Abstract
Microsomal triglyceride transfer protein (MTP) is essential for the assembly of apolipoprotein B-containing lipoproteins. Within the endoplasmic reticulum, it transfers lipid from the membrane to the forming lipoprotein. Recent evidence suggests that it may also function within the Golgi apparatus. To address this hypothesis, we developed a polyclonal antibody to MTP and used it in a series of studies on mouse liver and McArdle-RH7777 (McA) cells. Western blot analysis demonstrated the presence of MTP within mouse hepatic-Golgi apparatus-rich fractions. In addition, in vitro lipid transfer assays demonstrated the presence of triglyceride transfer activity within the Golgi fractions. Immunohistochemical studies with mouse liver demonstrated the presence of MTP within all hepatocytes, but not in nonparenchymal cells. The subcellular location of MTP in McA cells was investigated using confocal microscopy. MTP colocalized with the trans-Golgi network (TGN) 38 and Golgi SNARE (soluble N-ethylmalemide-sensitive factor attachment protein receptor) of 28 kDa (GS28), markers for the trans- and cis-Golgi apparatus, respectively. Morphometric analyses indicated that approximately 17% of the MTP signal colocalized with the TGN38, while 33% of the trans-Golgi marker colocalized with the MTP. Approximately 17% of the MTP signal colocalized with the GS28, whereas 53% of the cis-Golgi marker colocalized with the MTP. The results provide unequivocal evidence for the location of MTP within the Golgi apparatus, and further highlight the importance of this organelle in the assembly of lipoproteins.
Highlights
Microsomal triglyceride transfer protein (MTP) is essential for the assembly of apolipoprotein B-containing lipoproteins
The antibody recognized a 97 kDa protein in mouse hepatic microsomes that comigrated with a band recognized by an MTP antibody kindly provided by John Wetterau at Bristol Myers Squibb (Fig. 1A)
Using Western blots, we have identified MTP within Golgi apparatus-rich fractions of mouse liver (Fig. 1B) and demonstrated triglyceride transfer activity within the lumen of the Golgi (Fig. 2)
Summary
Microsomal triglyceride transfer protein (MTP) is essential for the assembly of apolipoprotein B-containing lipoproteins. These observations have led us to hypothesize that MTP is present within the Golgi apparatus and mediates the transfer of membrane triglyceride to nascent lipoproteins. We report the presence of MTP in Golgi apparatus-rich fractions from mouse liver, and we demonstrate triglyceride transfer activity in these fractions using an in vitro lipid transfer assay.
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