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Volume 270, Number 2, Issue of January 13, 1995 pp. 536-540
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
The 39-kDa Receptor-associated Protein Modulates Lipoprotein Catabolism by Binding to LDL Receptors

(Received for publication, June 7, 1994; and in revised form, September 26, 1994)

Jheem D. Medh Glenna L. Fry Susan L. Bowen Marc W. Pladet Dudley K. Strickland David A. Chappell

The 39-kDa receptor-associated protein (RAP) is co-synthesized and co-purifies with the low density lipoprotein receptor-related protein (LRP)/alpha(2)-macroglobulin receptor and is thought to modulate ligand binding to LRP. In addition to binding LRP, RAP binds two other members of the low density lipoprotein (LDL) receptor family, gp330 and very low density lipoprotein (VLDL) receptors. Here, we show that RAP binds to LDL receptors as well. In normal human foreskin fibroblasts, RAP inhibited LDL receptor-mediated binding and catabolism of LDL and VLDL with S 20-60 or 100-400. RAP inhibited I-labeled LDL and S100-400 lipoprotein binding at 4 °C with Kvalues of 60 and 45 nM, respectively. The effective concentrations for 50% inhibition (EC) of cellular degradation of 2.0 nMI-labeled LDL, 4.7 nMI-labeled S 20-60, and 3.6 nMI-labeled S 100-400 particles were 40, 70, and 51 nM, respectively. Treatment of cells with lovastatin to induce LDL receptors increased cellular binding, internalization, and degradation of RAP by 2.3-, 1.7-, and 2.6-fold, respectively. In solid-phase assays, RAP bound to partially purified LDL receptors in a dose-dependent manner. The dissociation constant (K) of RAP binding to LDL receptors in the solid-phase assay was 250 nM, which is higher than that for LRP, gp330, or VLDL receptors in similar assays by a factor of 14 to 350. Also, RAP inhibited I-labeled LDL and S100-400 VLDL binding to LDL receptors in solid-phase assays with K values of 140 and 130 nM, respectively. Because LDL bind via apolipoprotein (apo) B100 whereas VLDL bind via apoE, our results show that RAP inhibits LDL receptor interactions with both apoB100 and apoE. These studies establish that RAP is capable of binding to LDL receptors and modulating cellular catabolism of LDL and VLDL by this pathway.




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