BLUE LIGHT-INDUCED PHOSPHORYLATION OF A PLASMA-MEMBRANE PROTEIN IN PEA - A STEP IN THE SIGNAL-TRANSDUCTION CHAIN FOR PHOTOTROPISM

A 117 kDa polypeptide associated with the plasma membrane isolated fro m the growing zones of etiolated pea epicotyls is phosphorylated upon a brief exposure to blue light. The literature pertaining to this phot osensitive system to date is briefly summarised prior to a short exper imental section. In the experimental work the polypeptide has been stu died in its non-denatured and denatured state in order to investigate its possible role in blue light-induced signal transduction. The polyp eptide has a molecular weight near 117 kDa. Under non-denaturing condi tions, following solubilisation in non-ionic detergent, and after elec trophoresis, a complex migrating at approximately '335 kDa' still reta ins the ability to perceive the blue light signal and undergo phosphor ylation. This complex is resolved into a phosphorylated protein of app roximate molecular mass of 117 kDa by second dimensional analysis on d enaturing polyacrylamide gels. It is not at present possible to determ ine whether this protein is the only one involved or whether two or mo re different proteins are required for the light-inducible phosphoryla tion. This reaction has been established elsewhere by genetic and phys iological criteria to be an early step in the signal transduction path way for phototropism. Two other reactions that are inducible in vitro in isolated plasma membranes from etiolated seedings-activation of GTP ase activity and reduction of a b-type cytochrome-are almost certainly independent pathways unrelated to the phosphorylation reaction ar to each other. Neither has as yet been associated definitively with a kno wn physiological response.