The Facultative Role of Lipid Rafts and Membrane Microdomains in Controlling the Assembly of the Store-Operated Channel Complex

Abstract

Calcium influx is an important factor in a wide variety of cellular functions. The signal initiating calcium influx may originate from the activation of G-protein coupled receptors, or arise from inside the cell, via maneuvers that can lead to depletion of intracellular calcium storage compartments. While the role of members from the Transient Receptor Potential Cation (TRPC) family of channels as receptor-operated channels (ROC) is well established and supported by abundant studies, the role of this family of channels as store-operated channels (SOC) has been the focus of a heated controversy over the last few years. In the present study, we have explored the modulation of STIM1 on human TRPC1 channel. We have found that the association of STIM1 to TRPC1 favors the insertion of this channel into lipid rafts, where TRPC1 functions as a SOC in association to Orai channels forming a complex we have named SOCIC (store-operated calcium influx complex). In the absence of STIM1, TRPC1 associates to other members from the TRPC family of channels to form ROCs. A novel imaging method implemented for the present study based on multicolor LightGuide Total Internal Reflection Fluorescence Microscopy (LG-TIRFM), illustrates the relevance of the dynamic association between STIM1 and TRPC1 for the activation of SOC and the lipid raft localization of the STIM1-TRPC1-Orai complex. This study provides new evidence about the dual activity of TRPC1 (forming ROC or SOC) and the partners needed to determine TRPC1 functional fate. It highlights also the role of plasma membrane microdomains (lipid rafts) and ER-PM junctions in modulating TRPC1 channel function. Using LG-TIRFM, we identified new partners of the SOCIC, and studied the dynamics of SOCIC assembly and disassembly in real time.