Restricted lateral diffusion of acidic lipids in phospholipid vesicles aggregated by myelin basic protein

Abstract

The effect of myelin basic protein on the lateral diffusion of spin-labelled stearic acid in dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) vesicles has been investigated measuring bimolecular collision rates by electron spin resonance spectroscopy. In a molar fraction range of 0.02–0.15, in which the line shape is dominated by the effects of exchange narrowing and dipolar broadening, spectral simulations based on the modified Bloch equations have been used to quantitate the observed spectral changes. Best fitting simulated spectra required a combination of dipole-dipole and spin exchange contributions and least-squares optimizations were applied to obtain a unique combination of these factors. Lateral diffusion coefficients were calculated from collision frequency data in the frame of the lattice model; for pure DMPC and DMPG vesicles lateral diffusion coefficients of 1.7 · 10 −7 and 1.2 · 10 −7 cm s 2 · −1 were obtained. Upon adding myelin basic protein to these vesicles the diffusion coefficients of the negatively charged DMPG decreased to 4.1 · 10 −8 cm 2 · s −1 , whereas that of the zwitterionic DMPC remained unchanged. It is concluded that myelin basic protein must strongly restrict the lateral movement of the DMPG molecules and reduce their diffusion coefficient close to the values observed for proteins. The restricted lateral diffusion is explained by electrostatic interactions which lead to an altered molecular dynamics due to one-to-one lipid exchange and lipid-protein co-diffusion.