Modulation of the Arabidopsis KAT1 channel by an activator of protein kinase C in Xenopus laevis oocytes

The Arabidopsis thaliana K+ channel KAT1 has been suggested to play a key role in the regulation of the aperture of stomatal pores on the surface of plant leaves. Calcium-dependent and calcium-independent signaling pathways are involved in abscisic acid-mediated regulation of guard cell turgidity. Although the activity of the KAT1 channel is thought to be regulated by calcium-dependent protein kinases, the effect of phosphorylation on KAT1 and the phosphorylated target sites remain elusive. Because it has been proposed that the phosphorylation recognition sequence of plant calcium-dependent protein kinases resembles that of animal protein kinases C, in this study, we used the Xenopus laevis oocyte protein kinase C to identify the target sites of calcium-dependent protein kinases. KAT1 expressed in Xenopus oocytes was inhibited by the protein kinase C activator phorbol 12-myristate 13-acetate. On the basis of an in silico search, we selected S/T-X-K/R motifs facing the cytosol, as it has been reported that protein kinase C and calcium-dependent protein kinase share a common consensus sequence. Mutagenesis analyses revealed that six Ser/Thr residues were responsible for the reduction in activity after phorbol 12-myristate 13-acetate application. Simultaneous mutation of the five residues located in the carboxyl-terminus region of KAT1 led to a K+ channel mutant that was insensitive to protein kinase C. These results indicate that, in plant cells, a kinase analogous to protein kinase C might exist that may modulate KAT1 channel activity through calcium-dependent phosphorylation at some of the pinpointed residues in the cytosolic region of KAT1.