Phospholipid Transfer Activity of Microsomal Triacylglycerol Transfer Protein Is Sufficient for the Assembly and Secretion of Apolipoprotein B Lipoproteins

Paul Rava,George K Ojakian,Gregory S Shelness,M Mahmood Hussain

doi:10.1074/jbc.m512823200

Abstract

Human microsomal triacylglycerol transfer protein (hMTP) is essential for apolipoprotein B (apoB)-lipoprotein assembly and secretion and is known to transfer triacylglycerols, cholesterol esters, and phospholipids. To understand the relative importance of each lipid transfer activity, we compared the ability of hMTP and its Drosophila ortholog (dMTP) to assemble apoB lipoproteins and to transfer various lipids. apoB48 secretion was induced when co-expressed with either hMTP or dMTP in COS cells, and oleic acid supplementation further augmented secretion without altering particle density. C-terminal epitope-tagged dMTP (dMTP-FLAG) facilitated the secretion of apoB polypeptides in the range of apoB48 to apoB72 but was approximately 50% as efficient as hMTP-FLAG. Comparison of lipid transfer activities revealed that although phospholipid transfer was similar in both orthologs, dMTP was unable to transfer neutral lipids. We conclude that the phospholipid transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB lipoproteins and may represent its earliest function evolved for the mobilization of lipid in invertebrates. Identification of MTP inhibitors, which selectively affect transfer of a specific lipid class, may have therapeutic potential.

Highlights

Activity is necessary for apolipoprotein B (apoB) lipoprotein assembly and secretion
We asked whether apoB secretion supported by Drosophila microsomal triglyceride transfer protein (MTP) (dMTP) is modulated by oleic acid (OA) supplementation
In the absence of OA, apoB secreted from COS cells expressing human MTP (hMTP) or dMTP was present in fractions 3– 6 corresponding to a density of 1.1–1.15 g/ml (Fig. 1, C–E)

Summary

Introduction

Activity is necessary for apoB lipoprotein assembly and secretion (for reviews, see Refs. 2– 6). Activity is necessary for apoB lipoprotein assembly and secretion More recent evidence suggests that MTP lipid transfer activity is responsible for lipid accretion within the secretory pathway and that MTP potentially stabilizes lipid vesicles in the endoplasmic reticulum Kinetic studies indicate the presence of two lipid-binding sites: one site binds triacylglycerols, cholesterol esters, and phospholipids with a preference for neutral lipids, whereas a second site binds only phospholipids [16]. It is not clear whether all MTP lipid transfer activities are required for lipoprotein assembly. Phospholipid, but not the triacylglycerol transfer, activity may be necessary for the assembly of primordial lipoproteins

Methods

Results

Conclusion