This family includes eukaryotic, prokaryotic and archaeal proteins that bear similarity to a C-terminal region of human activator of 90 kDa heat shock protein ATPase homolog 1 (AHSA1/p38, Swiss:O95433). This protein is known to interact with the middle domain of Hsp90, and stimulate its ATPase activity [1]. It is probably a general upregulator of Hsp90 function, particularly contributing to its efficiency in conditions of increased stress [2]. p38 is also known to interact with the cytoplasmic domain of the VSV G protein, and may thus be involved in protein transport [3]. It has also been reported as being underexpressed in Down's syndrome. This region is found repeated in two members of this family (Swiss:Q8XY04 and Swiss:Q6MH87). The structure of YndB from Bacillus subtilis showed the helix-grip fold consisting of a beta-sheet with two small and one long alpha-helix which form a hydrophobic cavity that preferentially binds lipid-like molecules. This structure confirms its similarity with the eukaryote protein Aha1 and its classification as a member of the AHSA1 family) [4].

1: Aha1 binds to the middle domain of Hsp90, contributes to client protein activation and stimulates the ATPase activity of the molecular chaperone. Lotz GP, Lin H, Harst A, Obermann WM; J Biol Chem 2003;0:0-0. PMID:12604615

2: Activation of the ATPase activity of hsp90 by the stress-regulated cochaperone aha1. Panaretou B, Siligardi G, Meyer P, Maloney A, Sullivan JK, Singh S, Millson SH, Clarke PA, Naaby-Hansen S, Stein R, Cramer R, Mollapour M, Workman P, Piper PW, Pearl LH, Prodromou C; Mol Cell 2002;10:1307-1318. PMID:12504007

3: p38: A novel protein that associates with the vesicular stomatitis virus glycoprotein. Sevier CS, Machamer CE; Biochem Biophys Res Commun 2001;287:574-582. PMID:11554768

4: Solution structure and function of YndB, an AHSA1 protein from Bacillus subtilis. Stark JL, Mercier KA, Mueller GA, Acton TB, Xiao R, Montelione GT, Powers R; Proteins. 2010;78:3328-3340. PMID:20818668