Stim1 Binds to and Inhibits CaV1.2 Voltage Gated Calcium Channels

Abstract

CaV1.2 and other L-type voltage gated calcium channels play a key role in regulating cardiac contraction, synaptic plasticity, insulin secretion and a variety of other cellular events. Phospho-inositide linked receptors like the muscarinic acetylcholine receptor, inhibit L-type calcium channels and this inhibition is important for parasympathetic regulation of heart contraction as well as for learning and memory in the brain. The mechanisms by which PLC coupled receptors inhibit L-type channels are still controversial though several hypotheses including reduction of cAMP and depletion of PIP2 from the cell membrane have been proposed. We report a new and unexpected mechanism by which PLC-coupled receptors inhibit L-type calcium channels in cells. We have found that depletion of ER calcium stores either down stream of muscarinic receptors or following application of the ER calcium ATPase inhibitor, thapsigargin, inhibits CaV1.2 channels. CaV1.2 inhibition depends on binding to Stim1, an ER calcium sensor protein that activates the Orai family of store operated calcium channels. In cells expressing CaV1.2, Stim1 translocates to ER-plasma membrane junctions and co-localizes with clusters of CaV1.2. In vitro and in vivo studies indicate that the CAD domain of Stim1 binds to a coiled coil in the II-III loop of CaV1.2. Stim1 lacking the CAD domain is unable to bind to CaV1.2 and fails to inhibit CaV1.2 currents following depletion of ER calcium stores. These studies support a new mechanism by which phosphoinositide-linked receptors inhibit L-type calcium channels and suggest that Stim1 dynamically regulates the relative contributions of Orai and CaV1.2 channels to calcium influx in excitable cells.