A previous study demonstrated that activated α2-macroglobulin (α2M*) binding to the low-density receptor-related protein/α2-macroglobulin receptor (LRP/α2MR) is blocked by Ni2+ [Hussain, M. M., et al. (1995) Biochemistry 34, 16074-16081]. We now report that the effect of Ni2+ is on a region of the α2M molecule upstream of the carboxyl terminal receptor recognition domain. This observation is consistent with previous observations from this laboratory suggesting that α2M* binding to LRP/α2MR involves a region of the α2M molecule immediately upstream of the receptor recognition domain [Enghild, J.J. et al. (1989) Biochemistry 28, 1406-1412]. We further demonstrate that Ni2+ has no effect on the binding of α2M* or a cloned and expressed receptor binding fragment (RBF) to the recently described α2M signaling receptor as assessed by direct binding and signal transduction studies.