BLUE LIGHT-INDUCED PHOSPHORYLATION OF A PLASMA MEMBRANE-ASSOCIATED PROTEIN IN ZEA-MAYS L

Blue light induces a variety of photomorphogenic responses in higher p lants, among them phototropic curvature, the bending of seedlings towa rd a unidirectional light source. In dark-grown coleoptiles of maize ( Zea mays L.) seedlings, blue light induces rapid phosphorylation of a 114-kD protein at fluence levels that are sufficient to stimulate phot otropic curvature. Phosphorylation in response to blue light can be de tected in vivo in coleoptile tips preincubated in 32Pi or in vitro in isolated membranes supplemented with [gamma-P-32]ATP. Phosphorylation reaches a maximum level in vitro within 2 min following an inductive l ight pulse, but substantial labeling occurs within the first 15 s. Iso lated membranes remain activated for several minutes following an in v itro blue light stimulus, even in the absence of exogenous ATP. Phosph oamino acid analysis of the 114-kD protein detected phosphoserine and a trace of phosphothreonine. The kinase involved in phosphorylating th e protein in vitro is not dependent on calcium. The 114-kD protein its elf has an apparent binding site for ATP, detected by incubating with the nonhydrolyzable analog, 5'-p-fluorosulfonyl-benzoyladenosine. This result suggests that the 114-kD protein, which becomes phosphorylated in response to blue light, may also be capable of kinase activity.