ANALYSIS OF LIGAND-BINDING TO THE ALPHA(2)-MACROGLOBULIN RECEPTOR LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN - EVIDENCE THAT LIPOPROTEIN LIPASE AND THE CARBOXYL-TERMINAL DOMAIN OF THE RECEPTOR-ASSOCIATEDPROTEIN BIND TO THE SAME SITE

The endocytic alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha(2)MR/LRP) binds several classes of ext racellular ligands at independent sites. In addition, alpha(2)MR/LRP c an bind multiple copies of the 39-40-kDa receptor-associated protein ( RAP). Both amino-terminal and carboxyl-terminal fragments of RAP exhib it affinity, and the fragments apparently bind to different sites on t he receptor. RAP completely inhibits the binding of all presently know n extracellular ligands, whereas several ligands such as alpha(2)-macr oglobulin and tissue type plasminogen activator are poor inhibitors of RAP binding. Since RAP is largely an intracellular molecule that norm ally does not occupy alpha(2)MR/LRP at the cell surface, we hypothesiz ed that an established extracellular ligand might bind to those sites on the receptor capable of binding the RAP fragments. We found complet e cross competition between carboxyl-terminal RAP fragments and fragme nts of lipoprotein lipase containing the recently identified binding d omain for alpha(2)MR/LRP (Nykjaer, A., Nielsen, M., Lookene, A., Meyer , N., Roigaard, H., Etzerodt, M., Beisiegel, U., Olivecrona, G., and G liemann, J. (1994) J. Biol. Chem. 269, 31747-31755). Moreover, the lip oprotein lipase fragment completely inhibited the binding of several a lpha(2)MR/LRP ligands in a pattern similar to that of carboxyl-termina l RAP fragments. On the other hand, the amino-terminal RAP fragment wa s a poor competitor of binding of the lipoprotein lipase fragment, whe reas it competed effectively with pro-uPA for bind ing to the receptor . The results provide evidence that lipoprotein lipase binds to the si te on alpha(2)MR/LRP also available for binding of the carboxyl termin al domain of RAP and suggest that pro-uPA may bind to or overlap the s ite available for the amino-terminal domain of RAP.