A SMALL PROTEIN IN MODEL MEMBRANES - A TIME-RESOLVED FLUORESCENCE AND ESR STUDY ON THE INTERACTION OF M13 COAT PROTEIN WITH LIPID BILAYERS

Model membranes with unsaturated lipid chains containing various amounts of M13 coat protein in the alpha-helical form were studied using time-resolved fluorescence and ESR spectroscopy. The lipid-to-protein (L/P) ratios used were > 12 to avoid protein-protein contacts and irreversible aggregation leading to beta-polymeric coat protein. In the ESR spectra of the 12-SASL probe in dioleoyl phosphatidylcholine (DOPC) bilayers no second protein induced component is observed upon incorporation of M13 coat protein. However, strong effects are detected on the ESR lineshapes upon changing the protein concentration. The ESR lineshapes are simulated by assuming a fixed ratio between the parallel (D(parallel-to)) and perpendicular (D(perpendicular-to)) diffusion coefficients of 4, and an order parameter equal to zero. It is found that increasing the protein concentration from L/P infinity to L/P 15 results in a decrease of the rotational diffusion coefficient D(perpendicular-to) from 3.4 x 10(7) to 1.9 x 10(7) s-1. In the time-resolved fluorescence experiments with DPH-propionic acid as a probe, it is observed that increasing the M13 coat protein concentration causes an increase of the two fluorescent lifetimes, indicating an increase in bilayer order. Analysis of the time-resolved fluorescence anisotropy decay allows one to quantitatively determine the order parameters [P2] and [P4], and the rotational diffusion coefficient D(perpendicular-to) of the fluorescent probe. The order parameters [P2] and [P4] increase from 0.34 to 0.55 and from 0.59 to 0.77, respectively, upon adding M13 coat protein to DOPC bilayers with an L/P ratio of 35. The rotational diffusion coefficient D(perpendicular-to) of the DPH-propionic acid probe decreases on incorporating M13 coat protein, in accordance with the ESR results. It is concluded that M13 coat protein in the alpha-monomeric s-tate is not able to produce a long living lipid boundary shell and consequently an immobilization of the lipids. An overall effect on the lipids is induced, resulting in a reduction in the dynamics and an increase in average lipid order. The hydrophobic region of M13 coat protein is proposed to perfectly match the lipid bilayer, resulting in a relatively small distortion of the bilayer structure of the lipid system.