The role of protein disulphide isomerase in the microsomal triacylglycerol transfer protein does not reside in its isomerase activity.

The microsomal triacylglycerol transfer protein (MTP), an αβ dimer, is obligatory for the assembly of apoB-containing lipoproteins in liver and intestinal cells. The β subunit is identical with protein disulphide isomerase, a 58 kDa endoplasmic reticulum luminal protein involved in ensuring correct disulphide bond formation of newly synthesized proteins. We report here the expression of the human MTP subunits in Spodoptera frugiperda cells. When the α subunit was expressed alone, the polypeptide formed insoluble aggregates that were devoid of triacylglycerol transfer activity. In contrast, when the α and β subunits were co-expressed, soluble αβ dimers were formed with significant triacylglycerol transfer activity. Expression of the α subunit with a mutant protein disulphide isomerase polypeptide in which both -CGHC- catalytic sites had been inactivated also yielded αβ dimers that had comparable levels of lipid transfer activity relative to wild-type dimers. The results indicate that the role of the β subunit in MTP seems to be to keep the α subunit in a catalytically active, non-aggregated conformation and that disulphide isomerase activity of the β subunit is not required for this function.