EVIDENCE FOR THE ACTIVATION OF A MAP KINASE UPON PHOSPHATE-INDUCED CELL-CYCLE REENTRY IN TOBACCO CELLS

Mitogen-activated-protein (MAP) kinases are components of signal trans duction pathways a which respond to a variety of stimuli in different organisms. In quiescent mammalian cells, the reactivation of cell divi sion induced by different mitogenic signals is mediated by the rapid p hosphorylation and activation of MAP kinases. We have investigated whe ther a similar situation occurs in plants, arresting tobacco (Nicotian a tabacum L.) cells in the GI phase of the cell cycle by phosphate sta rvation, and then inducing them to re-enter the cell cycle by refeedin g with phosphate. The transient activation of a kinase activity with t he characteristics of a MAP kinase was observed during the first hour after refeeding, when the cells were still in G(1). Using myelin basic protein (MBP) as substrate, an increase in this phosphorylating activ ity, with a molecular mass of approximately 45 kDa, was detected in ce ll extracts between 35 and 55 min after induction, in in-gel phosphory lation assays and after immunoprecipitation with anti-MAP kinase antib odies. The specificity of the antibodies against recombinant tobacco M AP kinases suggested that the MAP kinase p45(ntf4) was responsible for the observed activity, These data provide experimental evidence for t he activation in vivo of a plant MAP kinase, possibly mediating the re activation of cell division in G(1)-arrested cells.