Thermotropic Behavior of Mixed Phosphatidylcholine-Phosphatidylglycerol Vesicles Reconstituted with the Matrix Protein of Vesicular Stomatitis Virus

The peripheral matrix (M) protein of vesicular stomatitis virus reconstituted with fused unilamellar vesicles containing equimolar amounts of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol has been studied by high-sensitivity differential scanning calorimetry and steady-state fluorescence spectroscopy. The association of the basic (pI ≃ 9.1) matrix protein with the mixed neutral/acidic phospholipid bilayer induced a dramatic upward shift in the phospholipid phase transition temperature even at M protein/phospholipid molar ratios as low as 1/12 000. Despite the large effect of the matrix protein on the phase transition temperature and the shape of the heat capacity function, the enthalpy change associated with the phospholipid gel to liquid-crystalline transition remained constant even at saturating protein concentrations. Steady-state fluorescence depolarization measurements indicated that association of the M protein with the phospholipid bilayer increased the apparent order of the bilayer both below and above the phospholipid phase transition temperature; this effect may be responsible for the observed changes in thermotropic behavior. At high protein concentrations, the matrix protein induced lipid phase separation, probably due to its tight association with the acidic phospholipid component of the membrane.