Dual Regulation of a Chimeric Plant Serine/Threonine Kinase by Calcium and Calcium/Calmodulin

Abstract

A chimeric Ca/calmodulin-dependent protein kinase (CCaMK) gene characterized by a catalytic domain, a calmodulin-binding domain, and a neural visinin-like Ca-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 Ca/calmodulin-dependent manner. The calmodulin-binding region of CCaMK has similarity to the calmodulin-binding region of the α-subunit of multifunctional Ca/calmodulin-dependent protein kinase (CaMKII). CCaMK exhibits basal autophosphorylation at the threonine residue(s) (0.098 mol of P/mol) that is stimulated 3.4-fold by Ca (0.339 mol of P/mol), while calmodulin inhibits Ca-stimulated autophosphorylation to the basal level. A deletion mutant lacking the visinin-like domain did not show Ca-stimulated autophosphorylation activity but retained Ca/calmodulin-dependent protein kinase activity at a reduced level. Ca-dependent mobility shift assays using E. coli-expressed protein from residues 358-520 revealed that Ca 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 Ca-induced conformational changes in the visinin-like domain. Autophosphorylation of CCaMK increases Ca/calmodulin-dependent protein kinase activity by about 5-fold, whereas it did not affect its Ca-independent activity. This report provides evidence for the existence of a protein kinase in plants that is modulated by Ca and Ca/calmodulin. The presence of a visinin-like Ca-binding domain in CCaMK adds an additional Ca-sensing mechanism not previously known to exist in the Ca/calmodulin-mediated signaling cascade in plants.