Lipid transporters E-Syt3 and ORP5 regulate epithelial ion transport by controlling phosphatidylserine enrichment at ER/PM junctions

Endoplasmic reticulum/plasma membrane (ER/PM) junctions are a major site of cellular signal transduction including in epithelia; however, whether their lipid membrane environment affects junctional ion transporters function remains unclear. Here, we show that epithelial secretion is governed by phosphatidylserine (PtdSer) levels in ER/PM nanodomains, specified by the antagonistic action of the lipid transfer proteins E-Syt3 and ORP5, which transduce cAMP signals to the chloride channel CFTR and activate the sodium-bicarbonate cotransporter NBCe1-B by IRBIT. Lipid transfer by E-Syt3, along with restricted plasma membrane localization by the E-Syt3 C2C domain, are essential for E-Syt3 function, as removal of PtdSer from junctions by E-Syt3 dissociated the cAMP signaling pathway complex, preventing CFTR activation, and prevented NBCe1-B activation by IRBIT. CFTR and NBCe1-B PtdSer sensor domains responded to PtdSer reduction by E-Syt3; which was reversed by exogenous PtdSer or by PtdSer supplied by ORP5. In mice, E-Syt3 depletion improved chloride flux and fluid secretion in salivary glands and isolated pancreatic ducts. These findings provide a framework for understanding the role of junctional lipids in the assembly of functional ion protein complexes and cellular communication at epithelial signaling hubs.