The low density receptor-related protein/alpha 2-macroglobulin receptor (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+ of the binding of different ligands to cells and to the purified LRP/alpha 2-MR. Ni2+ inhibited all of the specific binding of radiolabeled methylamine-activated alpha 2-macroglobulin (125I-alpha 2-M*) to rabbit aortic smooth muscle cells (SMC), rat hepatoma Fu5AH, and mouse fibroblast L cells. Ni2+ also inhibited the binding of trypsin-activated alpha 2-macroglobulin to SMC but did not affect the binding of RAP, Pseudomonas exotoxin A, or low-density lipoproteins. The inhibition of alpha 2-M* binding by Ni2+ was not due to its interaction with alpha 2-M*. Preincubation of SMC with Ni2+ followed by ligand binding suggested that Ni2+ binds to cell-surface molecules and inhibits the binding 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 demonstrated by the inhibition of this binding by the RAP. Moreover, the inhibition of alpha 2-M* binding to the LRP/alpha 2-MR by Ni2+ was demonstrated using purified receptor immobilized on microtiter plates. Two to three molecules of 63Ni2+ bound to the immobilized receptor with equal 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.(ABSTRACT TRUNCATED AT 250 WORDS)