Human apolipoprotein A-IV (apo-IV) is a plasma protein whose most distinctive property is its labile affinity for the surface of plasma lipoprotein. Although the specific function of apo A-IV in lipid metabolism has not yet been determined, studies, conducted by the Principal Investigator have contributed to a growing body of evidence which implicates apo A-IV as an important factor in the intravascular metabolism of high density lipoprotein (HDL). Recent studies now suggest that the labile binding of apo A-IV to lipid surfaces may be central to its physiological function. Specifically, we have proposed that the labile binding of apo A-IV to HDL constitutes a barostatic mechanism which maintains the surface pressure of HDL in a critical range required for optimal activity of lecithin-cholesterol acyltransferase (LCAT) and cholesterol ester transfer protein (CETP). The purpose of this proposal is to investigate the impact of molecular structure on the interaction of apo A-IV and phospholipid, with particular regard tot he molecular mechanism of the CETP reaction. We therefore propose four specific aims: 1) Recombinant apo A-IV deletion mutants will be studied by spectroscopic and surface balance techniques to examine the effect of specific amino acid domains on its ordered structure, thermodynamic stability, and lipid affinity; 2) The interaction of recombinant apo A-IV deletion mutants and phospholipid will be studied by physico-chemical techniques to examine the impact of protein structure on the properties and LCAT reactivity of apo A-IV/lipid complexes; 3) The interaction of CETP and HDL will be investigated using fluorescent labelled recombinant and native HDL to determine the mechanisms by which apo A-IV facilitates CETP-catalyzed lipid exchanges and the interconversion of HDL subspecies; 4) CETP will be studied using surface balance techniques to elucidate the mechanisms of the reaction and the role of apo A-IV facilitates CETP-catalyzed lipid exchanges and the interconversion of HDL subspecies; 4) CETP will be studied using surface balance techniques to elucidate the mechanisms of the reaction and the role of apo A-IV in lipid exchange phenomena. The investigation of the structure and function of the plasma apolipoproteins has led to major advances in the diagnosis, treatment, and prevention to atherosclerotic cardiovascular disease, the leading cause of death in the USA. The long term goal of this proposal is to delineate the physiological functions of human apo A-IV.
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