AN INCREASE IN CYTOSOLIC CALCIUM-ION CONCENTRATION PRECEDES HYPOOSMOTIC SHOCK-INDUCED ACTIVATION OF PROTEIN-KINASES IN TOBACCO SUSPENSION-CULTURE CELLS

Hypoosmotic shock induced a transient increase in cytosolic free calci um concentration ([Ca2+](cyt)) and subsequent activation of 50-, 75- a nd 80-kDa protein kinases in tobacco (Nicotiana tabacum L,) suspension culture cells, external calcium suppressed both the elevation of [Ca2 +](cyt) and the activation of protein kinases in response to hypoosmot ic shock, indicating that the elevation of [Ca2+](cyt) is prerequisite for the activation of protein kinases, Pharmacological analysis indic ated that the hypoosmotic shock-activated protein kinases were activat ed by phosphorylation, suggesting that the activities of these protein kinases are regulated by putative protein kinases, These results sugg est that the hypoosmotic signal is transduced to protein kinase cascad es,which are triggered by [Ca2+](cyt) elevation.