Putative hydrophobic ligand-binding SRPBCC domain of Micromonospora echinospora CalC, human Aha1, and related proteins
This family includes the SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain of Micromonospora echinospora CalC, human Aha1, and related proteins. Proteins in this group belong to the SRPBCC domain superfamily of proteins, which bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket. MeCalC confers resistance to the enediyne, calicheamicin gamma 1 (CLM), by a self sacrificing mechanism which results in inactivation of both CalC and the highly reactive diradical enediyne species. MeCalC can also inactivate two other enediynes, shishijimicin and namenamicin. A crucial Gly of the MeCalC CLM resistance mechanism is not conserved in this subgroup. This family also includes the C-terminal, Bet v1-like domain of Aha1, one of several co-chaperones, which regulate the dimeric chaperone Hsp90. Aha1 promotes dimerization of the N-terminal domains of Hsp90, and stimulates its low intrinsic ATPase activity, and may regulate the dwell time of Hsp90 with client proteins. Aha1 can act as either a positive or negative regulator of chaperone-dependent activation, depending on the client protein, but the mechanisms by which these opposing functions are achieved are unclear. Aha1 is upregulated in a number of tumor lines co-incident with the activation of several signaling kinases.