PHOSPHORYLATION OF PLANT-PROTEINS AND THE IDENTIFICATION OF PROTEIN-TYROSINE KINASE-ACTIVITY IN MAIZE SEEDLINGS

Phosphotyrosine was found to be 0.5% of the total phosphoamino acids l abelled with [P-32]orthophosphate in endogenous maize seedlings protei ns. Two peaks of protein kinase activity towards phosphorylation of sy nthetic peptide poly (Glu(80), Tyr(20)) were obtained after chromatogr aphy of protein extract of dark-grown etiolated maize seedlings on pho sphocellulose. The phosphorylation of synthetic peptide as well as end ogenous proteins was strongly stimulated by Mn2+. At least three endog enous proteins with molecular masses in the range of 40-65 kDa were pr edominantly phosphorylated. This phosphorylation was resistant to alka li treatment. Chemical, immunological and enzymatic data indicated the presence of tyrosine kinase activity and also phosphotyrosine in prot eins of maize seedlings. The plant enzyme(s) is reminiscent known mamm alian cytosolic tyrosine kinase(s).