Evidence for a second alpha 2-macroglobulin receptor.

alpha 2-Macroglobulin (alpha 2M)-methylamine binds to purified low density lipoprotein receptor-related protein (LRP), and it is assumed that LRP functions as the alpha 2M receptor in vivo. Binding of alpha 2M-methylamine to macrophage receptors elevates intracellular calcium ([Ca2+]i), inositol phosphates, and cyclic AMP. We have employed human alpha 2M-methylamine and recombinant receptor binding fragment (RBF) to study transduction mechanisms. Macrophages exposed to either ligand demonstrated a rapid rise in [Ca2+]i. Since the 39-kDa LRP/alpha 2M receptor-associated protein (RAP) blocks alpha 2M binding to LRP, we explored the effects of RAP upon signaling. Pretreatment of macrophages with RAP did not block the increase in [Ca2+]i elicited by alpha 2M-methylamine or RBF, suggesting a distinct binding site. RBF also elicited a transient 1.5-2.0-fold increase in inositol 1,4,5-triphosphate. In permeabilized macrophages, GTP gamma S and Gp-p(NH)p potentiated and sustained this inositol 1,4,5-triphosphate increase. Preincubation of permeabilized macrophages with GDP beta S abrogated the effects of GTP gamma S. Our results suggest that the signaling alpha 2M receptor is coupled to a pertussis toxin-insensitive G protein and possibly to a cholera toxin-sensitive G protein. We conclude that macrophages contain a second alpha 2M receptor that is G protein-coupled.