Mitochondrial Ca2+ uptake, a process crucial for bioenergetics and Ca2+ signaling, is catalyzed by the mitochondrial calcium uniporter. The uniporter is a multi-subunit Ca2+-activated Ca2+ channel, with the Ca2+ pore formed by the MCU protein and Ca2+-dependent activation mediated by MICU subunits. Recently, a mitochondrial inner membrane protein EMRE was identified as a uniporter subunit absolutely required for Ca2+ permeation. However, the molecular mechanism and regulatory purpose of EMRE remain largely unexplored. Here, we determine the transmembrane orientation of EMRE, and show that its known MCU-activating function is mediated by the interaction of transmembrane helices from both proteins. We also reveal a second function of EMRE: to maintain tight MICU regulation of the MCU pore, a role that requires EMRE to bind MICU1 using its conserved C-terminal polyaspartate tail. This dual functionality of EMRE ensures that all transport-competent uniporters are tightly regulated, responding appropriately to a dynamic intracellular Ca2+ landscape.DOI: http://dx.doi.org/10.7554/eLife.15545.001
ORCID IDs: 0000-0003-0202-3890 (C.L.); 0000-0002-9943-2975 (J.-M.Z.) Stomata are natural openings through which many pathogenic bacteria enter plants. Successful bacterial pathogens have evolved various virulence factors to promote stomatal opening. Here, we show that the Pseudomonas syringae type III effector protein AvrB induces stomatal opening and enhances bacterial virulence in a manner dependent on RPM1-INTERACTING4 (RIN4), which promotes stomatal opening by positively regulating the Arabidopsis plasma membrane H + -ATPase (AHA1), which is presumed to directly regulate guard cell turgor pressure. In support of a role of AHA1 in AvrBinduced stomatal opening, AvrB enhances ATPase activity in plants. Unexpectedly, AHA1 promotes the interaction between the jasmonate (JA) receptor CORONATINE INSENSITIVE1 (COI1) and JASMONATE ZIM-DOMAIN (JAZ) proteins and enhances JA signaling. JA signaling is required for optimum stomatal infection in AHA1-active plants. Similarly, AvrB also induces the COI1-JAZ9 interaction and the degradation of multiple JAZ proteins. AvrB-induced stomatal opening and virulence require the canonical JA signaling pathway, which involves the COI1 and NAC transcription factors. The findings thus point to a previously unknown pathway exploited by P. syringae that acts upstream of COI1 to regulate JA signaling and stomatal opening.
The mitochondrial calcium uniporter is a Ca-activated Ca channel complex mediating mitochondrial Ca uptake, a process crucial for Ca signaling, bioenergetics, and cell death. The uniporter is composed of the pore-forming MCU protein, the gatekeeping MICU1 and MICU2 subunits, and EMRE, a single-pass membrane protein that links MCU and MICU1 together. As a bridging subunit required for channel function, EMRE could paradoxically inhibit uniporter complex formation if expressed in excess. Here, we show that mitochondrial mAAA proteases AFG3L2 and SPG7 rapidly degrade unassembled EMRE using the energy of ATP hydrolysis. Once EMRE is incorporated into the complex, its turnover is inhibited >15-fold. Protease-resistant EMRE mutants produce uniporter subcomplexes that induce constitutive Ca leakage into mitochondria, a condition linked to debilitating neuromuscular disorders in humans. The results highlight the dynamic nature of uniporter subunit assembly, which must be tightly regulated to ensure proper mitochondrial responses to intracellular Ca signals.
Saline-alkali soil is a major environmental constraint impairing plant growth and crop productivity. In this study, we identified a Ca 21 sensor/kinase/plasma membrane (PM) H 1 -ATPase module as a central component conferring alkali tolerance in Arabidopsis (Arabidopsis thaliana). We report that the SCaBP3 (SOS3-LIKE CALCIUM BINDING PROTEIN3)/CBL7 (CALCINEURIN B-LIKE7) loss-of-function plants exhibit enhanced stress tolerance associated with increased PM H 1 -ATPase activity and provide fundamental mechanistic insights into the regulation of PM H 1 -ATPase activity. Consistent with the genetic evidence, interaction analyses, in vivo reconstitution experiments, and determination of H 1 -ATPase activity indicate that interaction of the Ca 21 sensor SCaBP3 with the C-terminal Region I domain of the PM H 1 -ATPase AHA2 (Arabidopsis thaliana PLASMA MEMBRANE PROTON ATPASE2) facilitates the intramolecular interaction of the AHA2 C terminus with the Central loop region of the PM H 1 -ATPase to promote autoinhibition of H 1 -ATPase activity. Concurrently, direct interaction of SCaPB3 with the kinase PKS5 (PROTEIN KINASE SOS2-LIKE5) stabilizes the kinase-ATPase interaction and thereby fosters the inhibitory phosphorylation of AHA2 by PKS5. Consistently, yeast reconstitution experiments and genetic analysis indicate that SCaBP3 provides a bifurcated pathway for coordinating intramolecular and intermolecular inhibition of PM H 1 -ATPase. We propose that alkaline stress-triggered Ca 21 signals induce SCaBP3 dissociation from AHA2 to enhance PM H 1 -ATPase activity. This work illustrates a versatile signaling module that enables the stress-responsive adjustment of plasma membrane proton fluxes.
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