of inflammatory arthritis.

Inhibitors

Inhibitors of BiP target the ATP-binding domain. Honokiol, a Magnolia grandiflora derivative, is a BiP inhibitor. Inducers of BiP were also found including, BiP inducer X (BIX) was identified in a screen for compounds that induce BiP expression.

See also

• Unfolded protein response
• Endoplasmic reticulum stress response (ER stress)

Further reading

• Li J, Lee AS (2006). "Stress induction of GRP78/BiP and its role in cancer". Curr. Mol. Med. 6 (1): 45–54. doi:10.2174/156652406775574523. PMID 16472112.

External links

• HSPA5+protein,+human at the U.S. National Library of Medicine Medical Subject Headings (MeSH)