Activity of phosphatidylinositol transfer protein is sensitive to ethanol and membrane curvature

Abstract

Phosphatidylinositol transfer protein (PITP) is critical for many cellular signalling and trafficking events that are influenced by ethanol. The influence of ethanol and membrane curvature on the activity of recombinant mouse PITP-α in vitro is evaluated by monitoring the transfer of phosphatidylinositol (PtdIns) from rat hepatic microsomes to unilamellar vesicles. Acute exposure to pharmacological levels of ethanol enhanced the function of PITP. Chloroform shared a similar ability to enhance function when both drug concentrations were normalized to their respective octanol/water partition coefficients, indicating that the effect is not unique to ethanol and might be common to hydrophobic solutes. Neither the PITP activity nor its ethanol enhancement was altered by using thermally pretreated (denatured) or protease-treated microsomes, indicating that the native microsomal protein structure was unlikely to be a determinant of transfer. Kinetic analyses indicated that ethanol acted by increasing the PITP-mediated flux of PtdIns from both microsomal and liposomal surfaces. The activity of PITP was strongly dependent on the lipid structure, with a steep dependence on the expressed curvature of the membrane. Activity was greatest for small, highly curved sonicated vesicles and decreased markedly for large, locally planar unilamellar vesicles. Ethanol enhanced PITP-mediated PtdIns transfer to all vesicles, but its effect was much smaller than the enhancement due to curvature, which is consistent with ethanol's comparatively modest ability to perturb membrane lipids. The ethanol efficacy observed is as pronounced as any previously described lipid-mediated ethanol action. In addition, these observations raise the possibility that PITP specifically delivers PtdIns to metabolically active membrane domains of convex curvature and/or low surface densities of lipid.