Protein disulfide isomerase appears necessary to maintain the catalytically active structure of the microsomal triglyceride transfer protein

Wetterau, John R.; Combs, Kelly A.; McLean, Larry R.; Spinner, Sara N.; Aggerbeck, Lawrence P.

doi:10.1021/bi00104a023

Cited by 187 publications

(152 citation statements)

References 33 publications

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“…Mammalian MTP is a heterodimeric complex of a 97-kDa "M" subunit and a 55-kDa PDI subunit (28,29). We therefore determined whether dMTP also associates with endogenous PDI.…”

Section: Resultsmentioning

confidence: 99%

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

Rava

Ojakian

Shelness

et al. 2006

Journal of Biological Chemistry

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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 ϳ50% 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.Lipoproteins are lipid-protein complexes that transport lipids, fatsoluble vitamins, and other hydrophobic molecules in the plasma. Apolipoprotein B (apoB) 2 is a structural protein embedded in the phospholipid monolayer on the surface of triglyceride-rich lipoproteins. It has been hypothesized that apoB contains amphipathic ␣-helical and ␤-sheet domains (1). Lipidation of the ␤-sheets is necessary for the assembly of larger apoB polypeptides into lipoprotein emulsion particles. Lipoprotein assembly begins co-translationally and microsomal triglyceride transfer protein (MTP) plays a pivotal role in this process (for reviews, see Refs. 2-6). MTP is absent in abetalipoproteinemia, a disease characterized by the deficit of plasma apoB lipoproteins and low plasma cholesterol levels (7,8). Reconstitution of MTP in heterologous systems rescues apoB secretion, whereas tissue-specific liver knock-out models recreate the lipoprotein deficiency present in abetalipoproteinemia (9, 10). The signature activity of MTP is its ability to transfer lipids between membranes. It has been demonstrated that MTP lipid transfer activity is necessary for apoB lipoprotein assembly and secretion (for reviews, see Refs. 2-6). More recent evidence suggests that MTP lipid transfer activity is also responsible for lipid accretion within the secretory pathway and that MTP potentially stabilizes lipid vesicles in the endoplasmic reticulum (reviewed in Refs. 2 and 3).MTP transfers several lipids including triacylglycerols, cholesterol esters, and phospholipids between vesicles in vitro (11)(12)(13)(14). It has been suggested that MTP transiently interacts with a membrane, extracts lipids, and then delivers them to another lipid acceptor or to nascent apoB lipoproteins (15). Kinetic studies in...

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“…Mammalian MTP is a heterodimeric complex of a 97-kDa "M" subunit and a 55-kDa PDI subunit (28,29). We therefore determined whether dMTP also associates with endogenous PDI.…”

Section: Resultsmentioning

confidence: 99%

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

Rava

Ojakian

Shelness

et al. 2006

Journal of Biological Chemistry

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“…Here again, PDI seems necessary to keep the complex in solution, as irreversible inactivation and aggregation ensues upon removal of the β-subunit [112].…”

Section: Subunit Associationmentioning

confidence: 99%

The protein disulphide-isomerase family: unravelling a string of folds

Ferrari

Söling

1999

Biochemical Journal

399

359

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The mammalian protein disulphide-isomerase (PDI) family encompasses several highly divergent proteins that are involved in the processing and maturation of secretory proteins in the endoplasmic reticulum. These proteins are characterized by the presence of one or more domains of roughly 95-110 amino acids related to the cytoplasmic protein thioredoxin. All but the PDI-D subfamily are composed entirely of repeats of such domains, with at least one domain containing and one domain lacking a redox-active -Cys-Xaa-Xaa-Cys-tetrapeptide. In addition to their known roles as redox catalysts and isomerases, the last few years have revealed additional functions of the PDI proteins,

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“…1 is a heterodimer consisting of protein disulfide isomerase and the 97-kDa-large subunit and is found mainly in the endoplasmic reticulum (ER) lumen of liver and intestinal cells (1)(2)(3). The essential role of MTP in the translocation of apolipoprotein B (apoB) across the ER membrane and the assembly of apoB with lipids has been demonstrated clearly by co-expressing apoB and MTP in cells that do not normally express either protein and by the characterization of mice lacking MTP.…”

Section: Microsomal Triglyceride Transfer Protein (Mtp)mentioning

confidence: 99%

Microsomal Triglyceride Transfer Protein Binding and Lipid Transfer Activities Are Independent of Each Other, but Both Are Required for Secretion of Apolipoprotein B Lipoproteins from Liver Cells

Liang

Ginsberg

2001

Journal of Biological Chemistry

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Recent studies indicate that microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB) interact physically via two specific binding sites located within the amino-terminal globular region of apoB100. The first site is thought to be within the first 5.8% of the amino-terminal sequence, and the second site is between 9 and 16% of the amino-terminal sequence. It is not clear from prior studies whether these sites have unique or overlapping functions. Furthermore, there are no data differentiating between lipid transfer and potential chaperone functions of MTP. In the present study we have attempted to further characterize the physiologic interaction between apoB and MTP and to determine the relationship between the binding and lipid transfer aspects of the interaction. HepG2 cells were transiently transfected with apoB cDNAs, and MTP binding to apoB polypeptides was determined by two-step immunoprecipitation. MTP bound equally well to apoB polypeptides with (apoB13, 16,␤, apoB34, and apoB42) or without (apoB16, apoB13, and 16 or apoB13, 13, and 16) ␤ sheet domains. When proteasomal degradation of newly synthesized apoB polypeptides was blocked, MTP binding to all of the apoB polypeptides was only modestly affected by lipid availability and was independent of MTP-associated lipid transfer. Furthermore, MTP did not bind directly to a portion of the first ␤ sheet domain. We created two apoB constructs (apoB16 del and apoB34 del ) by deleting the first 210 amino acids of apoB16 and apoB34. These apoB polypeptides, therefore, lacked the putative first MTP binding site. MTP binding to apoB16 del and apoB34 del was decreased significantly. However, the secretion of apoB16 del was not different from apoB16, whereas the secretion of apoB34 del was impaired significantly. Our results indicate that the interaction between MTP and apoB involves independent binding and lipid transfer activities but that both activities are required for the secretion of apolipoprotein B from liver cells. Microsomal triglyceride transfer protein (MTP)1 is a heterodimer consisting of protein disulfide isomerase and the 97-kDa-large subunit and is found mainly in the endoplasmic reticulum (ER) lumen of liver and intestinal cells (1-3). The essential role of MTP in the translocation of apolipoprotein B (apoB) across the ER membrane and the assembly of apoB with lipids has been demonstrated clearly by co-expressing apoB and MTP in cells that do not normally express either protein and by the characterization of mice lacking MTP. The expression of apoB alone in cells without MTP resulted in the intracellular degradation of nascent apoB and no secretion; co-expression of apoB with MTP resulted in the secretion of apoB on lipoprotein particles (4 -6). Furthermore, the inhibition of MTP activity in cultured liver cells blocked the assembly and secretion of apoB-containing lipoproteins (7-11). We demonstrated that inhibition of MTP activity was associated with increased ubiquitination of apoB, particularly when proteasomal degradation was ...

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Protein disulfide isomerase appears necessary to maintain the catalytically active structure of the microsomal triglyceride transfer protein

Cited by 187 publications

References 33 publications

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

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

The protein disulphide-isomerase family: unravelling a string of folds

Microsomal Triglyceride Transfer Protein Binding and Lipid Transfer Activities Are Independent of Each Other, but Both Are Required for Secretion of Apolipoprotein B Lipoproteins from Liver Cells

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