Resistance of lipophilin, a hydrophobic myelin protein, to denaturation by urea and guanidinium salts.

The influence of urea, guanidinium chloride (GdmCl), and granidinium thiocyanate (GdmSCN) on the solution structure of lipophilin was examined by circular dischroism and fluorescence techniques. According to the CD results, lipophilin retained at least 60% of organized secondary structure in 8 M urea and 6 M GdmCl (measurements were not possible in GdmSCN). This partial denaturation was of a complex, irreversible nature, and was not appreciably enhanced by prolonged incubation (8 days), by heating to 70 degrees C, by disulfide bond reduction, or by pH variation in the range pH 1.5 to 11. Fluorescence studies demonstrated that the tryptophan residues were only slightly perturbed by 8 M and 6 M GdmCl and remained well buried to the permeant quenching agent acrylamide. A greater, but still far from complete, disruption of lipophilin was achieved in 6 M GdmSCN, and fluorescence polarization provided evidence for some form of cooperative structural change induced by increasing concentrations of this reagent. Transfer of the protein from 2-chloroethanol, in which the tryptophan residues are fully exposed, into 6 M GdmSCN by dialysis resulted in reburial of the fluorophores owing to development of tertiary structure. The combined evidence suggests that the extraordinary resistance of lipophilin to these denaturants is due to the presence of an impervious hydrophobic core. In lipophilin and some other membrane-associated proteins, extended sequences of apolar residues might provide the nuclei for such structural domains.