Co-expression of calcium-dependent protein kinase with the inward rectified guard cell K+ channel KAT1 alters current parameters in Xenopus laevis oocytes

Increased guard cell cytosolic [Ca2+] is known to be involved in signal tra nsduction pathways leading to stomatal closure, and inhibit the inward rect ifying guard cell K+ channel KAT1, Guard cell calcium-dependent protein kin ase (CDPK) has been shown to phosphorylate KAT1; such phosphorylation is kn own to modulate other K+ channels involved in signal transduction cascades. The work reported here focused on demonstrating CDPK-dependent inhibition of KAT1 currents. A cDNA encoding soybean CDPK was generated and it's trans lation product was shown to be functional; demonstrating Ca2+-dependent aut ophosphorylation and phosphorylation of a target protein. Ion currents were monitored using voltage clamp techniques upon expression of KAT1 in Xenopu s laevis oocytes, Coexpression of recombinant CDPK with KAT1 in oocytes alt ered the kinetics and magnitude of induced K+ currents; at a given hyperpol arizing command voltage, the magnitude of KAT1 currents was reduced and the half-time for channel activation was increased. This finding supports a mo del of Ca2+-dependent ABA inhibition of inward K+ currents in guard cells a s being mediated by CDPK phosphorylation of KAT1.