Bipartite structure of the alpha-lactalbumin molten globule.

Molten globules are thought to be general intermediates in protein folding. Apparently conflicting studies have failed to clarify whether one of the best characterized molten globules, that of alpha-lactalbumin, resembles an expanded native-like protein or a nonspecific collapsed polypeptide. Here we show that the molten globule properties of alpha-lactalbumin are largely confined to one of its two domains. The alpha-helical domain forms a helical structure with a native-like tertiary fold, while the beta-sheet domain is largely unstructured. Molten globules thus possess a native-like backbone topology, but this topology does not necessarily encompass the entire polypeptide chain. Our studies indicate that molten globules provide an approximate solution to, and considerable simplification of the protein folding problem.