nucleotide-binding domain (NBD) of heat shock 70 kDa protein 4L (HSPA4L) and similar proteins
HSPA4L, also called heat shock 70-related protein APG-1, heat-shock protein family A member 4-like protein, HSPA4-like protein, osmotic stress protein 94, or HSPH3, possesses chaperone activity in vitro where it inhibits aggregation of citrate synthase. It is expressed ubiquitously and predominantly in the testis. It is required for normal spermatogenesis and plays a role in osmotolerance. HSPA4L 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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