Identification of a 40 kDa Protein Kinase Activated by Stress in a Halotolerant Green Alga Dunaliella tertiolecta

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Activation of a 40 kDa protein kinase in Dunaliella tertiolecta was detected by in-gel kinase assay when the cells were subjected to various forms of stress, such as heat shock, acidic stress and H₂O₂. The substrate specificity of the 40 kDa kinase resembles that of HAP kinase (high osmotic pressure-activated protein kinase). An anti-phosphotyrosine monoclonal antibody did not co-precipitate with the 40 kDa kinase in the cell extracts of Dunaliella subjected to stress. Treatment of Dunaliella cells with mastoparan or AlF₄^-, trimeric G-protein activators, stimulated another protein kinase with a relative molecular mass of 46 kDa. This protein kinase has a different substrate specificity from that of HAP kinase and the stress-activated 40 kDa kinase. These results suggest that the stress-activated 40 kDa kinase is closely related to HAP kinase and that the 46 kDa kinase is associated with a putative trimeric G-protein.