PIE-FCCS Study of the Effects of Polycationic Macromolecules on Phosphatidylserine and Phosphatidylinositol Phosphate Lipid Mobility

Abstract

The interaction between proteins and anionic lipids in the plasma membrane is a common motif in cell communication at the plasma membrane. Such interactions can gate membrane protein function and have also been proposed to sequester membrane lipids during quiescent phases of signaling. To date, however, the molecular structure and dynamics of the lipid-protein interface is poorly understood. To isolate these interactions in a biophysical assay, we have investigated the behavior of a polycationic polymer, quaternized polyvinylpyridine (QPVP), on supported lipid bilayers doped with tail-labeled phosphatidylserine (PS) or phosphatidylinositol phosphate (PIP) lipids. To measure the mobility and association of the lipid and adsorbed polymer, we use a time-resolved fluorescence technique, pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is a dual-color fluorescence spectroscopy that translates fluctuations in fluorescence signal into a measurement of diffusion and co-localization. With PIE-FCCS we have investigated the polymer adsorption-dependent translational mobility of the lipids and systematically studied the influence of lipid head-group charge and solvent ionic strength. Our results indicate that alteration of anionic lipid lateral mobility is dependent on the net charge of the lipid head group and is significantly screened by the ionic strength of the solution. At physiological salt concentration we observe that the lipid lateral mobility is nearly unaffected by mobile, adsorbed polymers and that there is no evidence of stable lipid-polymer complexes.