LYSINE AND ARGININE RESIDUES IN THE N-TERMINAL 18-PERCENT OF APOLIPOPROTEIN-B ARE CRITICAL FOR ITS BINDING TO MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein ( MTP) are essential for the efficient assembly and secretion of triglyc eride-rich lipoproteins. We have presented evidence for a high-affinit y interaction between these proteins [Hussain, M. M., et al. (1997) Bi ochemistry 36, 13060-13067]. In this study, we used chemically modifie d low-density lipoproteins (LDL) and recombinant human apoB18 to ident ify amino acid residues in apoB that are critical for its interactions with MTP. Acetoacetylation of 74% of lysine residues and cyclohexaned ione 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 o f modified LDL to MTP. Carboxyethylation of all the histidine residues decreased, but did not abolish, apoB-MTP interactions. In contrast, g lycine methyl ester modifications of aspartic and glutamic acid residu es, 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 w ith MTP. These studies indicated that lysine and arginine, but not asp artic and glutamic acid, residues are critical for apoB-MTP interactio ns, whereas histidine residues are not as critical. Since lysine and a rginine 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 inhibitor y effect on apoB-MTP interactions, suggesting that the lysine and argi nine residues crucial for apoB-MTP interactions are different from tho se that interact with the LDL receptor and heparin. The lysine and arg inine residues in apoB18 may directly interact with negatively charged residues in the MTP molecule, or they may function to maintain the co nformation of the recognition site.