Identification of microsomal triglyceride transfer protein in intestinal brush-border membrane.

Microsomal triglyceride transfer protein (MTP) is a heterodimeric complex consisting of a unique large 97-kDa protein and the multifunctional 58-kDa protein disulfide isomerase (PDI). It plays an essential role in the assembly of lipoproteins by shuttling lipids between phospholipid membranes. Based on cell fractionation, early studies have suggested the endoplasmic reticulum (ER) as the exclusive site of MTP. Focusing on the plasma membrane in this study, our attempts with immunoelectron microscopy and specific antibodies surprisingly revealed that labeling was not exclusively confined to the microsomes of rat absorptive cells. Immunogold labeling was also detected over the microvillus membrane of enterocytes. Western blot analysis and biochemical activity measurement confirmed MTP protein expression in brush-border membrane vesicles (BBMV) isolated from the intestinal epithelial cells of various species. Furthermore, MTP was coexpressed in microvilli membrane with PDI that is crucial to maintain the structure and activity of the MTP complex. The treatment of Caco-2 cells with nocodazole and colchicine blocked the appearance of MTP in the apical membrane. Similarly, the addition of BMS-197636, a known inhibitor of MTP transfer activity, suppressed the latter. In conclusion, the present studies suggest that MTP is present in the brush-border membrane of the enterocyte. Understanding the possible physiological role of MTP in this location may reveal additional functions.