BLUE-LIGHT INDUCES PHOSPHORYLATION AT SERYL RESIDUES ON A PEA (PISUM-SATIVUM L) PLASMA-MEMBRANE PROTEIN

We have partially characterized the blue-light-stimulated in vitro phosphorylation of a membrane protein from etiolated Pisum sativum L. stems. Properties of the response have implicated its involvement in signal transduction of phototropic stimuli (T.W. Short, W.R. Briggs [1990] Plant Physiol 92: 179-185; P. Reymond, T.W. Short, W.R. Briggs [1992] Proc Natl Acad Sci USA 89: 4718-4721). Analysis of proteolysis products and phosphoamino acid analysis indicate that the substrate protein is phosphorylated on multiple seryl residues. Kinetics of the in vitro reaction show phosphorylation to be complete within 2 to 5 min at 30 degrees C in either light-exposed or dark-control plasma membrane preparations, regardless of whether the membranes were first solubilized in Triton X-100. Nucleotide competition assays show the kinase to be ATP specific. The pH optimum covers a broad range with a maximum near 7.5. A wide array of salts inhibits the phosphorylation at high concentrations, but millimolar concentrations of Mg2+ are required to form Mg.ATP complexes for maximal activity, whereas excess free Mg2+ Or Ca2+ are not required for the reaction.