nucleotide-binding domain (NBD) of heat shock 105kDa/110kDa protein 1 (HSPH1) and similar proteins
HSPH1, also called heat shock protein 105 kDa, antigen NY-CO-25, heat shock 110 kDa protein, acts as a nucleotide-exchange factor (NEF) for chaperone proteins HSPA1A and HSPA1B, promoting the release of ADP from HSPA1A/B thereby triggering client/substrate protein release. It prevents the aggregation of denatured proteins in cells under severe stress, on which the ATP levels decrease markedly. It inhibits HSPA8/HSC70 ATPase and chaperone activities. HSPH1 belongs to the 105/110 kDa heat shock protein (HSP105/110) subfamily of the HSP70-like family. HSP105/110s are believed to function generally as co-chaperones of HSP70 chaperones, acting as nucleotide exchange factors (NEFs), to remove ADP from their HSP70 chaperone partners during the ATP hydrolysis cycle. HSP70 chaperones assist in protein folding and assembly, and can direct incompetent "client" proteins towards degradation. Like HSP70 chaperones, HSP105/110s have an N-terminal nucleotide-binding domain (NBD) and a C-terminal substrate-binding domain (SBD). For HSP70 chaperones, 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 also regulated by J-domain proteins.
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