Light-mediated changes in two proteins found associated with plasma membrane fractions from pea stem sections

Irradiation of etiolated pea (Pisum sativum L.) seedlings with white light affects two proteins, both of monomer molecular mass near 120 kDa. Both proteins have been detected in association with plasma membrane fractions. The first is identifiable in that it becomes heavily phosphorylated when the membranes are incubated with exogenous ATP. The second of these proteins is phytochrome, as determined by electrophoretic transfer (Western) blot analysis. Measurable phosphorylation and phytochrome (the latter detected by antigenicity) decline when the tissue is irradiated with white light prior to membrane isolation and in vitro phosphorylation. The phosphorylated protein is probably not phytochrome for three reasons. (i) It shows a slightly different distribution in sucrose gradients. (ii) Red light causes a gradual decline in the phytochrome that is associated with membrane fractions but has a negligible effect on the phosphorylatable protein; blue light, on the other hand, causes significantly slower loss of phytochrome than does red light but brings about a rapid decline in the phosphorylation signal. (iii) The molecular masses are not identical. The association of both proteins with membrane fractions is probably neither ionic nor, at least for the phosphorylatable protein, the consequence of entrapment of soluble proteins in vesicles formed during tissue extraction. Phytochrome is lost from the membrane fractions during irradiation, as judged by loss of antigenicity. Whether the phosphorylatable protein is lost, a specific kinase is lost, phosphatase activity increases, or phosphorylatable sites are blocked as a consequence of blue light treatment is not known.