Blue Light-Induced Phosphorylation of a Plasma Membrane Protein in Pea: a Step in the Signal Transduction Chain for Phototropism

Abstract

A 117 kDa polypeptide associated with the plasma membrane isolated from the growing zones of etiolated pea epicotyls is phosphorylated upon a brief exposure to blue light. The literature pertaining to this photosensitive system to date is briefly summarised prior to a short experimental section. In the experimental work the polypeptide has been studied in its non-denatured and denatured state in order to investigate its possible role in blue light-induced signal transduction. The polypeptide has a molecular weight near 117 kDa. Under non-denaturing conditions, following solubilisation in non-ionic detergent, and after electrophoresis, a complex migrating at approximately '335 kDa' still retains the ability to perceive the blue light signal and undergo phosphorylation. This complex is resolved into a phosphorylated protein of approximate molecular mass of 117 kDa by second dimensional analysis on denaturing polyacrylamide gels. It is not at present possible to determine whether this protein is the only one involved or whether two or more different proteins are required for the light-inducible phosphorylation. This reaction has been established elsewhere by genetic and physiological criteria to be an early step in the signal transduction pathway for phototropism. Two other reactions that are inducible in vitro in isolated plasma membranes from etiolated seedings - activation of GTPase activity and reduction of a b-type cytochrome-are almost certainly independent pathways unrelated to the phosphorylation reaction or to each other. Neither has as yet been associated definitively with a known physiological response.