Plasma phospholipid transfer protein (PLTP) plays a key role in lipid and lipoprotein metabolism. Known[unreadable] functions of PLTP include transfer of phospholipids (PL) and cholesterol among lipoproteins and between[unreadable] lipoproteins and cells, modulation of HDL level, size, and composition, and enhancement of cellular[unreadable] cholesterol efflux. Current data indicate that most PLTP in human plasma associates with distinct HDL[unreadable] particles and lacks the ability to transfer PL from liposomes to HDL in vitro. The physiological significance[unreadable] of this inactive form of PLTP is unknown. We recently found that PLTP interacts with cell surface ABCA1[unreadable] and promotes transport of cell cholesterol and PL to HDL particles. The structural domains of PLTP and[unreadable] ABCA1 involved in this process remain to be determined. The major goals of this proposal are to establish[unreadable] the structural basis and the metabolic and pathologic relevance of inactive PLTP, and to define the interaction[unreadable] of PLTP with cellular ABCA1 and its role in reverse cholesterol transport. Our overall hypothesis is that[unreadable] specific structural features of PLTP are critical for lipid transport and for interacting with ABCA1. To test[unreadable] this hypothesis, we will determine the biochemical characteristics of lipoprotein particles containing active[unreadable] and inactive PLTP, delineate their potential molecular differences, examine the possibility of their[unreadable] interconversion, and study their roles in modulating plasma lipoproteins and in cellular lipid homeostasis. We[unreadable] will also characterize the mechanisms of PLTP-mediated lipid removal by the ABCA1 pathway, and define[unreadable] molecular properties of PLTP and ABCA1 required for their interaction and lipid efflux using PLTP domainspecific[unreadable] peptides, and PLTP and ABCA1 mutants. Furthermore, we will investigate the pathological[unreadable] relevance of the balance between active and inactive PLTP in the pathogenesis of FCHL and the quantitative[unreadable] trait loci for PLTP activity and active and inactive PLTP mass in families with FCHL in collaboration with[unreadable] Project 1. The proposed studies should provide novel insights into the mechanisms of action and the role of[unreadable] PLTP in lipoprotein metabolism and pathophysiology. A better understanding of the basic function and[unreadable] mechanisms of action of PLTP could lead to more targeted treatment and prevention of lipoprotein disorders[unreadable] that contribute to the development of cardiovascular disease.