Spectroscopic analysis of acylated and deacylated myelin proteolipid protein

The effect of covalently bound fatty acid on the conformation of the myelin proteolipid protein has been studied by ultraviolet and intrinsic fluorescence spectroscopy. With dimethyl sulfoxide used as a perturbant, the exposure of Trp and Tyr residues in various mixtures of chloroform-methanol was evaluated by difference spectroscopy of the proteolipid protein (APL) and its chemically deacylated form (d-APL). The fraction of chromophoric groups exposed increased with the proportion of chloroform with 25% of the groups exposed in 1:2 chloroform-methanol and 98% in 3:1 chloroform-methanol. These conformational changes correlate well with changes in intrinsic viscosity. Values for the deacylated form were indistinguishable from those of the acylated protein, suggesting that fatty acids do not affect protein conformation in organic solvents. In water, UV difference spectroscopy indicated that the number of Tyr and Trp groups exposed in both APL and d-APL was relatively small and was independent of the molecular size of the perturbant. However, differences in the environment of the Trp groups in the two forms of the protein could be demonstrated by intrinsic fluorescence. When the protein was excited at 295 nm, the maximum emission wavelength for the acylated protein was 330 nm, whereas it was 335 nm for the deacylated form. Furthermore, the Trp groups in d-APL were more easily quenched by acrylamide than in APL, indicating that they were more exposed, or in a more hydrophilic environment, following deacylation. Protein aggregation appears to be independent of the presence of fatty acids, suggesting that the fluorescence differences between APL and d-APL are related to factors other than aggregation.(ABSTRACT TRUNCATED AT 250 WORDS)