CUTTING ACTIVATES A 46-KILODALTON PROTEIN-KINASE IN PLANTS

Using SDS/polyacrylamide gels that contained myelin basic protein, we identified a 46-kDa protein kinase in tobacco that is transiently acti vated by cutting, Although the activity of the kinase was rarely detec table in mature leaves, marked activity became apparent within several minutes after isolation of leaf discs and subsided within 30 min, In the presence of cycloheximide (CHX), the kinase activity did not dimin ish after the isolation over the course of 2 hr, suggesting that prote in synthesis was not required for the activation of the kinase, A seco nd cutting of leaf discs between 30 min and 60 min after the isolation failed to activate the kinase, whereas a second cutting given 3 hr af ter isolation apparently activated the kinase, These results suggest t hat the 46-kDa protein kinase is desensitized immediately after the fi rst activation, which can be blocked by CHX, but the response ability recovers with time. When protein extracts containing the active kinase were treated with serine/threonine-specific or tyrosine-specific prot ein phosphatase, the kinase activity was abolished, After immunoprecip itation with antibody against phosphotyrosine, activity of the kinase was recovered in the immunoprecipitate, These results suggest that the active form of the kinase is phosphorylated at both serine/threonine and tyrosine residues, It seems likely that the 46-kDa protein kinase can be activated by dual phosphorylation. The activity of a 46-kDa pro tein kinase was also detected in leaves of a wide variety of plant spe cies including dicotyledonous and monocotyledonous plants, We propose the name PMSAP (plant multisignal-activated protein) kinase for this k inase because the kinase was also activated by various signals other t han cutting.