THE INTERACTION OF THE MOLECULAR CHAPERONE, ALPHA-CRYSTALLIN, WITH MOLTEN GLOBULE STATES OF BOVINE ALPHA-LACTALBUMIN

Small heat shock proteins function in a chaperone-like manner to preve nt the precipitation of proteins under conditions of stress (e.g. heat ). alpha-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 prote in when it forms a high molecular weight complex with alpha-crystallin is investigated by examining, using NMR spectroscopy and size exclusi on high performance liquid chromatography, the interaction of alpha-cr ystallin with alpha-lactalbumin and its various intermediately folded (molten globule) states. The complex is formed following reduction of alpha-lactalbumin by dithiothreitol in the presence of alpha-crystalli n, and this interaction has been monitored in real time by H-1 NMR spe ctroscopy. It is concluded that alpha-crystallin interacts with a diso rdered molten globule state of alpha-lactalbumin while it is on an irr eversible pathway toward aggregation and precipitation. alpha-Crystall in does not interact, however, with molten globule states of alpha-lac talbumin that are stable in solution, e.g. the reduced and carboxy-ami dated species. It is proposed that alpha-crystallin distinguishes betw een the various molten globule states of alpha-lactalbumin on the basi s of the lifetimes of these states, i.e. the protein must be in a diso rdered molten globule state for a significant length of time and on th e pathway to aggregation and precipitation for interaction to occur.