THE ROLE OF PROTEIN DISULFIDE-ISOMERASE IN THE MICROSOMAL TRIACYLGLYCEROL TRANSFER PROTEIN DOES NOT RESIDE IN ITS ISOMERASE ACTIVITY

The microsomal triacylglycerol transfer protein (MTP), an alpha beta d imer, is obligatory for the assembly of apoB-containing lipoproteins i n liver and intestinal cells. The beta subunit is identical with prote in disulphide isomerase, a 58 kDa endoplasmic reticulum luminal protei n involved in ensuring correct disulphide bond formation of newly synt hesized proteins. We report here the expression of the human MTP subun its in Spodoptera frugiperda cells. When the alpha subunit was express ed alone, the polypeptide formed insoluble aggregates that were devoid of triacylglycerol transfer activity. In contrast, when the alpha and beta subunits were co-expressed, soluble alpha beta dimers were forme d with significant triacylglycerol transfer activity. Expression of th e alpha subunit with a mutant protein disulphide isomerase polypeptide in which both -CGHC- catalytic sites had been inactivated also yielde d alpha beta dimers that had comparable levels of lipid transfer activ ity relative to wild-type dimers. The results indicate that the role o f the beta subunit in MTP seems to be to keep the alpha subunit in a c atalytically active, non-aggregated conformation and that disulphide i somerase activity of the beta subunit is not required for this functio n.