Immunosuppressants implicate protein phosphatase regulation of K⁺ channels in guard cells

The elevation of Ca²⁺ levels in the cytoplasm inactivates inward-rectifying K⁺ channels that play a central role in regulating the apertures of stomatal pores in higher plants. However, the mechanism for the Ca²⁺-mediated inhibition of K⁺-channel function is unknown. Using patch-clamp techniques, we show that cyclophilin-cyclosporin A and FK506-binding protein-FK506 complexes, which are highly specific inhibitors of protein phosphatase 2B (calcineurin), block Ca²⁺-induced inactivation of K⁺ channels in Vicia faba guard cells. A constitutively active calcineurin fragment that is Ca²⁺-independent inhibits K⁺-channel activity in the absence of Ca²⁺. We have also identified an endogenous Ca²⁺-dependent phosphatase activity from V. faba that is inhibited by the cyclophilin-cyclosporin A and FK506-binding protein-FK506 complexes. Our findings implicate a Ca²⁺-dependent, calcineurin-like protein phosphatase in a Ca²⁺ signal-transduction pathway of higher plants.