Branched synthetic peptide constructs mimic cellular binding and efflux ofapolipoprotein AI in reconstituted high density lipoproteins

This study investigates the suitability of the trimeric apolipoprotein (apo ) AI(145-183) peptide that we recently described, to serve as a model to pr obe the relationship between apo AI structure and function. Three copies of the apoAI(145-183) unit, composed each of two amphipatic alpha-helical seg ments, were branched onto a covalent core matrix and the construct was reco mbined with phospholipids. A similar construct was made with the apoAI(102- 140) peptide and used as a comparison with dimyristoylglycerophosphocholine (DMPC)-apoAI complexes. The DMPC-trimeric-apoAI(145-183) complexes had sim ilar immunological reactivity with monoclonal antibodies directed against t he 149-186 apoAI sequence (A44), suggesting that the A44 epitope is exposed similarly in both the synthetic peptide and the native apoAI complexes. Th e complexes generated with the trimeric-apoAI(145-183) bind specifically to HeLa cells with comparable affinity to the DMPC-apoAI complexes; they are a good competitor for binding of apoAI to both HeLa cells and Fu5AH rat hep atoma cells; finally, these complexes promote cholesterol efflux from Fu5AH cells with an efficiency comparable with the apo AI/lipid complexes. To st udy LCAT activation by the trimeric apo AI(145-183) construct, complexes we re prepared with dipalmitoylphosphatidylcholine (DPPC), cholesterol (C) and either the trimeric construct or apoAI. LCAT activation by the trimeric co nstruct was much lower than by apo AI, possibly because the conformation of the trimeric 145-183 peptide in DPPC/C/peptide complexes does not mimic th at of apoAI in the corresponding complexes. In comparison, the complexes ge nerated with the multimeric apoAI(102-140) construct had a poor capacity to mimic the physico-chemical and biological properties of apoAI. The apoAI(1 02-140) construct had low affinity for lipid compared with the (145-183) co nstruct. After association with lipids, it was a poor competitor of DMPC-ap oAI complexes for cellular binding and had only limited capacity to promote cholesterol efflux. These results suggest trimeric constructs can serve as an appropriate models for apoAI, enabling further investigations and new e xperimental approaches to determine the structure-function relationship of apoAI. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.