Structural Basis of the Ca2+ Inhibitory Mechanism of Drosophila Na+/Ca2+ Exchanger CALX and Its Modification by Alternative Splicing

The Na+/Ca2+ exchanger CALX promotes Ca2+ efflux in Drosophila sensory neuronal cells to facilitate light-mediated Ca2+ homeostasis. CALX activity is negatively regulated by specific Ca2+ interaction within its two intracellular Ca2+ regulatory domains CBD1 and CBD2, yet how the Ca2+ binding is converted to molecular motion to operate the exchanger is unknown. Here, we report crystal structures of the entire Ca2+ regulatory domain CBD12 from two alternative splicing isoforms, CALX 1.1 and 1.2, exhibiting distinct regulatory Ca2+ dependency. The structures show an open V-shaped conformation with four Ca2+ ions bound on the CBD domain interface, confirmed by LRET Einalysis. The structures together with Ca2+-binding analysis support that the Ca2+ inhibition of CALK is achieved by interdomain conformational changes induced by Ca2+ binding at CBD1. The conformational difference between the two isoforms also indicates that alternative splicing adjusts the interclomain orientation angle to modify the Ca2+ regulatory property of the exchangers.