Microsomal Triglyceride Transfer Protein (MTP)

Microsomal triglyceride transfer protein (MTP) is a heterodimer chaperone consisting of a 97-kDa lipid-binding and -transfer subunit and a 55-kDa protein disulfide isomerase (PDI). MTP was first identified as an endoplasmic reticulum (ER) resident protein that helps in the transfer of neutral lipids to nascent apolipoprotein B (apoB). MTP is predominantly expressed in hepatocytes and enterocytes and is required for the assembly and secretion of VLDL and chylomicrons by the liver and intestine, respectivel. Its critical role in the assembly and secretion of apoB-containing lipoproteins was identified in abetalipoproteinemia patients who have mutations in MTP and completely lack apoB-containing lipoproteins in the circulation. It has been established now that MTP not only is involved in the transfer of neutral lipids but also plays a role in cholesterol ester and cluster of differentiation 1d (CD1d) biosynthesis. Besides neutral lipids, MTP may also help in the transfer of sphingolipids such as ceramides and sphingomyelin to the apoB-containing lipoproteins.