A 48-kDa protein kinase was detected in Vicia faba guard cell protoplasts by an in-gel protein kinase assay using a recombinant peptide (KAT1C) of the carboxyl-terminus of an inward-rectifying voltage-dependent K+ channel cloned from Arabidopsis thaliana, KAT1. This protein kinase (ABR* kinase) was activated by pretreatment of guard cell protoplasts with ABA, but not by pretreatment with IAA, 2,4-D, kinetin or GA3. The activation of ABR* kinase was dependent on the time and concentration of ABA. The kinase activity was sensitive to staurosporine and K-252a, protein kinase inhibitors, and insensitive to Ca2+. No ABR* kinase activity was detected in mesophyll cell protoplasts. These characteristics of ABR* kinase are consistent with those of an ABA-responsive protein kinase (ABR kinase) reported previously [Mori and Muto (1997), Plant Physiol. 113: 833]. These results indicate that ABR* kinase phosphorylates the inward-rectifying K+ channel in response to treatment of stomatal guard cells with ABA. The data reported here provide evidence that this ABA-responsive protein kinase may promote ABA signaling by directly phosphorylating guard cell ion channels.