Dual regulation of a chimeric plant serine/threonine kinase by calcium and calcium/calmodulin.
A chimeric Ca2+/calmodulin-dependent protein kinase (CCaMK) gene characterized by a catalytic domain, a calmodulin-binding domain, and a neural visinin-like Ca2+-binding domain was recently cloned from plants (Patil, S., Takezawa, D., and Poovaiah, B. W. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 4797-4801). The Escherichia coli-expressed CCaMK phosphorylates various protein and peptide substrates in a Ca2+/calmodulin-dependent manner. The calmodulin-binding region of CCaMK has similarity to the calmodulin-binding region of the alpha-subunit of multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII). CCaMK exhibits basal autophosphorylation at the threonine residue(s) (0.098 mol of 32P/mol) that is stimulated 3.4-fold by Ca2+ (0.339 mol of 32P/mol), while calmodulin inhibits Ca2+-stimulated autophosphorylation to the basal level. A deletion mutant lacking the visinin-like domain did not show Ca2+-stimulated autophosphorylation activity but retained Ca2+/calmodulin-dependent protein kinase activity at a reduced level. Ca2+-dependent mobility shift assays using E. coli-expressed protein from residues 358 520 revealed that Ca2+ binds to the visinin-like domain. Studies with site-directed mutants of the visinin-like domain indicated that EF-hands II and III are crucial for Ca2+-induced conformational changes in the visinin-like domain. Autophosphorylation of CCaMK increases Ca2+/calmodulin-dependent protein kinase activity by about 5-fold, whereas it did not affect its Ca2+-independent activity. This report provides evidence for the existence of a protein kinase in plants that is modulated by Ca2+ and Ca2+/calmodulin. The presence of a visinin-like Ca2+-binding domain in CCaMK adds an additional Ca2+-sensing mechanism not previously known to exist in the Ca2+/calmodulin-mediated signaling cascade in plants.
Takezawa
D
D
Department of Horticulture, Washington State University, Pullman, Washington 99164-6414, USA.
Ramachandiran
S
S
Paranjape
V
V
Poovaiah
B W
BW
D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000737:chemistry* / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism*; D017931:DNA Primers / Q000737:chemistry; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D011994:Recombinant Proteins; D013329:Structure-Activity Relationship
D002147:Calmodulin; D017931:DNA Primers; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D017346:Protein Serine-Threonine Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002118:Calcium