Background: Ferritin stores iron by ferroxidation to form a mineral core, enabling diatom blooms upon iron input.Results: Ferrous iron binds solely to ferroxidase site A anaerobically. Ferroxidation kinetics has two observed phases.Conclusion: Ferrous iron and dioxygen binding to the di-iron ferroxidase site is stepwise.Significance: Iron storage by ferritins requires a coordinated binding of iron and dioxygen.

The transmembrane docking of endoplasmic reticulum (ER) Ca-sensing STIM proteins with plasma membrane (PM) Orai Ca channels is a critical but poorly understood step in Ca signal generation. STIM1 protein dimers unfold to expose a discrete STIM-Orai activating region (SOAR1) that tethers and activates Orai1 channels within discrete ER-PM junctions. We reveal that each monomer within the SOAR dimer interacts independently with single Orai1 subunits to mediate cross-linking between Orai1 channels. Superresolution imaging and mobility measured by fluorescence recovery after photobleaching reveal that SOAR dimer cross-linking leads to substantial Orai1 channel clustering, resulting in increased efficacy and cooperativity of Orai1 channel function. A concatenated SOAR1 heterodimer containing one monomer point mutated at its critical Orai1 binding residue (F394H), although fully activating Orai channels, is completely defective in cross-linking Orai1 channels. Importantly, the naturally occurring STIM2 variant, STIM2.1, has an eight-amino acid insert in its SOAR unit that renders it functionally identical to the F394H mutant in SOAR1. Contrary to earlier predictions, the SOAR1-SOAR2.1 heterodimer fully activates Orai1 channels but prevents cross-linking and clustering of channels. Interestingly, combined expression of full-length STIM1 with STIM2.1 in a 5:1 ratio causes suppression of sustained agonist-induced Ca oscillations and protects cells from Ca overload, resulting from high agonist-induced Ca release. Thus, STIM2.1 exerts a powerful regulatory effect on signal generation likely through preventing Orai1 channel cross-linking. Overall, STIM-mediated cross-linking of Orai1 channels is a hitherto unrecognized functional paradigm that likely provides an organizational microenvironment within ER-PM junctions with important functional impact on Ca signal generation.

Background: Iron storage by ferritin enables diatom bloom upon iron input.Results: Ferroxidase center variants show faster iron mineralization and rate of post-oxidation reorganization of iron.Conclusion: Glu-130 and Glu-44 regulate the flux of iron through the ferroxidase center.Significance: Optimization of ferritin for iron oxidation but not mineralization suggests an iron buffering function in addition to long-term iron storage.