Distinct Roles of N-Terminal Fatty Acid Acylation of the Salinity-Sensor Protein SOS3

The Salt-Overly-Sensitive (SOS) pathway controls the net uptake of sodium by roots and the xylematic transfer to shoots in vascular plants. SOS3/CBL4 is a core component of the SOS pathway that senses calcium signaling of salinity stress to activate and recruit the protein kinase SOS2/CIPK24 to the plasma membrane to trigger sodium efflux by the Na/H exchanger SOS1/NHX7. However, despite the well-established function of SOS3 at the plasma membrane, SOS3 displays a nucleo-cytoplasmic distribution whose physiological meaning is not understood. Here, we show that the N-terminal part of SOS3 encodes structural information for dual acylation with myristic and palmitic fatty acids, each of which commands a different location and function of SOS3. N-myristoylation at glycine-2 is essential for plasma membrane association and recruiting SOS2 to activate SOS1, whereas S-acylation at cysteine-3 redirects SOS3 toward the nucleus. Moreover, a poly-lysine track in positions 7–11 that is unique to SOS3 among other Arabidopsis CBLs appears to be essential for the correct positioning of the SOS2-SOS3 complex at the plasma membrane for the activation of SOS1. The nuclear-localized SOS3 protein had limited bearing on the salt tolerance of Arabidopsis. These results are evidence of a novel S-acylation dependent nuclear trafficking mechanism that contrasts with alternative subcellular targeting of other CBLs by S-acylation.

Targeting the bacterial SOS response for new antimicrobial agents: drug targets, molecular mechanisms and inhibitors

Antimicrobial resistance is a pressing threat to global health, with multidrug-resistant pathogens becoming increasingly prevalent. The bacterial SOS pathway functions in response to DNA damage that occurs during infection, initiating several pro-survival and resistance mechanisms, such as DNA repair and hypermutation. This makes SOS pathway components potential targets that may combat drug-resistant pathogens and decrease resistance emergence. This review discusses the mechanism of the SOS pathway; the structure and function of potential targets AddAB, RecBCD, RecA and LexA; and efforts to develop selective small-molecule inhibitors of these proteins. These inhibitors may serve as valuable tools for target validation and provide the foundations for desperately needed novel antibacterial therapeutics.

Salinity Stress-Mediated Suppression of Expression of Salt Overly Sensitive Signaling Pathway Genes Suggests Negative Regulation by AtbZIP62 Transcription Factor in Arabidopsis thaliana

Salt stress is one of the most serious threats in plants, reducing crop yield and production. The salt overly sensitive (SOS) pathway in plants is a salt-responsive pathway that acts as a janitor of the cell to sweep out Na+ ions. Transcription factors (TFs) are key regulators of expression and/or repression of genes. The basic leucine zipper (bZIP) TF is a large family of TFs regulating various cellular processes in plants. In the current study, we investigated the role of the Arabidopsis thaliana bZIP62 TF in the regulation of SOS signaling pathway by measuring the transcript accumulation of its key genes such as SOS1, 2, and 3, in both wild-type (WT) and atbzip62 knock-out (KO) mutants under salinity stress. We further observed the activation of enzymatic and non-enzymatic antioxidant systems in the wild-type, atbzip62, atcat2 (lacking catalase activity), and atnced3 (lacking 9-cis-epoxycarotenoid dioxygenase involved in the ABA pathway) KO mutants. Our findings revealed that atbzip62 plants exhibited an enhanced salt-sensitive phenotypic response similar to atnced3 and atcat2 compared to WT, 10 days after 150 mM NaCl treatment. Interestingly, the transcriptional levels of SOS1, SOS2, and SOS3 increased significantly over time in the atbzip62 upon NaCl application, while they were downregulated in the wild type. We also measured chlorophyll a and b, pheophytin a and b, total pheophytin, and total carotenoids. We observed that the atbzip62 exhibited an increase in chlorophyll and total carotenoid contents, as well as proline contents, while it exhibited a non-significant increase in catalase activity. Our results suggest that AtbZIP62 negatively regulates the transcriptional events of SOS pathway genes, AtbZIP18 and AtbZIP69 while modulating the antioxidant response to salt tolerance in Arabidopsis.

Calcineurin B-Like Proteins CBL4 and CBL10 Mediate Two Independent Salt Tolerance Pathways in Arabidopsis

In Arabidopsis, the salt overly sensitive (SOS) pathway, consisting of calcineurin B-like protein 4 (CBL4/SOS3), CBL-interacting protein kinase 24 (CIPK24/SOS2) and SOS1, has been well defined as a crucial mechanism to control cellular ion homoeostasis by extruding Na+ to the extracellular space, thus conferring salt tolerance in plants. CBL10 also plays a critical role in salt tolerance possibly by the activation of Na+ compartmentation into the vacuole. However, the functional relationship of the SOS and CBL10-regulated processes remains unclear. Here, we analyzed the genetic interaction between CBL4 and CBL10 and found that the cbl4 cbl10 double mutant was dramatically more sensitive to salt as compared to the cbl4 and cbl10 single mutants, suggesting that CBL4 and CBL10 each directs a different salt-tolerance pathway. Furthermore, the cbl4 cbl10 and cipk24 cbl10 double mutants were more sensitive than the cipk24 single mutant, suggesting that CBL10 directs a process involving CIPK24 and other partners different from the SOS pathway. Although the cbl4 cbl10, cipk24 cbl10, and sos1 cbl10 double mutants showed comparable salt-sensitive phenotype to sos1 at the whole plant level, they all accumulated much lower Na+ as compared to sos1 under high salt conditions, suggesting that CBL10 regulates additional unknown transport processes that play distinct roles from the SOS1 in Na+ homeostasis.

A computational analysis of Salt Overly Sensitive 1 homologs in halophytes and glycophytes

Soil salinity is one of the most serious impediments to global agricultural productivity. Although most terrestrial plants are glycophytes which cannot tolerate high salt concentrations, a small fraction of species are halophytes. Exactly what allows these extremophile plants to survive in saline conditions is not yet well understood. Several studies have established the Salt Overly Sensitive (SOS) pathway as the canonical model for the mechanism responsible for salt tolerance. The SOS pathway involves interplay among Na+-H+ antiporters for transporting sodium, and the activation of the kinase that phosphorylates the transporter. Among them, SOS1, a plasma membrane Na+-H+ antiporter, has been shown to be a critical component for maintaining salt homeostasis by pumping sodium out of cells upon activation. Therefore, it is of great interest to evaluate any differences of SOS1 in halophytes as compared to glycophytes. Here we report a computational analysis of the primary and secondary structures of eight halophytes and seven glycophytes. ClustalW alignment of the protein sequences as a whole reveals no regions conserved specifically in only halophytes or in only glycophytes. In addition, the key regulatory residues at the C-terminus of SOS1, S1136 and S1138, which were shown to be the phosphorylation sites by the kinase SOS2, were completely conserved in all 15 halophytes and glycophytes. The four amino acids, G136, R365, G777, and G784, in which alterations affect the function of SOS1, are mostly conserved in the 15 species. The 14-3-3 binding site in the C-terminus which is important in the phosphorylation step of SOS1 in the SOS signal transduction cascade is also well conserved. Furthermore, the number of transmembrane helices for each species is between 9 and 12 and there is no significant difference between halophytes and glycophytes. If halophytes present any special feature of SOS1, it likely involves the presence (halophytes) or absence (glycophytes) of a SOS1-interacting component.

A computational analysis of Salt Overly Sensitive 1 homologs in halophytes and glycophytes

Soil salinity is one of the most serious impediments to global agricultural productivity. Although most terrestrial plants are glycophytes which cannot tolerate high salt concentrations, a small fraction of species are halophytes. Exactly what allows these extremophile plants to survive in saline conditions is not yet well understood. Several studies have established the Salt Overly Sensitive (SOS) pathway as the canonical model for the mechanism responsible for salt tolerance. The SOS pathway involves interplay among Na+-H+ antiporters for transporting sodium, and the activation of the kinase that phosphorylates the transporter. Among them, SOS1, a plasma membrane Na+-H+ antiporter, has been shown to be a critical component for maintaining salt homeostasis by pumping sodium out of cells upon activation. Therefore, it is of great interest to evaluate any differences of SOS1 in halophytes as compared to glycophytes. Here we report a computational analysis of the primary and secondary structures of eight halophytes and seven glycophytes. ClustalW alignment of the protein sequences as a whole reveals no regions conserved specifically in only halophytes or in only glycophytes. In addition, the key regulatory residues at the C-terminus of SOS1, S1136 and S1138, which were shown to be the phosphorylation sites by the kinase SOS2, were completely conserved in all 15 halophytes and glycophytes. The four amino acids, G136, R365, G777, and G784, in which alterations affect the function of SOS1, are mostly conserved in the 15 species. The 14-3-3 binding site in the C-terminus which is important in the phosphorylation step of SOS1 in the SOS signal transduction cascade is also well conserved. Furthermore, the number of transmembrane helices for each species is between 9 and 12 and there is no significant difference between halophytes and glycophytes. If halophytes present any special feature of SOS1, it likely involves the presence (halophytes) or absence (glycophytes) of a SOS1-interacting component.

The SOS response increases bacterial fitness, but not evolvability, under a sublethal dose of antibiotic

Exposure to antibiotics induces the expression of mutagenic bacterial stress–response pathways, but the evolutionary benefits of these responses remain unclear. One possibility is that stress–response pathways provide a short-term advantage by protecting bacteria against the toxic effects of antibiotics. Second, it is possible that stress-induced mutagenesis provides a long-term advantage by accelerating the evolution of resistance. Here, we directly measure the contribution of the Pseudomonas aeruginosa SOS pathway to bacterial fitness and evolvability in the presence of sublethal doses of ciprofloxacin. Using short-term competition experiments, we demonstrate that the SOS pathway increases competitive fitness in the presence of ciprofloxacin. Continued exposure to ciprofloxacin results in the rapid evolution of increased fitness and antibiotic resistance, but we find no evidence that SOS-induced mutagenesis accelerates the rate of adaptation to ciprofloxacin during a 200 generation selection experiment. Intriguingly, we find that the expression of the SOS pathway decreases during adaptation to ciprofloxacin, and this helps to explain why this pathway does not increase long-term evolvability. Furthermore, we argue that the SOS pathway fails to accelerate adaptation to ciprofloxacin because the modest increase in the mutation rate associated with SOS mutagenesis is offset by a decrease in the effective strength of selection for increased resistance at a population level. Our findings suggest that the primary evolutionary benefit of the SOS response is to increase bacterial competitive ability, and that stress-induced mutagenesis is an unwanted side effect, and not a selected attribute, of this pathway.