Role of the microtubule cytoskeleton in the function of the store-operated Ca²⁺ channel activator STIM1

We examined the role of the microtubule cytoskeleton in the localization and store-operated Ca²⁺ entry (SOCE) function of the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) in HEK 293 cells. STIM1 tagged with an enhanced yellow fluorescent protein (EYFP-STIM1) exhibited a fibrillar localization that colocalized with endogenous α-tubulin. Depolymerization of microtubules with nocodazole caused a change from a fibrillar EYFP-STIM1 localization to one that was similar to that of the ER. Treatment of HEK 293 cells with nocodazole had a detrimental impact on SOCE and the associated Ca²⁺ release-activated Ca²⁺ current (I_CRAC). This inhibition was significantly reversed in cells overexpressing EYFP-STIM1, implying that the primary inhibitory effect of nocodazole is related to STIM1 function. Surprisingly, nocodazole treatment alone induced significant SOCE and I_CRAC in cells expressing EYFP-STIM1, and this was accompanied by an increase in EYFP-STIM1 fluorescence near the plasma membrane. We conclude that microtubules play a facilitative role in the SOCE signaling pathway by optimizing the localization of STIM1.