DOMINANT FORCES IN THE RECOGNITION OF A TRANSIENT FOLDING INTERMEDIATE OF ALPHA-LACTALBUMIN BY GROEL

GroEL is known to retard the refolding of apo-alpha-lactalbumin by int eracting with the molten globule state of the protein. In order to inv estigate the dominant forces in this interaction, the GroEL-affected k inetic refolding of apo-alpha-lactalbumin from its acidic molten globu le state was studied at different temperatures and in the presence of different kinds of monovalent cations at a fixed temperature (25 degre es C), by stopped-flow fluorescence measurements. The binding constant between GroEL and alpha-lactalbumin in the molten globule state was e valuated quantitatively from the kinetic refolding curves in the absen ce and presence of GroEL. The binding was found to be entropy-driven a t room temperature and the heat capacity change for the binding was fo und to be largely negative (-3.6 kJ mol(-1). K-1), indicating that Gro EL binds to alpha-lactalbumin through hydrophobic interactions. The st udy of the effect of different monovalent cations at various ionic str engths shows that the binding is strengthened by electrostatic screeni ng by ions, demonstrating the importance of electrostatic interactions . The relationship of these results with a putative target recognition site of GroEL will be discussed. (C) 1996 Academic Press Limited