Lysine and Arginine Residues in the N-Terminal 18 of Apolipoprotein B Are Critical for Its Binding to Microsomal Triglyceride Transfer Protein

Abstract

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly and secretion of triglyceride-rich lipoproteins. We have presented evidence for a high-affinity interaction between these proteins [Hussain, M. M., et al. (1997) Biochemistry 36, 13060−13067]. In this study, we used chemically modified low-density lipoproteins (LDL) and recombinant human apoB18 to identify amino acid residues in apoB that are critical for its interactions with MTP. Acetoacetylation of 74% of lysine residues and cyclohexanedione modification of 54% of arginine residues completely abolished the interactions between LDL and MTP. Regeneration of lysine and arginine residues by hydroxylamine treatment completely restored the binding of modified LDL to MTP. Carboxyethylation of all the histidine residues decreased, but did not abolish, apoB−MTP interactions. In contrast, glycine methyl ester modifications of aspartic and glutamic acid residues, up to 38−44%, had no effect on LDL−MTP interactions. Furthermore, modification of lysine and arginine, but not the aspartic and glutamic acid, residues in apoB18 also completely abolished its interactions with MTP. These studies indicated that lysine and arginine, but not aspartic and glutamic acid, residues are critical for apoB−MTP interactions, whereas histidine residues are not as critical. Since lysine and arginine residues in apoB are known to interact with the LDL receptors and heparin, we studied the effect of different glycosaminoglycans on apoB−MTP interactions. Glycosaminoglycans had no significant inhibitory effect on apoB−MTP interactions, suggesting that the lysine and arginine residues crucial for apoB−MTP interactions are different from those that interact with the LDL receptor and heparin. The lysine and arginine residues in apoB18 may directly interact with negatively charged residues in the MTP molecule, or they may function to maintain the conformation of the recognition site.