Interaction of Human Apolipoprotein A-I with Model Membranes Exhibiting Lipid Domains》.pdfVIP

Biophysical Journal Volume 89 July 2005 285–295 285 Interaction of Human Apolipoprotein A-I with Model Membranes Exhibiting Lipid Domains ´ y y Cristina Arnulphi,* Susana A. Sanchez, M. Alejandra Tricerri,* Enrico Gratton, and Ana Jonas* *Department of Biochemistry, and yLaboratory for Fluorescence Dynamics, Department of Physics, University of Illinois at Urbana-Champaign, Champaign, Illinois ABSTRACT Several mechanisms for cell cholesterol efflux have been proposed, including membrane microsolubilization, suggesting that the existence of specific domains could enhance the transfer of lipids to apolipoproteins. In this work isothermal titration calorimetry, circular dichroism spectroscopy, and two-photon microscopy are used to study the interaction of lipid-free apolipoprotein A-I (apoA-I) with small unilamellar vesicles (SUVs) of 1-palmitoyl, 2-oleoyl phosphatidylcholine (POPC) and sphingomyelin (SM), with and without cholesterol. Below 30C the calorimetric results show that apoA-I interaction with POPC/SM SUVs produces an exothermic reaction, characterized as nonclassical hydrophobic binding. The heat capacity change (DC) is p small and positive, whereas it was larger and negative for pure POPC bilayers, in the absence of SM. Inclusion of cholesterol in the membranes induces changes in the observed thermodynamic pattern of binding and counteracts the formation of a-helices in the protein. Above 30C the reactions are endothermic. Giant unilamellar vesicles (GUVs) of identical composition to the SUVs, and two-photon fluorescence microscopy techniques, we