Addition of the M13 virus coat protein to dimyristoylphosphatidylcholine vesicles decreases the amplitude of the lipid bilayer phase transition observed with parinaric acid fluorescence intensity. According to these measurements, the solid-phase bilayer is disordered by the protein, while the fluid phase is not appreciably affected. The decrease in the relative amplitude of the transition as a function of protein concentration is shown to be consistent with a perturbation of about 70 lipids by the coat protein and a preferential partitioning of trans-parinaric acid into the bulk bilayer rather than the perturbed annular region. The fluorescence of the single tryptophan residue of the coat protein was found to be very efficiently quenched by addition of parinaric acid. The cis and trans isomers of parinaric acid are equally efficient as quenchers at 30 °C when the bilayer is in its fluid state, but at 10 °C the quenching by the cis isomer is much more efficient than for the trans isomer. Tquenching efficiency does not depend on the coat protein concentration. These results are consistent with a perturbation of the bilayer by the coat protein at low temperature. The structure of the bilayer near the protein results in a spatial distribution of trans-parinaric acid such that this quencher avoids the donor protein. The intensity and partitioning/quenching behavior used to characterize the effect of the protein on the solid bilayer do not reveal any bilayer structural changes for the fluid-phase bilayer. However, fluorescence polarization measurements with parinaric acid reveal a marked decrease in the rotational mobility of the probe relative to that of the fluid bilayer. The utility of parinaric acid fluorescence methods in the study of membrane-bound proteins is emphasized.
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