nucleotide-binding domain (NBD) of hypoxia up-regulated protein 1 (HYOU1) and similar proteins
This subgroup includes human HYOU1 (also known as human hypoxia up-regulated 1, 170 kDa glucose-regulated protein/GRP170; HSP12A; 150 kDa oxygen-regulated protein/ORP150; GRP-170; ORP-150) and Saccharomyces cerevisiae Lhs1p (also known as Cer1p, SsI1). Mammalian HYOU1 has a pivotal role in cytoprotective cellular mechanisms triggered by oxygen deprivation. It may play a role as a molecular chaperone and participate in protein folding. HYOU1 functions as a nucleotide exchange factor (NEF) for HSPA5 (also known as BiP, Grp78 or HspA5) and may also act as a HSPA5-independent chaperone. S. cerevisiae Lhs1p, does not have a detectable endogenous ATPase activity like canonical HSP70s, but functions as a NEF for Kar2p; it's interaction with Kar2p is stimulated by nucleotide-binding. In addition, Lhs1p has a nucleotide-independent holdase activity that prevents heat-induced aggregation of proteins in vitro. Members in this subgroup belong 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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