The molecular chaperone alpha-crystallin is in kinetic competition with aggregation to stabilize a monomeric molten-globule form of alpha-lactalbumin

In vice, alpha -crystallin and other small heat-shock proteins (sHsps) act as molecular chaperones to prevent the precipitation of 'substrate' protein s under stress conditions through the formation of a soluble sHsp-substrate complex. Using a range of different salt conditions, the rate and extent o f precipitation of reduced alpha -lactalbumin have been altered. The intera ction of alpha -crystallin with reduced alpha -lactalbumin under these vari ous salt conditions was then studied using a range of spectroscopic techniq ues. Under conditions of low salt, alpha -lactalbumin aggregates but does n ot precipitate. alpha -Crystallin is able to prevent this aggregation, init ially by stabilization of a monomeric molten-globule species of alpha -lact albumin. It is proposed that this stabilization occurs through weak transie nt interactions between alpha -crystallin and alpha -lactalbumin. Eventuall y a stable, soluble high-molecular-mass complex is formed between the two p roteins. Thus it appears that a tendency for alpha -lactalbumin to aggregat e (but not necessarily precipitate) is the essential requirement for alpha -crystallin-alpha -lactalbumin interaction. In other words, alpha -crystall in interacts with a non-aggregated form of the substrate to prevent aggrega tion. The rate of precipitation of alpha -lactalbumin is increased signific antly in the presence of Na2SO4 compared with NaCl. However, in the former case, alpha -crystallin is unable to prevent this aggregation and precipita tion except in the presence of a large excess of alpha -crystallin, i.e. at mass ratios more than 10 times greater than in the presence of NaCl. It is concluded that a kinetic competition exists between aggregation and intera ction of unfolding proteins with alpha -crystallin.