The regulation of STIM1 translocation to the plasma membrane

A rise in intracellular Ca2+ concentration is key to controlling both short term and long term Ca2+ dependent processes which include secretion, metabolism and gene expression, cell growth and proliferation. Store operated Ca2+ channels (SOCs), which are activated by the depletion of Ca2+ from internal Ca2+ stores, the main store being the endoplasmic reticulum (ER), are the major route for Ca2+ influx in non-excitable cell types. Stromal interacting molecule 1 (STIM1) is a Ca2+ sensing protein located in the endoplasmic reticulum (ER). Depletion of ER calcium stores triggers oligomerisation and subsequent translocation of STIM1 from its reticular location to specialized endoplasmic reticulum-plasma membrane (ER-PM) junctions where it forms STIM1 puncta and interacts with the SOC channel, Orai1. This induces the clustering of Orai1 into a functional tetrameric pore which is permeable to Ca2+ ions, enabling Ca2+ entry into the cell. The precise mechanism by which STIM1 is recruited to the plasma membrane to activate SOCs and the plasma membrane components involved in targeting STIM1 to the plasma membrane are largely unknown. In this study the mechanisms underlying movement of STIM1 to the plasma membrane and its accumulation at ER-plasma membrane junctions was explored in HeLa cells. In the initial part of this study I investigated whether the movement of STIM1 to the plasma membrane is an ATP-dependent process. I found that depletion of cytosolic ATP can stimulate STIM1 puncta formation in HeLa cells and that the formation of STIM1-Orai1 complexes at the plasma membrane is unaffected in these conditions. Inhibition of ATP synthesis also initiated the loss of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) from the plasma membrane. ATP depletion did not affect the structure of the microtubule cytoskeleton. These results suggest that the translocation of STIM1 and the formation of STIM1-Orai1 complexes is an ATP independent process which is not due to the disruption of microtubules and support a diffusional model for STIM1 puncta formation. It has been suggested that an additional interaction of the C-terminal polybasic domain of STIM1 with plasma membrane phosphoinositides could contribute to STIM1 puncta formation prior to binding to Orai1. I investigated the role of phosphoinositides in the formation of STIM1 puncta and SOCE in response to store depletion. Treatment of HeLa cells with inhibitors of the phosphatidylinositol 3-kinase (PI3K) and phosphatidylinositol 4-kinase (wortmannin and LY294002) partially inhibited formation of STIM1 puncta. Additional rapid depletion of PtdIns(4,5)P2 resulted in more substantial inhibition of the translocation of STIM1-EYFP into puncta. The inhibition was extensive at a concentration of LY294002 (50 μM) that should primarily inhibit PI3K consistent with a major role for PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in puncta formation. Depletion of phosphoinositides also partially inhibited SOCE. Overexpression of Orai1 resulted in a recovery of translocation of STMI1 into puncta following phosphoinositide depletion and under these conditions SOCE was increased to above control levels. These observations support the idea that phosphoinositides are not essential but contribute to STIM1 accumulation at ER-PM junctions with a second translocation mechanism involving direct STIM1/Orai1 interactions. It was recently reported that STIM1 and Orai1 may function within a macromolecular complex involving other unidentified proteins. In this study I have identified that Golli-BG21, a member of the myelin basic protein (MBP) family, can directly interact with STIM1. Golli interacts with the C-terminal domain of STIM1 in both in vitro and in vivo binding assays and this interaction may be modulated by intracellular Ca2+ concentration. Golli also colocalises with full length STIM1 and Orai1 complexes in HeLa cells following store depletion. Overexpression of Golli reduces SOCE in HeLa cells but this inhibition is overcome by overexpressing STIM1. We therefore suggest that Golli binds to STIM1-Orai1 complexes to negatively regulate the activity of SOCs.