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

Abstract

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.