Protein phosphorylation is one of the major mechanisms by which eukaryotic cells transduce extracellular signals into intracellular responses. Calcium/calmodulin (Ca2+/CaM)-dependent protein phosphorylation has been implicated in various cellular processes, yet little is known about Ca2+/CaM-dependent protein kinases (CaMKs) in plants. From an Arabidopsis expression library screen using a horseradish peroxidase-conjugated soybean calmodulin isoform (SCaM-1) as a probe, we isolated a full-length cDNA clone that encodes AtCK (Arabidopsis thaliana calcium/calmodulin-dependent protein kinase). The predicted structure of AtCK contains a serine/threonine protein kinase catalytic domain followed by a putative calmodulin-binding domain and a putative Ca2+-binding domain. Recombinant AtCK was expressed in E. coli and bound to calmodulin in a Ca2+-dependent manner. The ability of CaM to bind to AtCK was confirmed by gel mobility shift and competition assays. AtCK exhibited its highest levels of autophosphorylation in the presence of 3 mM Mn2+. The phosphorylation of myelin basic protein (MBP) by AtCK was enhanced when AtCK was under the control of calcium-bound CaM, as previously observed for other Ca2+/CaM-dependent protein kinases. In contrast to maize and tobacco CCaMKs (calcium and Ca2+/CaM-dependent protein kinase), increasing the concentration of calmodulin to more than 3 ?M suppressed the phosphorylation activity of AtCK. Taken together our results indicate that AtCK is a novel Arabidopsis Ca2+/CaM-dependent protein kinase which is presumably involved in CaM-mediated signaling.

Keywords: Arabidopsis thaliana, Calcium, Calmodulin, Protein Kinase, Signaling