STIM1 Is a MT-Plus-End-Tracking Protein Involved in Remodeling of the ER

Stromal interaction molecule 1 (STIM1) is a transmembrane protein that is essential for store-operated Ca²⁺ entry, a process of extracellular Ca²⁺ influx in response to the depletion of Ca²⁺ stores in the endoplasmic reticulum (ER) (reviewed in [1-4]). STIM1 localizes predominantly to the ER; upon Ca²⁺ release from the ER, STIM1 translocates to the ER-plasma membrane junctions and activates Ca²⁺ channels (reviewed in [1-4]). Here, we show that STIM1 directly binds to the microtubule-plus-end-tracking protein EB1 and forms EB1-dependent comet-like accumulations at the sites where polymerizing microtubule ends come in contact with the ER network. Therefore, the previously observed tubulovesicular motility of GFP-STIM1 [5] is not a motor-based movement but a traveling wave of diffusion-dependent STIM1 concentration in the ER membrane. STIM1 overexpression strongly stimulates ER extension occurring through the microtubule "tip attachment complex" (TAC) mechanism [6, 7], a process whereby an ER tubule attaches to and elongates together with the EB1-positive end of a growing microtubule. Depletion of STIM1 and EB1 decreases TAC-dependent ER protrusion, indicating that microtubule growth-dependent concentration of STIM1 in the ER membrane plays a role in ER remodeling.