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 protein-protein interactions. Biochemical studies confirmed two previously established interactions and identified five 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.
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