CONTRIBUTION OF INDIVIDUAL RESIDUES TO FORMATION OF THE NATIVE-LIKE TERTIARY TOPOLOGY IN THE ALPHA-LACTALBUMIN MOLTEN GLOBULE

Molten globules are partially folded forms of proteins that have nativ elike secondary structure and a compact geometry, but often without ri gid, specific side-chain packing. Recently, the molten globule of alph a-lactalbumin (alpha-LA) has been shown to adopt a native-like tertiar y topology, mainly localized in the alpha-helical domain. This native- like topology is reflected by the high effective concentration (C-eff) for formation of the 28-111. disulfide bond, which is approximately 1 0 to 40 times higher than the C-eff for formation of any non-native di sulfide bond in the alpha-helical domain. In order to understand the m echanism for formation of the native-like tertiary topology, we substi tuted alanine for each of the 23 buried residues in the alpha-helical domain of alpha-LA and determined the effect of these substitutions on the C-eff for formation of the 28-111 disulfide bond. Our results ind icate that a subset of hydrophobic residues is most important for form ation of the native-like topology. These residues form a densely packe d core in the three-dimensional structure of alpha-LA. In contrast, th e less important residues consist of both hydrophobic and hydrophilic amino acids located at peripheral positions. These results suggest tha t a relatively small number of hydrophobic residues may be sufficient for specifying the overall structure of a protein during early stages of protein folding. (C) 1998 Academic Press.