ABSCISIC-ACID ACTIVATES A 48-KILODALTON PROTEIN-KINASE IN GUARD-CELL PROTOPLASTS

A 49- and a 46-kD Ca2+-independent protein kinase and a 53-kD Gaze-dep endent protein kinase were detected in Vicia faba guard cell protoplas ts (GCPs) by an in-gel protein kinase assay using myelin basic protein as a substrate. A 48-kD protein kinase designated as abscisic acid (A BA)-responsive protein kinase (ABR kinase) appeared when GCPs were tre ated with ABA. The activation of ABR kinase was suppressed by the prot ein kinase inhibitor staurosporine, indicating that a putative activat or protein kinase phosphorylates and activates ABR kinase. The treatme nt of GCPs with ,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic aci d, a calcium chelator, suppressed the activation of ABR kinase, sugges ting that an influx of extracellular Ca2+ is required for the activati on. Staurosporine and K-252a inhibited both the activity of ABR kinase and the stomatal closure induced by ABA treatment of V. faba epiderma l peels. These results suggest that ABR kinase and its activator kinas e may consist of a protein kinase cascade in a signal transduction pat hway linking ABA perception to stomatal closure. The mobility of the 5 3-kD Ca2+-dependent protein kinase in sodium dodecyl sulfate-polyacryl amide gel was shifted upon Ca2+ binding to the enzyme, thus exhibiting the characteristics of a Ca2+-dependent or calmodulin-like domain pro tein kinase. This kinase may be the activator of ABR kinase.