ABA and NaCl activate myelin basic protein kinase in the chloronema cells of the moss Funaria hygrometrica

Mitogen-activated protein kinases (MAPKs) are components of highly conserved signal transduction pathways. We show that in pH 5-grown chloronema cells of the moss Funaria hygrometrica, abscisic acid (ABA) and NaCl caused a rapid increase in the activity of a 38 kDa protein kinase (PK38) that uses myelin-basic protein (MBP) as a substrate. The activity was dose-dependent, and seemed to saturate with 20 μM ABA or 200 mM NaCl. It was also strongly stimulant-dependent, as the withdrawal of ABA or NaCl reduced the activity drastically. PK38 also utilized a synthetic peptide specific for extracellular-regulated kinases (ERKs: EC 2.7.1.37). The responses to NaCl and ABA were additive, suggesting that the two stimuli act via independent, parallel pathways. The cation requirement and ERK substrate specificity suggest that PK38 could be a stress-activated PK.