Truncated apo B-70.5-containing lipoproteins bind to megalin but not the LDL receptor

Apo B-100 of LDL can bind to both the LDL receptor and megalin, but the mol ecular interactions of apo B-100 with these 2 receptors are not completely understood Naturally occurring mutant forms of apo B may be a source of val uable information on these interactions. Apo B-70.5 is uniquely useful beca use it contains the NH2-terminal portion of apo B-100, that includes only o ne of the two putative LDL receptor-binding sites (site A). The lipoprotein containing apo B-70.5 (Lp B-70.5) was purified from apo B100/apo B-70.5 he terozygotes by sequential ultracentrifugation combined with immunoaffinity chromatography. Cell culture experiments, Ligand blot analysis, and in vivo studies all consistently showed that Lp B-70.5 is not recognized by the LD L receptor. The kidney was identified as a major organ in catabolism of Lp B-70.5 in New Zealand white rabbits. Autoradiographic analysis revealed tha t renal proximal tubular cells selectively removed Lp B-70.5. On ligand blo tting: of renal cortical membranes, Lp B-70.5 bound only to megalin. The ab ility of megalin to mediate cellular endocytosis of Lp B-70.5 was confirmed using retinoic acid/dibutyryl cAMP-treated F9 cells. This study suggests t hat the putative LDL receptor-binding site A on apo B-100 might not by itse lf be a functional binding domain and that the apo B-binding sites recogniz ed by the LDL receptor and by megalin may be different. Moreover, megalin m ay play an important role in renal catabolism of apo B truncations, includi ng apo B-70.5.