A Novel Abetalipoproteinemia Missense Mutation Highlights the Importance of N-Terminal β-Barrel in Microsomal Triglyceride Transfer Protein Function

Background —The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias due to associated hepatosteatosis and gastrointestinal adverse effects. Comprehensive knowledge about the structure-function of MTP might help design new molecules that avoid steatosis. Characterization of mutations in MTP causing abetalipoproteinemia have revealed that the central α-helical and C-terminal β-sheet domains are important for protein disulfide isomerase (PDI) binding and lipid transfer activity. Our aim was to identify and characterize mutations in the N-terminal domain to understand its function. Methods and Results —We identified a novel missense mutation (D169V) in a 4-month old Turkish male with severe signs of ABL. To study the effect of this mutation on MTP function, we created mutants via site-directed mutagenesis. Although D169V was expressed in the endoplasmic reticulum and interacted with apoB17, it was unable to bind PDI, transfer lipids, and support apoB secretion. Computational modeling suggested that D169 could form an internal salt bridge with K187 and K189. Mutagenesis of these lysines to leucines abolished PDI heterodimerization, lipid transfer, and apoB secretion, without affecting apoB17 binding. Further, mutants with preserved charges (D169E, K187R, K189R) rescued these activities. Conclusions —D169V is detrimental because it disrupts an internal salt bridge leading to loss of PDI binding and lipid transfer activities; however, it does not affect apoB-binding. Thus, the N-terminal domain of MTP is also important for its lipid transfer activity.