Arrangement of apolipoprotein A-I in discoidal high density lipoproteins: Spontaneously-formed versus detergent-dialysis prepared discs

PRIETO E D; GARDA H A; SOULAGES J L

Rosario

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

Sociedad Argentina de Biofísica

Discoidal high density lipoproteins (dHDL) result from the interaction of apolipoprotein A-I (apoAI) with cells and they are important intermediates in HDL metabolism. Although dHDL can be spontaneously formed by the reaction of apoAI with phospholipid vesicles at the phase transition temperature, knowledge about the structure and apoAI arrangement on these particles comes from studies in cholate-dialysis reconstituted samples. There is a general agreement that in dHDL with 2 apoAI molecules, a phospholipid bilayer disc is surrounded at its edge by the amphipathic helices of both protein molecules, which are antiparallel and in an extended conformation. However, alternative configurations differing in the alignement (or helix registry) of one apoAI molecule with respet to the another, are obseved in cholate-dialysis reconstituted samples (1, 2). At the SAB 2005 meeting, we presented evidence obtained by energy transfer measurements with single triptophan mutants. ApoAI configuration in DMPC discs spontaneously formed by the reaction at 24ºC differs from that in discs prepared by cholate-dialysis . We have enlarged those studies by using a cross-linking /tripsinolysis approach followed by HPLC and MALDI-TOF, to map those lysine residue lying close together each other. Althoug most cross-links remains to be identified, the results confirm the different arrangement of apoAI in discs formed by cholate-dialysis or the spontaneous reaction. Data support a unique arrangement of apoAI in spontaneously formed discs with the 3-4 helix pair of each apoAI molecule close together each other, opposite to the variable registry observed in cholate-dialysis prepared samples. Is likely to the spontaneous reaction mechanism have similarities with the "in vivo" mechanism, so these observations could be of great physiological relevance. References: 1) Li HH, Lyles DS, Pan W, Alexander E, Thomas MJ and Sorci-Thomas MG (2002) ApoA-I structure on discs and spheres. Variable helix registry and conformational states. J Biol Chem 277, 39093-39101. 2) Silva RA, Hilliard GM, Li L, Segrest JP and Davidson WS (2005) A mass spectrometric determination of the conformation of dimeric apolipoprotein A-I in discoidal high density lipoproteins. Biochemistry 44, 8600-8607.