Abstract
The microsomal triglyceride transfer protein (MTTP) is essential for the assembly of VLDLs. We recently observed that a polymorphism in the MTTP promoter (-493G>T), which is in allelic association with an isoleucine-to-theronine substitution at position 128 (Ile128Thr) in the expressed protein, confers an increased risk of coronary heart disease. Two variant proteins comprising amino acids 16-297 of intact MTTP, MTTP(N)-Ile128 and MTTP(N)-Thr128, had similar native secondary structure content, as judged by circular dichroism. However, the thermal stability of MTTP(N)-Thr128 was greatly reduced, and this protein was also more extensively cleaved in limited proteolysis experiments compared with MTTP(N)-Ile128; both of these findings support a less compact fold. On adding LDL, which includes natively folded apolipoprotein B (apoB), decreased stability of the MTTP(N)-Thr128-LDL complex was observed compared with that of the MTTP(N)-Ile128-LDL complex. In a refined model of the N-terminal domain of MTTP, residue 128 is located in a surface-exposed position, in the same region as an identified MTTP binding site in the homologous apoB protein. Thus, the Ile128Thr polymorphism confers reduced structural stability, leading to decreased binding of MTTP to LDL particles. Because the major MTTP binding target on LDL is apoB, the Ile128Thr polymorphism could target the MTTP-apoB interaction.
Highlights
The microsomal triglyceride transfer protein (MTTP) is essential for the assembly of VLDLs
No physiological polymorphisms in the expressed MTTP protein have yet been functionally analyzed, several amino acid substitutions that are associated with changes in the structure and/or function of proteins involved in lipoprotein metabolism have been reported
We aimed to evaluate whether the Ile128Thr polymorphism might have any impact on the structure and/or function on the N-terminal domain of MTTP
Summary
The microsomal triglyceride transfer protein (MTTP) is essential for the assembly of VLDLs. We recently observed that a polymorphism in the MTTP promoter (2493G.T), which is in allelic association with an isoleucine-to-theronine substitution at position 128 (Ile128Thr) in the expressed protein, confers an increased risk of coronary heart disease. The Ile128Thr polymorphism confers reduced structural stability, leading to decreased binding of MTTP to LDL particles. The MTTP subunit confers lipid transfer activity, whereas protein disulfide isomerase holds the complex in a stable, active conformation [4]. A third example is the alanine-threonine substitution at position 176 in the lipoprotein lipase protein Individuals who have this genetic variant show abnormal heparin binding and loss of enzymatic activity [14]
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