Mitochondria supply ATP to the ER through a mechanism antagonized by cytosolic Ca2+

The endoplasmic reticulum (ER) imports ATP and uses energy from ATP hydrolysis for protein folding and trafficking. However, little is known about this vital ATP transport process across the ER membrane. Here, using three commonly used cell lines (CHO, INS1 and HeLa), we report that ATP enters the ER lumen through a cytosolic Ca2+-antagonized mechanism, or CaATiER (Ca2+-Antagonized Transport into ER) mechanism for brevity. Significantly, we observed that a Ca2+ gradient across the ER membrane is necessary for ATP transport into the ER. Therefore Ca2+ signaling in the cytosol is inevitably coupled with ATP supply to the ER. We propose that under physiological conditions, cytosolic Ca2+ inhibits ATP import into the ER lumen to limit ER ATP consumption. Furthermore, the ATP level in the ER is readily depleted by oxidative phosphorylation (OxPhos) inhibitors, and that ER protein misfolding increases ATP trafficking from mitochondria into the ER. These findings suggest that ATP usage in the ER may increase mitochondrial OxPhos while decreasing glycolysis, i.e., an “anti-Warburg” effect. Significance Statement We report that ATP enters the ER lumen through an AXER-dependent, cytosolic Ca2+-antagonized mechanism, or CaATiER (Ca2+-Antagonized Transport into ER) mechanism. In addition, our findings suggest that ATP usage in the ER may render an “anti-Warburg” effect by increasing ATP regeneration from mitochondrial OxPhos while decreasing the portion of ATP regeneration from glycolysis.