A 30-AMINO ACID TRUNCATION OF THE MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN LARGE SUBUNIT DISRUPTS ITS INTERACTION WITH PROTEIN DISULFIDE-ISOMERASE AND CAUSES ABETALIPOPROTEINEMIA

The microsomal triglyceride transfer protein (MTP) is a heterodimer co mposed of the multifunctional enzgme, protein disulfide-isomerase, and a unique large, 97 kDa, subunit. It is found as a soluble protein wit hin the lumen of the endoplasmic reticulum of liver and intestine and is required for the assembly of very low density lipoproteins and chyl omicrons. Mutations in MTP which result in an absence of MTP function have been shown to cause abetalipoproteinemia. Here, the gene encoding the MTP 97-kDa subunit of an abetalipoproteinemic subject, which we h ave previously demonstrated lacks MTP activity and protein (Wetterau, J. R., Aggerbeck, L. P., Bouma, M.-E., Eisenberg, C., Munck, A., Hermi er, M., Schmitz, J., Gay, G., Rader, D. J., and Gregg, R. E. (1992) Sc ience 258, 999-1001), was isolated and sequenced. A nonsense mutation, which predicts the truncation of the protein by 30 amino acids, was i dentified. To investigate if this apparently subtle change in MTP coul d explain the observed absence of MTP, protein disulfide-isomerase was co-expressed with either the normal or mutant MTP 97-kDa subunit in S f9 insect cells using a baculovirus expression system. Although there were high levels of expression of both the normal and mutant forms of the MTP 97-kDA subunit, only the normal subunit was able to form a sta ble, soluble complex with protein disulfide-isomerase. These results i ndicate that the carboxyl-terminal 30 amino acids of the MTP 97-kDa su bunit plays an important role in its interaction with protein disulfid e-isomerase.