Influence of the Peripheral Matrix Protein of Vesicular Stomatitis Virus on the Membrane Dynamics of Mixed Phospholipid Vesicles: Fluorescence Studies

In an effort to characterize the association of a peripheral membrane protein with mixed lipid bilayers, the basic (pI ≃ 9.1) matrix (M) protein of vesicular stomatitis virus was reconstituted with detergent-dialyzed vesicles and with preformed sonicated vesicles, each containing phospholipids with acidic head groups. The gel to liquid-crystalline phase transition of these reconstituted vesicles was studied by using steady-state fluorescence depolarization and differential polarized phase fluorometry of the hydrophobic membrane probes 1,6-diphenyl-1,3,5-hexatriene, 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene, and trans-parinaric acid. Reconstitution of the M protein with detergentdialyzed vesicles composed of 50 mol % dimyristoylphosphatidylserine (DMPS) and 50 mol % dipalmitoylphosphatidylcholine (DPPC) resulted in no significant effect on the phase transition temperature as measured by all three probes. However, the M protein appeared to increase the order of the mixed lipid bilayer gel state, as evidenced by increased fluorescence anisotropy of the three probes below the transition temperature. In addition, the phase transition of the vesicle bilayer was sharpened following reconstitution with the M protein. M protein reconstituted with preformed sonicated vesicles composed of 50 mol % DPPC and 50 mol % dipalmitoylphosphatidylglycerol (DPPG) caused a large increase in the phase transition temperature as monitored by all three probes. Depending on the probe used, the observed T_m was increased from 2 to 7°C. The M protein dramatically increased the order of the mixed lipid gel state and sharpened the lipid phase transition. Differential polarized phase fluorometry of both vesicle systems demonstrated increased order of the gel state lipid in the presence of M protein and supported the steady-state fluorescence results. These results demonstrate that binding of the peripheral membrane protein M to lipid bilayers containing acidic phospholipids results in profound alterations in the dynamics of lipid behavior in the membrane.