The interaction of the molecular chaperone, α-crystallin, with molten globule states of bovine α-lactalbumin

Abstract

Small heat shock proteins function in a chaperone-like manner to prevent the precipitation of proteins under conditions of stress (e.g. heat). α- Crystallin, the major mammalian lens protein, is a small heat shock protein. The mechanism of chaperone action of these proteins is poorly understood. In this paper, the conformational state of a protein when it forms a high molecular weight complex with α-crystallin is investigated by examining, using NMR spectroscopy and size exclusion high performance liquid chromatography, the interaction of α-crystallin with α-lactalbumin and its various intermediately folded (molten globule) states. The complex is formed following reduction of α-lactalbumin by dithiothreitol in the presence of α-crystallin, and this interaction has been monitored in real time by 1H NMR spectroscopy. It is concluded that α-crystallin interacts with a disordered molten globule state of α-lactalbumin while it is on an irreversible pathway toward aggregation and precipitation. α-Crystallin does not interact, however, with molten globule states of α-lactalbumin that are stable in solution, e.g. the reduced and carboxy-amidated species. It is proposed that α-crystallin distinguishes between the various molten globule states of α-lactalbumin on the basis of the lifetimes of these states, i.e. the protein must be in a disordered molten globule state for a significant length of time and on the pathway to aggregation and precipitation for interaction to occur.