nucleotide-binding domain (NBD) of 70-kDa heat shock protein 1 (HSPA1) and similar proteins
This subfamily includes human HSPA1A (70-kDa heat shock protein 1A, also known as HSP72; HSPA1; HSP70I; HSPA1B; HSP70-1; HSP70-1A), HSPA1B (70-kDa heat shock protein 1B, also known as HSPA1A; HSP70-2; HSP70-1B), and HSPA1L (70-kDa heat shock protein 1-like, also known as HSP70T; hum70t; HSP70-1L; HSP70-HOM), HSPA2 (70-kDa heat shock protein 2, also known as HSP70-2; HSP70-3), HSPA6 (also known as heat shock 70kDa protein 6; HSP70B'), HSPA7 (heat shock 70kDa protein 7 , also known as HSP70B), and HSPA8 (heat shock 70kDa protein 8, also known as Lipopolysaccharide-associated protein 1/LAP1; HSC70; HSP73; HSPA10). They are molecular chaperones implicated in a wide variety of cellular processes, including protection of the proteome from stress, folding and transport of newly synthesized polypeptides, activation of proteolysis of misfolded proteins and the formation and dissociation of protein complexes. They play pivotal roles in the protein quality control system, ensuring the correct folding of proteins, the re-folding of misfolded proteins and controlling the targeting of proteins for subsequent degradation. This is achieved through cycles of ATP binding, ATP hydrolysis and ADP release, mediated by co-chaperones. The subfamily also includes Saccharomyces cerevisiae heat shock protein Ssa1-4, which may play a role in the transport of polypeptides both across the mitochondrial membranes and into the endoplasmic reticulum. This subfamily belongs to the heat shock protein 70 (HSP70) family of chaperones that assist in protein folding and assembly and can direct incompetent "client" proteins towards degradation. Typically, HSP70s have a nucleotide-binding domain (NBD) and a substrate-binding domain (SBD). The nucleotide sits in a deep cleft formed between the two lobes of the NBD. The two subdomains of each lobe change conformation between ATP-bound, ADP-bound, and nucleotide-free states. ATP binding opens up the substrate-binding site; substrate-binding increases the rate of ATP hydrolysis. HSP70 chaperone activity is regulated by various co-chaperones: J-domain proteins and nucleotide exchange factors (NEFs).
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