Identification of Two Classes of Lipid Molecule Binding Sites on the Microsomal Triglyceride Transfer Protein

Abstract

The gene for the microsomal triglyceride transfer protein (MTP) is defective in subjects with the genetic disease abetalipoproteinemia, indicating that MTP is essential for the assembly of apolipoprotein B containing lipoproteins. In vitro, MTP is a lipid molecule binding protein that catalyzes lipid transport between membranes by a shuttle mechanism. In this study, the lipid binding properties of MTP were examined. MTP was incubated with donor phosphatidylcholine vesicles of varying neutral lipid composition. MTP was subsequently reisolated by ultracentrifugation, and MTP-bound lipid was quantitated. When the triolein content of the vesicles was increased up to 4 mol %, neutral lipid binding to MTP increased proportionately, while phosphatidylcholine binding appeared to remain constant around two molecules per MTP. Using phosphatidylcholine emulsions containing 60 mol % triolein as the donor particles resulted in only a slight increase in triolein binding to MTP. The highest triolein:MTP ratio observed was (0.20-0.25):1. Differences in the neutral and phospholipid binding properties of MTP were observed by measuring the transport of lipid from MTP to acceptor vesicles. Transport of triolein was rapid and complete, while phosphatidylcholine transport was biphasic, containing rapid and slow phases. These results indicated that MTP contains more than one class of lipid molecule binding site. Measurements of fluorescent lipid transport from donor vesicles to MTP supported this hypothesis. The transport of pyrene-labeled triglyceride from donor particles to MTP was rapid, while phosphatidylcholine transfer had fast and slow phases. From these data, we propose that MTP contains at least two distinct classes of lipid molecule binding sites that differ in function. The fast site or sites are responsible for lipid transport.