Interaction of â2-glycoprotein I with Negaticely charged phospholipid membranes.

MARIANA PAOLOROSSI; GUILLERMO MONTICH

Rosario

Congreso; XXXV Reunión anual de la Sociedad Argentina de Biofísica; 2006

¥â2-Glycoprotein I (¥â2-GPI) is a soluble glycoprotein present in plasma. It participates in several physiological and pathogenic events. It was proposed, for example, that ¥â2-GPI binds to the exposed anionic phospholipids of apoptotic cells producing auto-antigenic lipid-protein complexes. The binding to the membrane is mediated by both electrostatic and hydrophobic forces. Even when the electrostatic interaction is absolutely required, the sole presence of a negative charge does not determine the strength of binding and different affinities are observed for different anionic lipids. We addressed the study of membrane interactions of ¥â2-GPI to understand the balance and mutual influence of electrostatic and hydrophobic driving forces and whether these interactions are coupled to conformational changes of the protein in the interface.  ¥â2-GPI was purified from human plasma by heparin affinity chromatography. The interaction of ¥â2-GPI with pure phospholipid membranes composed of palmitoyl oleoyl phosphatidylglycerol (POPG), palmitoyl oleoyl phosphatidylcholine (POPC) and bovine heart cardiolipin (CL) was studied by differential scanning calorimetry (DSC) and infrared spectroscopy (FT-IR). In solution, ¥â2-GPI exhibited a reversible two-state transition with a Tm of  64 ¨¬C. In the presence of large unilamellar vesicles (LUVs) composed of POPC, the thermal unfolding was similar to that observed for pure ¥â2-GPI. The thermal transition of the protein in the presence of CL or POPG LUVs presented remarkable differences when compared with that of the pure protein solution. The addition of CL or POPG LUVs induced an increase in turbidity of the solution of pure ¥â2-GPI immediately after liposomes addition. DSC thermograms show a sharp peak at 60 ¨¬C in the presence of POPG vesicles, while the addition of CL liposomes abolished the thermotropic transition of ¥â2-GPI. The secondary structure of ¥â2-GPI was studied by FT-IR. The presence of POPG or CL multilamellar vesicles did not induce remarkable changes in the spectrum shape of the native conformation at 25 ¨¬C.