Spontaneous apolipoprotein A-I lipidation. Influence of cholesterol and negative phospholipids on the reaction rate and size of lipoprotein products

CUELLAR LA; TOLEDO JD; GARDA HA

Montevideo, Uruguay

Congreso; 6th International Conference of Biological Physics. 5th Southern Cone Biophysical Congress. 34th Annual Meeting of the Argentinean Biophysical Society; 2007

Interntional Union of Pure and Applied Physics. Sociedad Argentina de Biofísica

Apolipoprotein A-I (apoAI), the major protein of high density lipoproteins (HDL), plays a key role in cellular lipid efflux. ApoAI lipidation at the cell surface is mediated by a lipid translocase, ATP-binding cassette A1 (ABCA1). With artificial phospholipid membranes, apoAI lipidation is highly dependent on the phase state and on the size of laterally separated lipid domains (Tricerri et al. J Lipid Res 2006). When effective, this reaction results in the formation of discoidal lipoprotein complexes of different size. To know the requeriments for this reaction "in vitro" is highly relevant to understand the mechanism of "in vivo" ABCA1-dependent lipidation. We have here studied the influence of variable amounts of cholesterol and negatively charged phospholipids in mixtures with dimirystoylphosphatidylcholine (DMPC) on the rate of the micellization reaction and on the size of the lipoprotein products. At the phase transition temperature (Tt) of DMPC (24 ºC), small amounts of cholesterol (up to 10%) increases the micellization rate and determine the appearance of a new population of large size discs. In opposition to zwitterionic phospholipids, which react only at Tt, negatively charged phospholipids like phosphatidylinositol (PI) reacts well in the liquid crystalline state. In this case, however, the lipoprotein complexes formed are of smaller size (≈ 70 kD) in comparison with those of DMPC (> 100 kD). In mixtures with DMPC, small amounts of PI (10%) increase the micellization rate. This fact is particularly relevant for the apoAI lipidation at the cell membrane, since it was reported that ABCA1 preferentially translocates negatively charged phospholipids. * Funded by ANPCyT, CONICET and CIC-Pcia de Bs Aires.