RASSF4 controls SOCE and ER-PM junctions through regulation of PI(4,5)P₂.

RAS association domain family 4 (RASSF4) is involved in tumorigenesis and regulation of the Hippo pathway. In this study, we identify new functional roles of RASSF4. First, we discovered that RASSF4 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 RASSF4 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 RASSF4-knockdown cells. Furthermore, we demonstrated that RASSF4 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 RASSF4 controls SOCE and ER-PM junctions through ARF6-dependent regulation of PM PI(4,5)P₂ levels, pivotal for a variety of physiological processes. © 2017 Chen et al.

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