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 labelled with [P-32]orthophosphate in endogenous maize seedlings proteins. Two peaks of protein kinase activity towards phosphorylation of synthetic peptide poly (Glu(80), Tyr(20)) were obtained after chromatography of protein extract of dark-grown etiolated maize seedlings on phosphocellulose. The phosphorylation of synthetic peptide as well as endogenous proteins was strongly stimulated by Mn2+. At least three endogenous proteins with molecular masses in the range of 40-65 kDa were predominantly phosphorylated. This phosphorylation was resistant to alkali treatment. Chemical, immunological and enzymatic data indicated the presence of tyrosine kinase activity and also phosphotyrosine in proteins of maize seedlings. The plant enzyme(s) is reminiscent known mammalian cytosolic tyrosine kinase(s).