Hyperosmotic stress induces the rapid phosphorylation of a soybean phosphatidylinositol transfer protein homolog through activation of the protein kinases SPK1 and SPK2.
Although phosphatidylinositol transfer proteins (PITPs) are known to serve critical functions in regulating a varied array of signal transduction processes in animals and yeast, the discovery of a similar class of proteins in plants occurred only recently. Here, we report the participation of Ssh1p, a soybean PITP-like protein, in the early events of osmosensory signal transduction in plants, a function not attributed previously to animal or yeast PITPs. Exposure of plant tissues to hyperosmotic stress led to the rapid phosphorylation of Ssh1p, a modification that decreased its ability to associate with membranes. An osmotic stress-activated Ssh1p kinase activity was detected in several plant species by presenting recombinant Ssh1p as a substrate in in-gel kinase assays. Elements of a similar osmosensory signaling pathway also were conserved in yeast, an observation that facilitated the identification of soybean protein kinases SPK1 and SPK2 as stress-activated Ssh1p kinases. This study reveals the activation of SPK1 and/or SPK2 and the subsequent phosphorylation of Ssh1p as two early successive events in a hyperosmotic stress-induced signaling cascade in plants. Furthermore, Ssh1p is shown to enhance the activities of a plant phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinase, an observation that suggests that the ultimate function of Ssh1p in cellular signaling is to alter the plant's capacity to synthesize phosphoinositides during periods of hyperosmotic stress.
Monks
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DE
Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27695, USA.
Aghoram
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K
Courtney
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PD
DeWald
D B
DB
Dewey
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D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins; D064447:Checkpoint Kinase 2; D004789:Enzyme Activation; D008565:Membrane Proteins; D008954:Models, Biological; D009997:Osmotic Pressure; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D049294:Phospholipid Transfer Proteins; D010766:Phosphorylation; D010940:Plant Proteins / Q000378:metabolism*; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D015398:Signal Transduction; D013025:Glycine max; D014026:Nicotiana
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D008565:Membrane Proteins; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D049294:Phospholipid Transfer Proteins; D010940:Plant Proteins; D029701:Saccharomyces cerevisiae Proteins; C055525:phosphatidylinositol 3-phosphate; C037178:phosphatidylinositol 4-phosphate; D064447:Checkpoint Kinase 2; D017346:Protein Serine-Threonine Kinases; C067022:RAD53 protein, S cerevisiae