Phospholipid Transfer Protein in Human Plasma Associates with Proteins Linked to Immunity and Inflammation

Abstract

Phospholipid transfer protein (PLTP), which associates with apolipoprotein A-I (the major HDL protein) plays a key role in lipoprotein remodeling. Because its level in plasma increases during acute inflammation, it may also play previously unsuspected roles in the innate immune system. To gain further insight into its potential physiological functions, we isolated complexes containing PLTP from plasma by immunoaffinity chromatography, and determined their composition. Shotgun proteomics revealed that only 6 of the 24 proteins detected in the complexes were apolipoproteins. The most abundant proteins were clusterin (apoJ), PLTP itself, coagulation factors, complement factors, and apoA-I. Remarkably, 20 of the 24 proteins had known proteinprotein interactions. Biochemical studies confirmed two previously established interactions and identified 5 new ones between PLTP and proteins. Moreover, clusterin, apoA-I, and apoE preserved the lipid-transfer activity of recombinant PLTP in the absence of lipid, indicating that these interactions may have functional significance. Unexpectedly, lipids accounted for only 3% of the mass of the PLTP complexes. Collectively, our observations indicate that PLTP in human plasma resides on lipid-poor complexes dominated by clusterin and proteins implicated in host defense and inflammation. They further suggest that protein-protein interactions drive the formation of PLTP complexes in plasma.Phospholipid transfer protein (PLTP) is an 80-kDa glycoprotein that binds phospholipids and facilitates their transfer between lipoproteins in plasma. It is expressed by macrophages and many other tissues (1,2). PLTP associates with apoA-I and apoE (3,4), as well as with several unidentified proteins (4). Two forms of PLTP have been detected in human plasma: an "active" form that transfers phosphatidylcholine from phospholipid vesicles to high density lipoproteins (HDL), and an "inactive" form that lacks this capability (4-6). The apparent molecular weight of the "active" form is similar to that of small HDL particles (~160 kDa), while the inactive complexes appear to be much larger (apparent MW ~520 kDa) (4-6). However, the plasma fraction showing the greatest PLTP activity has a density † This study was supported by grants (H030086, HL086798) from the National Institutes of Health. TV was supported by a Pilot and Feasibility Award from the Diabetes and Endocrinology Research Center (NIH 5 P30 DK17047), and Scientist Development Grant Award from American Heart Association 0830231N. Mass spectrometry experiments were supported by the Mass Spectrometry Resource, Department of Medicine, and the Mass Spectrometry Core, Diabetes and Endocrinology Research Center, University of Washington. * To whom correspondence should be addressed: Northwest Lipid Metabolism and Diabetes Research Laboratories, Department of Medicine, University of Washington, 401 Queen Anne Avenue North, Seattle, 685-3279. jja@u.washington.edu. ∥ These authors contributed equally to the work. NIH Public Access Author Manusc...

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