Characterization of the Lipid Binding Pocket in GM2AP and SapB with EPR Spectroscopy

Resumo

Electron paramagnetic resonance (EPR) spectroscopy of spin-labeled lipids in complex with the sphingolipid activator proteins, GM2AP and SapB, was utilized to characterize the hydrophobic binding pocket of these lipid transfer proteins. Specifically, the EPR line shapes reveal that the mobility of the labeled lipids within the binding pockets of the transfer proteins are more restricted than when in a lipid bilayer environment and that lipids in GM2AP are slightly more restricted than in SapB. EPR accessibility based relaxation measurements show that the relative ratios of oxygen and water accessibility to sites along the acyl chains in lipids in complex with GM2AP are similar to the profiles obtained for a lipid bilayer albeit with lowered values. The results for SapB are quite different, with the oxygen profile mimicking a lipid bilayer, but there is a higher degree of water accessibility to the acyl chains in the SapB complex, likely because of the location of the lipid at the dimer interface in SapB coupled to dynamics of the dimer.