RAS SF4 controls SOCE and ER-PM junctions through regulation of PI(4,5)P₂

RAS association domain family 4 (RAS SF4) is involved in tumorigenesis and regulation of the Hippo pathway. In this study, we identify new functional roles of RAS SF4. First, we discovered that RAS SF4 regulates store-operated Ca²⁺ entry (SOCE), a fundamental Ca²⁺ signaling mechanism, by affecting the translocation of the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) to ER-plasma membrane (PM) junctions. It was further revealed that RAS SF4 regulates the formation of ER-PM junctions and the ER-PM tethering function of extended synaptotagmins E-Syt2 and E-Syt3. Moreover, steady-state PM phosphatidylinositol 4,5-bisphosphate (PI[4,5]P₂) levels, important for localization of STIM1 and E-Syts at ER-PM junctions, were reduced in RAS SF4-knockdown cells. Furthermore, we demonstrated that RAS SF4 interacts with and regulates the activity of adenosine diphosphate ribosylation factor 6 (ARF6), a small G protein and upstream regulator of type I phosphatidylinositol phosphate kinases (PIP5Ks) and PM PI(4,5)P₂ levels. Overall, our study suggests that RAS SF4 controls SOCE and ER-PM junctions through ARF6-dependent regulation of PM PI(4,5)P₂ levels, pivotal for a variety of physiological processes.