Determination of Protein copy number, slow feedback loops, and protein-protein interaction in the store operated calcium signalling network

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Calcium signaling plays a fundamental role in cellular processes that range from muscle contraction and memory formation to triggering activation and differentiation of immune cells. Recent studies in Tobias Meyer’s laboratory at Stanford University have identified STIM as the Ca2+ sensor that monitors endoplasmic reticulum Ca2+ levels and controls calcium influx at ER-plasma membrane junction sites. The discovery of STIM and subsequent identification of the Ca2+ influx channel it activates completed previous molecular models for receptor-triggered Ca2+signaling. However, there are two major shortfalls of the models. 1) The absolute concentrations of the components are not known, and 2) the models are not sufficiently robust, failing to explain how cells can retain signaling capabilities even when significant perturbations are applied. My first specific aim is to use a selective reaction monitoring (SRM) mass spectrometry approach to improve the calcium signaling models. I will quantitate protein copy number of the key components in the calcium signaling network. Then I will carry out system perturbations to understand how feedback loops alter the expression of the signal transduction components in a manner that stabilizes basal Ca2+ signals and allows receptor-triggered Ca2+ signaling to be robustly induced. My second specific aim is to combine SRM mass spectrometry with a cross-linking approach to address another fundamental problem in calcium signaling network – identifying which proteins interact with STIM when it translocates to endoplasmic reticulum (ER)-plasma membrane junctions upon depletion of calcium in the ER. Together, these results will help to discover novel regulatory principles that will significantly improve the modeling and reconstruction of the signaling events controlling calcium signaling in cells.