Hydrophobic photolabeling as a new method for structural characterization of molten globule and related protein folding intermediates.

Recent advances in attempts to unravel the protein folding mechanism have indicated the need to identify the folding intermediates. Despite their transient nature, in a number of cases it has been possible to detect and characterize some of the equilibrium intermediates, for example, the molten globule (MG) state. The key features of the MG state are retention of substantial secondary structure of the native state, considerable loss of tertiary structure leading to increased hydrophobic exposure, and a compact structure. NMR, circular dichroism, and fluorescence spectroscopies have been most useful in characterizing such intermediates. We report here a new method for structural characterization of the MG state that involves probing the exposed hydrophobic sites with a hydrophobic photoactivable reagent--2[3H]diazofluorene. This carbene-based reagent binds to hydrophobic sites, and on photolysis covalently attaches itself to the neighboring amino acid side chains. The reagent photolabels alpha-lactalbumin as a function of pH (3-7.4), the labeling at neutral pH being negligible and maximal at pH 3. Chemical and proteolytic fragmentation of the photolabeled protein followed by peptide sequencing permitted identification of the labeled residues. The results obtained indicate that the sequence corresponding to B (23-34) and C (86-98) helix of the native structure are extensively labeled. The small beta-domain (40-50) is poorly labeled, Val42 being the only residue that is significantly labeled. Our data, like NMR data, indicate that in the MG state of alpha-lactalbumin, the alpha-domain has a greater degree of persistent structure than the beta-domain. However, unlike the NMR method, the photolabeling method is not limited by the size of the protein and can provide information on several new residues, for example, Leu115. The current method using DAF thus allows identification of stable and hydrophobic exposed regions in folding intermediates as the reagent binds and on photolysis covalently links to these regions.