An accessible hydrophobic surface is a key element of the molecular chaperone action of Atp11p

Atp11p is a soluble protein of mitochondria that binds unassembled beta sub units of the F-1-ATPase and prevents them from aggregating in the matrix. I n this report, we show that Atp11p protects the insulin B chain from aggreg ating in vitro and therefore acts as a molecular chaperone. The chaperone a ction of Atp11p is mediated by hydrophobic interactions. An accessible hydr ophobic surface in Atp11p was identified with the environment-sensitive flu orescent probe 1,1'-bis(4-anilino-5-napth-thalenesulfonic acid (bis-ANS). T he spectral changes of bis-ANS in the presence of Atp11p indicate that the probe binds to a nonpolar region of the protein. Furthermore, the dye quenc hes the fluorescence of Atp11p tryptophan residues in a concentration-depen dent manner. Although up to three molecules of bis-ANS can bind cooperative ly to Atp11p, the binding of only one dye molecule is sufficient to virtual ly eliminate the chaperone activity of the protein.