Activation of the plasma membrane Na/H antiporter Salt-Overly-Sensitive 1 (SOS1) by phosphorylation of an auto-inhibitory C-terminal domain

Abstract

The plasma membrane sodium/proton exchanger Salt-OverlySensitive 1 (SOS1) is a critical salt tolerance determinant in plants. The SOS2-SOS3 calcium-dependent protein kinase complex upregulates SOS1 activity, but the mechanistic details of this crucial event remain unresolved. Here we show that SOS1 is maintained in a resting state by a C-terminal auto-inhibitory domain that is the target of SOS2-SOS3. The auto-inhibitory domain interacts intramolecularly with an adjacent domain of SOS1 that is essential for activity. SOS1 is relieved from auto-inhibition upon phosphorylation of the auto-inhibitory domain by SOS2-SOS3. Mutation of the SOS2 phosphorylation and recognition site impeded the activation of SOS1 in vivo and in vitro. Additional amino acid residues critically important for SOS1 activity and regulation were identified in a genetic screen for hypermorphic alleles.ion transport | salinity | sodium tolerance S alinity is a major problem in agriculture because the total area of salt-affected soils, including saline and sodic soils, exceeds 900 million ha (1). Salt-affected soils reduce both the ability of crops to take up water and the availability of mineral nutrients. Often, the high sodium (Na) content relative to other cations is the main factor affecting plant growth by causing a set of metabolic derangements (2). Because most crop species have only very limited capacities to cope with excess Na, the elucidation of Na tolerance mechanisms in plants is of paramount importance (2). Plant ion transporters mediating Na fluxes have recently been cloned and characterized, and the knowledge of the regulatory mechanisms of transporter abundance and activity in response to environmental, hormonal, and developmental signals is critical for understanding salinity tolerance (3). The plasma membrane Na/H antiporter SOS1 is essential for the salt tolerance of various model plants, including Arabidopsis thaliana (4) and its halophytic relative Thellungiella salsuginea (5), tomato (6), and the moss Physcomitrella patens (7). SOS1 is thought to mediate Na efflux at the root epidermis and longdistance transport from roots to shoots (4, 6) while protecting individual cells from Na toxicity (7-9). SOS1 is also indirectly required for the uptake of potassium (K) in the presence of Na, although the mechanistic basis is not fully understood (7,8,10). Both the protein kinase SOS2 and its associated calcium-sensor subunit SOS3 are required for the posttranslational activation of SOS1 Na/H exchange activity in Arabidopsis (11,12), and a similar regulatory module operates also in cereals (13).To understand further the mechanism(s) of SOS1 regulation, we identified the SOS2-dependent phosphorylation site and began to dissect the structure-function relationship in the SOS1 protein.Our results indicate that the SOS1 C-terminal domain comprises an auto-inhibitory domain the activity of which is counteracted by SOS2-dependent phosphorylation upon salinity stress. Results SOS1 ResiduesPhosphorylated by the SOS2 Protein Kinase. We have ...

Release of SOS2 kinase from sequestration with GIGANTEA determines salt tolerance in Arabidopsis

“…2e). This conclusion is supported by studies in yeast mutants unable to excrete Na þ that have clearly established the Na þ /H þ antiporter activity of SOS1 7,9 , and by the observation that active SOS1 protein is a requirement for salt tolerance in Arabidopsis 32 .…”

“…To curtail salt-induced damage, salt-exposed plants maintain low cytosolic Na þ concentrations by controlling influx, activating efflux, enhancing intracellular compartmentalization and coordinating tissue distribution of the ion. Efficient efflux of Na þ is achieved in plants by the plant-specific SOS pathway, which re-establishes ion and, in part, water homeostasis after exposure to high salinity 7,8 . Among the three known proteins in this pathway, SOS1 is a Na þ /H þ antiporter regulated positively by a protein kinase complex comprised of the Ca 2 þ activated protein SOS3 and the kinase SOS2, which phosphorylates SOS1 in response to salinity stress 7,9 .…”

Salinity Stress and Salt Tolerance

“…4). Both the protein kinase SOS2 and its associated calcium-sensor subunit SOS3 are required for the posttranslational activation of SOS1 Na + /H + exchange activity in Arabidopsis, (Qiu, Guo, Dietrich, Schumaker, & Zhu, 2002;Quintero, Martinez-Atienza, Villalta, Jiang, Kim, Ali, et al, 2011), and in rice (Martínez-Atienza, Jiang, Garciadeblas, Mendoza, Zhu, Pardo, et al, 2007). In yeast, co-expression of SOS1, SOS2, and SOS3 increases the salt tolerance of transformed yeast cells much more than expression of one or two SOS proteins (Quintero, Ohta, Shi, Zhu, & Pardo, 2002), suggesting that the full activity of SOS1 depends on the SOS2/SOS3 complex.…”

Structural basis of the regulatory mechanism of the plant CIPK family of protein kinases controlling ion homeostasis and abiotic stress

“…Available data suggest a mechanism in which calcium mediates the formation of stable CIPK-CBL complexes that regulate the phosphorylation state and activity of various ion transporters involved in the maintenance of cell ion homeostasis and abiotic stress responses in plants. Among them, the Arabidopsis thaliana CIPK24/SOS2-CBL4/SOS3 complex activates the Na + /H + antiporter SOS1 to maintain intracellular levels of the toxic Na + low under salt stress (7)(8)(9), the CIPK11-CBL2 pair regulates the plasma membrane H + -ATPase AHA2 to control the transmembrane pH gradient (10), the CIPK23-CBL1/9 (11,12) regulates the activity of the K + transporter AKT1 to increase the plant K + uptake capability under limiting K + supply conditions (12,13), and CIPK23-CBL1 mediates nitrate sensing and uptake by phosphorylation of the nitrate transporter CHL1 (14). Together these findings show that understanding the molecular mechanisms underling CIPKs function provides opportunities to increase plant tolerance to abiotic stress and to improve plants for human benefit.…”