Phosphorescence studies of the interaction of myelin basic protein with phosphatidylserine vesicles

Phosphorescence from the lone tryptophan residue has been studied to monitor the interaction of myelin basic protein with phosphatidylserine vesicles. Spectral shifts in the phosphorescence of the protein in a glycerol-buffer (70:30 w/w) solvent at low temperature are consistent with fluorescence data obtained under ambient conditions, indicating that the tryptophan side chain is exposed to the solvent in the free protein but is buried on interaction with a lipid bilayer. Measurements of the phosphorescence intensity and lifetime as a function of temperature reveal a marked protection of the tryptophan to thermally induced quenching in the presence of phosphatidylserine vesicles. Steady-state anisotropy measurements on the tryptophan phosphorescence were used to follow the slow motions of the protein associated with the synthetic bilayer. The observations that the rotational correlation time for the membrane-associated protein is 4 X 10(3) times that anticipated for a molecule the size of basic protein reflects its partial intrinsic character in the membrane.