Abstract
Calcium is a universal secondary messenger that triggers many cellular responses. However, it is unclear how a calcium signal is coordinately decoded by different calcium sensors, which in turn regulate downstream targets to fulfill a specific physiological function. Here we show that SOS2-LIKE PROTEIN KINASE5 (PKS5) can negatively regulate the Salt-Overly-Sensitive signaling pathway in Arabidopsis. PKS5 can interact with and phosphorylate SOS2 at Ser294, promote the interaction between SOS2 and 14-3-3 proteins, and repress SOS2 activity. However, salt stress promotes an interaction between 14-3-3 proteins and PKS5, repressing its kinase activity and releasing inhibition of SOS2. We provide evidence that 14-3-3 proteins bind to Ca2+, and that Ca2+ modulates 14-3-3-dependent regulation of SOS2 and PKS5 kinase activity. Our results suggest that a salt-induced calcium signal is decoded by 14-3-3 and SOS3/SCaBP8 proteins, which selectively activate/inactivate the downstream protein kinases SOS2 and PKS5 to regulate Na+ homeostasis by coordinately mediating plasma membrane Na+/H+ antiporter and H+-ATPase activity.
Highlights
Calcium is a universal secondary messenger that triggers many cellular responses
We found that the saltenhanced 14-3-3/PROTEIN KINASE5 (PKS5) and SOS3/SOS2 interactions were rescued by the addition of LaCl3, a calcium channel blocker (Supplementary Fig. 5a–d) and that the effects of Ca2+ on the repression of PKS5 and activation of SOS2 activities were rescued by EGTA (Supplementary Fig. 5e, f)
The salt-induced calcium signal is decoded by various Ca2 +-binding proteins, including 14-3-3s, SOS3, and SCaBP8, which may result in increased repression of PKS5 activity that reduces SOS2Ser[294] phosphorylation and in turn, relieves the repression of SOS2 activity by 14-3-3 proteins
Summary
Calcium is a universal secondary messenger that triggers many cellular responses. it is unclear how a calcium signal is coordinately decoded by different calcium sensors, which in turn regulate downstream targets to fulfill a specific physiological function. Fluctuations in the concentration of cytosolicfree Ca2+ ([Ca2+]cyt) triggered by internal or external stimuli are decoded by different Ca2+ sensors, such as calmodulin (CaM)[1,2,3], Ca2+-dependent protein kinases (CDPKs)[4,5], and SOS3-like Ca2+-binding protein/calcineurin B-like protein (SCaBP/CBL)[6,7,8,9,10,11] It is unclear how different calcium sensors decode a calcium signal and coordinately regulate the activity of various cellular targets to achieve a specific physiological response. GI, interacts with and represses SOS2 activity under normal growth conditions[22] It is unknown which kinase phosphorylates SOS2Ser[294] and how 14-3-3 proteins are regulated to either bind or release SOS2 in the absence or presence of salt stress, respectively. We provide a model whereby 143-3 proteins act as a Ca2+-dependent switch to coordinately regulate SOS2 and PKS5, thereby activating both the PM Na+/H+ antiporter and PM H+-ATPase and mediating the plant’s response to salt stress
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