NICKEL IS A SPECIFIC INHIBITOR FOR THE BINDING OF ACTIVATED ALPHA(2)-MACROGLOBULIN TO THE LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN ALPHA(2)-MACROGLOBULIN RECEPTOR

The low density receptor-related protein/alpha(2)-macroglobulin recept or (LRP/alpha(2)-MR) binds to several ligands involved in lipoprotein and protease clearance. The receptor-associated protein (RAP) inhibits the binding of all known ligands. We studied the inhibition by Ni2+ o f the binding of different ligands to cells and to the purified LRP/al pha(2)-MR. Ni2+ inhibited all of the specific binding of radiolabeled methylamine-activated alpha(2)-macroglobulin (I-125-alpha(2)-M) to ra bbit aortic smooth muscle cells (SMC), rat hepatoma Fu5AH, and mouse f ibroblast L cells. Ni2+ also inhibited the binding of trypsin-activate d alpha(2)-macroglobulin to SMC but did not affect the binding of RAP, Pseudomonas exotoxin A, or low density lipoproteins. The inhibition o f alpha(2)-M binding by Ni2+ was not due to its interaction with alph a 2-M. Preincubation of SMC with Ni2+ followed by ligand binding sugg ested that Ni2+ binds to cell-surface molecules and inhibits the bindi ng of alpha(2)-M but does not affect RAP binding. Most of the binding of alpha(2)-M to SMC was due to its binding to the LRP/alpha(2)-MR, as opposed to the recently described signaling receptor, as demonstrat ed by the inhibition of this binding by the RAP. Moreover, the inhibit ion of alpha(2)-M binding to the LRP/alpha(2)-MR by Ni2+ was demonstr ated using purified receptor immobilized on microtiter plates. Two to three molecules of Ni-63(2+) bound to the immobilized receptor with eq ual affinity but not to alpha(2)-M. The specific binding of alpha(2)- M to the immobilized receptor was inhibited in the presence of nickel . Furthermore, preincubation of the immobilized LRP/alpha(2)-MR with N i2+ inhibited the binding of alpha(2)-M but did not inhibit RAP or ex otoxin A binding. These data suggest that Ni2+ is a site-specific inhi bitor for the alpha 2-M binding site present on the LRP/alpha(2)-MR. Nickel may be a useful tool for investigating different ligand binding domains of the LRP/alpha(2)-MR.