APOLIPOPROTEIN-B BINDING TO MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN DECREASES WITH INCREASES IN LENGTH AND LIPIDATION - IMPLICATIONS IN LIPOPROTEIN BIOSYNTHESIS

Microsomal triglyceride transfer protein (MTP), a heterodimer of 97 kD a and protein disulfide isomerase, is required for the assembly of apo lipoprotein B (apoB)-containing triglyceride-rich lipoproteins. These proteins have been shown to interact with each other during early stag es of lipoprotein biosynthesis. Our studies indicated that binding bet ween apoB and heterodimeric MTP was of high affinity (K-d 10-30 nM) du e to ionic interactions. In contrast to MTP, protein disulfide isomera se alone interacted very poorly with lipoproteins, indicating the impo rtance of the heterodimer in these bindings. Preincubation of lipoprot eins with detergents enhanced their interaction with MTP. Native VLDL bound poorly to MTP, but its preincubation with Tween-20 resulted in s ignificantly increased binding to MTP. Furthermore, binding of LDL was enhanced by preincubation with taurocholate, indicating that partial delipidation of apoB-containing lipoproteins results in increased bind ing to MTP. Subsequently, attempts were made to study interactions bet ween C-terminally truncated apoB polypeptides and MTP. Binding of all the polypeptides to MTP was enhanced in the presence of taurocholate. Comparisons revealed that the binding of different apoB polypeptides t o MTP was in the order of apoB18 > apoB28 > apoB42 > apoB100. These st udies indicated that optimum interactions occur between apoB18 and MTP , and that the increase in apoB length beyond apoB18 has a negative ef fect on these interactions. Since apoB18 does not assemble triglycerid e-rich lipoproteins, these studies suggest that apoB may interact with MTP before its lipidation. It is proposed that steps in lipoprotein b iosynthesis may be dictated by the sequential display of different fun ctional domains on the apoB polypeptide.