Calcium-independent activation of salicylic acid-induced protein kinase and a 40-kilodalton protein kinase by hyperosmotic stress.
Reversible protein phosphorylation/dephosphorylation plays important roles in signaling the plant adaptive responses to salinity/drought stresses. Two protein kinases with molecular masses of 48 and 40 kD are activated in tobacco cells exposed to NaCl. The 48-kD protein kinase was identified as SIPK (salicylic acid-induced protein kinase), a member of the tobacco MAPK (mitogen-activated protein kinase) family that is activated by various other stress stimuli. The activation of the 40-kD protein kinase is rapid and dose-dependent. Other osmolytes such as Pro and sorbitol activate these two kinases with similar kinetics. The activation of 40-kD protein kinase is specific for hyperosmotic stress, as hypotonic stress does not activate it. Therefore, this 40-kD kinase was named HOSAK (high osmotic stress-activated kinase). HOSAK is a Ca(2+)-independent kinase and uses myelin basic protein (MBP) and histone equally well as substrates. The kinase inhibitor K252a rapidly activates HOSAK in tobacco cells, implicating a dephosphorylation mechanism for HOSAK activation. Activation of both SIPK and HOSAK by high osmotic stress is Ca(2+) and abscisic acid (ABA) independent. Furthermore, mutation in SOS3 locus does not affect the activation of either kinase in Arabidopsis seedlings. These results suggest that SIPK and 40-kD HOSAK are two new components in a Ca(2+)- and ABA-independent pathway that may lead to plant adaptation to hyperosmotic stress.
Hoyos
M E
ME
Department of Biochemistry, University of Missouri-Columbia, 117 Schweitzer Hall, Columbia, Missouri 65211, USA.
Zhang
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D002118:Calcium / Q000378:metabolism*; D004789:Enzyme Activation; D008970:Molecular Weight; D009997:Osmotic Pressure; D011494:Protein Kinases / Q000096:biosynthesis / Q000378:metabolism*; D020156:Salicylic Acid / Q000494:pharmacology*
D011494:Protein Kinases; D020156:Salicylic Acid; D002118:Calcium