Ca²⁺-regulated Ca²⁺ channels with an RCK gating ring control plant symbiotic associations

A family of plant nuclear ion channels, including DMI1 (Does not Make Infections 1) and its homologs CASTOR and POLLUX, are required for the establishment of legume-microbe symbioses by generating nuclear and perinuclear Ca²⁺ spiking. Here we show that CASTOR from Lotus japonicus is a highly selective Ca²⁺ channel whose activation requires cytosolic/nucleosolic Ca²⁺, contrary to the previous suggestion of it being a K⁺ channel. Structurally, the cytosolic/nucleosolic ligand-binding soluble region of CASTOR contains two tandem RCK (Regulator of Conductance for K⁺) domains, and four subunits assemble into the gating ring architecture, similar to that of large conductance, Ca²⁺-gated K⁺ (BK) channels despite the lack of sequence similarity. Multiple ion binding sites are clustered at two locations within each subunit, and three of them are identified to be Ca²⁺ sites. Our in vitro and in vivo assays also demonstrate the importance of these gating-ring Ca²⁺ binding sites to the physiological function of CASTOR as well as DMI1.