INTERACTION OF 70-KDA HEAT-SHOCK COGNATE PROTEIN WITH PEPTIDES AND MYOINOSITOL MONOPHOSPHATASE

Fluorescence techniques have been used to investigate the interaction of bovine 70-kDA heat shock cognate protein (Hsc 70) with small molecu lar weight peptides and myo-inositol monophosphatase. The emission pro perties of Hsc 70 remain invariant upon addition of ATP. The results o f steady-state fluorescence indicate that the tryptophan residues of H sc 70 are exposed to the rapidly relaxing aqueous solvent. Binding of residues 1-20 of ribonuclease A (RNase S-peptide) to Hsc 70 causes pro tein fluorescence quenching which was used to determine a dissociation constant K(d) = 2.7 muM for the binary Hsc 70.RNase S-peptide complex . The octapeptide corresponding to the NH2-terminal portion of sickle cell hemoglobin recognizes Hsc 70 and binds with a K(d) = 9.3 muM. Bin ding of RNase S-peptide to Hsc 70 produces a small enhancement of ATPa se activity. Unfolded myoinositol monophosphatase, tagged with the flu orescent probe (2-iodoacetamido)ethylamino]-1-naphthalenesulfonic acid , recognizes Hsc 70; the formation of a stable complex was detected by steady-state emission anisotropy measurements. The rate and extent of recovery of catalytic activity of unfolded myo-inositol monophosphata se is not influenced by Hsc 70. It is suggested that interaction of Hs c 70 with unfolded proteins in the cell may be able to delay the forma tion of misfolded structures.