BPS2026 – Structure and mechanism of the human mitochondrial MRS2 channel

The mitochondrial RNA splicing 2 protein (MRS2) is essential for mitochondrial ion homeostasis, and its dysfunction has been linked to impaired oxidative phosphorylation and cell death. It belongs to the CorA protein family, which includes Thermotoga maritima CorA (TmCorA), a well-characterized Mg 2+ -selective channel. Currently, the molecular mechanism of MRS2 has remained elusive despite its importance in cellular physiology. Here, using cryo-electron microscopy and electrophysiology, we show that human MRS2 (hMRS2) assembles as a pentameric channel with a central pore containing an Arg ring (R332) that is absent in CorA. This Arg ring is anticipated to create an energy barrier for ion permeation. Indeed, substituting R332 with serine greatly enhanced channel activity and enabled the first direct recordings of MRS2 currents. Cell-based assays further showed that R332 functions to limit ion flow via MRS2 to prevent MRS2 from depolarizing the mitochondrial inner membrane and eliminating the driving force for ATP synthesis. With the more conductive R332S mutant, we were able to systematically characterize hMRS2 and found it to be a non-selective cation channel permeable to Mg 2+ , Ca 2+ , Na + , and K + , in contrast to the Mg 2+ -selective TmCorA. The hMRS2 also underwent rapid Ca 2+ -dependent inactivation, whereas TmCorA is inactivated by Mg 2+ , suggesting functional adaptation to calcium signaling in higher eukaryotes. To gain mechanistic insight into the evolution of hMRS2’s regulation, inactivation, and conductivity properties, we characterized the fungal homolog Cryptococcus neoformans MRS2 (CnMRS2), showing it is an Mg 2+ -selective channel. Chimeric analysis revealed that differences between CnMRS2 and hMRS2 in transmembrane helix 2 underlie their differences in ion selectivity. These findings provide novel mechanistic insights into MRS2, creating a conceptual foundation for targeting this channel for pathological conditions linked to abnormal mitochondrial cation homeostasis.