Rapid blue-light-induced phosphorylation of plasma-membrane-associated proteins in wheat

Abstract

When the microsomal membranes isolated from 4-day-old etiolated wheat seedlings are irradiated in vitro with blue light and subjected to phosphorylation with [γ-³²P]ATP in vitro, the polypeptides of 110, 102, 82 and 70 kDa are heavily phosphorylated. The induction of phosphorylation is extremely rapid as it can be triggered by even 5 sec blue light irradiation, although maximal response is obtained with 15 sec blue irradiation. The response is observed most strongly in the microsomal fraction isolated from the highly photosensitive tip portion of the seedlings, and its intensity is higher in the leaf than in the tubular coleoptile fraction. The aqueous two-phase partitioning of the microsomal fraction has shown that all these four blue-light-responsive phosphopolypeptides are associated with the plasma membrane. The kinase involved in phosphorylating these polypeptides in vitro is sensitive to staurosporine. The presence of blue-light-responsive phosphopolypeptides in both the coleoptile sheath and the leaf encased inside, and the kinetics of induction of this response indicate that the phosphorylation of plasma membrane proteins may be one of the early steps in blue-light-elicited signal transduction chain, associated not only with phototropism but perhaps also with low-fluence blue responses in general.