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

Abstract

BACKGROUND:

The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias because of 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 has revealed that the central α-helical and C-terminal β-sheet domains are important for protein disulfide isomerase 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 child with severe signs of abetalipoproteinemia. 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 apolipoprotein B (apoB) 17, it was unable to bind protein disulfide isomerase, 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 protein disulfide isomerase heterodimerization, lipid transfer, and apoB secretion, without affecting apoB17 binding. Furthermore, mutants with preserved charges (D169E, K187R, and K189R) rescued these activities.

CONCLUSIONS:

D169V is detrimental because it disrupts an internal salt bridge leading to loss of protein disulfide isomerase 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.

• Authors: Meghan T Walsh, Jahangir Iqbal, Joby Josekutty, James Soh, Enza Di Leo, Eda Özaydin, Mehmet Gündüz, Patrizia Tarugi, M Mahmood Hussain
• Journal: Circulation. Cardiovascular genetics (Circ Cardiovasc Genet) Vol. 8 Issue 5 Pg. 677-87 (Oct 2015) ISSN: 1942-3268 [Electronic] United States
• PMID: 26224785 (Publication Type: Case Reports, Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, Non-P.H.S.)
• Copyright: © 2015 American Heart Association, Inc.

Chemical References

• Apolipoproteins B
• Carrier Proteins
• microsomal triglyceride transfer protein

Topics

• Abetalipoproteinemia (genetics, metabolism)
• Animals
• Apolipoproteins B (metabolism)
• Binding Sites
• COS Cells
• Carrier Proteins (chemistry, metabolism)
• Chlorocebus aethiops
• DNA Mutational Analysis
• Endoplasmic Reticulum (metabolism)
• Humans
• Infant
• Lipid Metabolism
• Male
• Mutation, Missense
• Protein Binding
• Protein Structure, Tertiary
• Transfection