Ca-regulated Ca 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 Ca spiking. Here we show that CASTOR from Lotus japonicus is a highly selective Ca channel whose activation requires cytosolic/nucleosolic Ca, 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, Ca-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 Ca sites. Our in vitro and in vivo assays also demonstrate the importance of these gating-ring Ca binding sites to the physiological function of CASTOR as well as DMI1.