Interaction of the Calcium-sensing Receptor and Filamin, a Potential Scaffolding Protein

In many cases, the biologic responses of cells to extracellular signals and the specificity of the responses cannot be explained solely on the basis of the interactions of known signaling proteins. Recently, scaffolding and adaptor proteins have been identified that organize signaling proteins in cells and that contribute to the nature and specificity of signaling pathways. In an effort to identify proteins that might organize the signaling system(s) activated by the extracellular Ca2+ receptor (CaR), we used a bait construct representing the intracellular C terminus of the human CaR and the yeast two hybrid system to screen a human kidney cDNA library. We identified a clone representing the C-terminal 1042 amino acids (aa) of the cytoskeletal protein filamin (ABP-280). Analysis of truncation and deletion constructs of the CaR C terminus and the filamin cDNA clone demonstrated that the CaR and filamin interact via regions containing aa 907–997 of the CaR C terminus and aa 1566–1875 of filamin. Interaction of the two proteins in mammalian HEK-293 cells was demonstrated by co-immunoprecipitation and colocalization of them using immunofluorescence microscopy. The functional importance of their interaction was documented by transiently expressing the CaR in M2 melanoma cells that lack filamin, or in A7 melanoma cells that stably express filamin, and demonstrating that the CaR activated ERK only in the presence of filamin. Co-expression of the CaR with a peptide derived from the region of the CaR C terminus that interacts with filamin reduced the ability of the CaR to activate p42ERK in a dose-dependent manner, but did not inhibit the ability of the ETA receptor to activate ERK. The fact that filamin interacts with the CaR and other cell signaling proteins including mitogen-activated protein kinases and small GTPases, indicates that it may act as a scaffolding protein to organize cell signaling systems involving the CaR.