Receptor signaling at the plasma membrane often releases calcium from intracellular stores. For example, inositol triphosphate (IP3) produced by receptor-coupled phospholipase C activates an intracellular store calcium channel, the IP3R. Conversely, stores can induce extracellular calcium to enter the cell through plasma membrane channels, too. How this "reverse" coupling works was unclear, but store IP3Rs were proposed to bind and regulate plasma membrane TRP cation channels. Here, we demonstrate that the adaptor protein, termed Homer, facilitates a physical association between TRPC1 and the IP3R that is required for the TRP channel to respond to signals. The TRPC1-Homer-IP3R complex is dynamic and its disassembly parallels TRPC1 channel activation. Homer's action depends on its ability to crosslink and is blocked by the dominant-negative immediate early gene form, H1a. Since H1a is transcriptionally regulated by cellular activity, this mechanism can affect both short and long-term regulation of TRPC1 function.