S-acylation stabilizes ligand-induced receptor kinase complex formation during plant pattern-triggered immune signaling

Plant receptor kinases are key transducers of extracellular stimuli, such as the presence of beneficial or pathogenic microbes or secreted signaling molecules. Receptor kinases are regulated by numerous post-translational modifications.1,2,3 Here, using the immune receptor kinases FLS24 and EFR,5 we show that S-acylation at a cysteine conserved in all plant receptor kinases is crucial for function. S-acylation involves the addition of long-chain fatty acids to cysteine residues within proteins, altering their biochemical properties and behavior within the membrane environment.6 We observe S-acylation of FLS2 at C-terminal kinase domain cysteine residues within minutes following the perception of its ligand, flg22, in a BAK1 co-receptor and PUB12/13 ubiquitin ligase-dependent manner. We demonstrate that S-acylation is essential for FLS2-mediated immune signaling and resistance to bacterial infection. Similarly, mutating the corresponding conserved cysteine residue in EFR suppressed elf18-triggered signaling. Analysis of unstimulated and activated FLS2-containing complexes using microscopy, detergents, and native membrane DIBMA nanodiscs indicates that S-acylation stabilizes, and promotes retention of, activated receptor kinase complexes at the plasma membrane to increase signaling efficiency.

Juxta-membrane S-acylation of plant receptor-like kinases is likely fortuitous and does not necessarily impact upon function

S-acylation is a common post-translational modification of membrane protein cysteine residues with many regulatory roles. S-acylation adjacent to transmembrane domains has been described in the literature as affecting diverse protein properties including turnover, trafficking and microdomain partitioning. However, all of these data are derived from mammalian and yeast systems. Here we examine the role of S-acylation adjacent to the transmembrane domain of the plant pathogen perceiving receptor-like kinase FLS2. Surprisingly, S-acylation of FLS2 adjacent to the transmembrane domain is not required for either FLS2 trafficking or signalling function. Expanding this analysis to the wider plant receptor-like kinase family we find that S-acylation adjacent to receptor-like kinase domains is common, affecting ~25% of Arabidopsis receptor-like kinases, but poorly conserved between orthologues through evolution. This suggests that S-acylation of receptor-like kinases at this site is likely the result of chance mutation leading to cysteine occurrence. As transmembrane domains followed by cysteine residues are common motifs for S-acylation to occur, and many S-acyl transferases appear to have lax substrate specificity, we propose that many receptor-like kinases are fortuitously S-acylated once chance mutation has introduced a cysteine at this site. Interestingly some receptor-like kinases show conservation of S-acylation sites between orthologues suggesting that S-acylation has come to play a role and has been positively selected for during evolution. The most notable example of this is in the ERECTA-like family where S-acylation of ERECTA adjacent to the transmembrane domain occurs in all ERECTA orthologues but not in the parental ERECTA-like clade. This suggests that ERECTA S-acylation occurred when ERECTA emerged during the evolution of angiosperms and may have contributed to the neo-functionalisation of ERECTA from ERECTA-like proteins.

Variable Effects of C-Terminal Fusions on FLS2 Function: Not All Epitope Tags Are Created Equal

Receptor-like kinases (RLKs) are the largest family of proteins in plants and are responsible for perceiving the vast majority of extracellular stimuli. Thus, RLKs function in diverse processes, including sensing pathogen attacks, regulating symbiotic interactions, transducing hormone and peptide signals, and monitoring cell wall status. However, despite their fundamental role in plant biology, very few antibodies are available against RLKs, which necessitates the use of epitope tags and fluorescent protein fusions in biochemical analyses such as immunoblot analysis and intracellular visualization. Epitope tags are widely used and are typically assumed to be benign, with no influence on protein function. FLAGELLIN SENSITIVE2 (FLS2) is the receptor for bacterial flagellin and often is used as a model for RLK function. Previous work implies that carboxyl-terminal epitope fusions to FLS2 maintain protein function. Here, a detailed complementation analysis of Arabidopsis (Arabidopsis thaliana) fls2 mutant plants expressing various FLS2 C-terminal epitope fusions revealed highly variable and unpredictable FLS2-mediated signaling outputs. In addition, only one out of four FLS2 epitope fusions maintained the ability to inhibit plant growth in response to flg22 treatment comparable to that in the wild type or control untagged transgenic lines. These results raise concerns over the widespread use of RLK epitope tag fusions for functional studies. Many of the subtleties of FLS2 function, and by extension those of other RLKs, may have been overlooked or inappropriately interpreted through the use of RLK epitope tag fusions.

Maleimide scavenging enhances determination of protein S-palmitoylation state in acyl-exchange methods

S-palmitoylation (S-acylation) is emerging as an important dynamic post-translational modification of cysteine residues within proteins. Current assays for protein S-palmitoylation involve either in vivo labeling or chemical cleavage of S-palmitoyl groups to reveal a free cysteine sulfhydryl that can be subsequently labeled with an affinity handle (acyl-exchange). Assays for protein S-palmitoylation using acyl-exchange chemistry therefore require blocking of non-S-palmitoylated cysteines, typically using N-ethylmaleimide (NEM), to prevent non-specific detection. This in turn necessitates multiple precipitation-based clean-up steps to remove reagents between stages, often leading to variable sample loss, reduced signal, or protein aggregation. These combine to reduce the sensitivity, reliability, and accuracy of these assays, which also require a substantial amount of time to perform. By substituting these precipitation steps with chemical scavenging of NEM by 2,3-dimethyl-1,3-butadiene in an aqueous Diels-Alder 4+2 cyclo-addition reaction, it is possible to greatly improve sensitivity and accuracy while reducing the hands-on time and overall time required for the assay.

Current perspective on protein S-acylation in plants: more than just a fatty anchor?

Membranes are an important signalling platform in plants. The plasma membrane is the point where information about the external environment must be converted into intracellular signals, while endomembranes are important sites of protein trafficking, organization, compartmentalization, and intracellular signalling. This requires co-ordinating the spatial distribution of proteins, their activation state, and their interacting partners. This regulation frequently occurs through post-translational modification of proteins. Proteins that associate with the cell membrane do so through transmembrane domains, protein-protein interactions, lipid binding motifs/domains or use the post-translational addition of lipid groups as prosthetic membrane anchors. S-acylation is one such lipid modification capable of anchoring proteins to the membrane. Our current knowledge of S-acylation function in plants is fairly limited compared with other post-translational modifications and S-acylation in other organisms. However, it is becoming increasingly clear that S-acylation can act as more than just a simple membrane anchor: it can also act as a regulatory mechanism in signalling pathways in plants. S-acylation is, therefore, an ideal mechanism for regulating protein function at membranes. This review discusses our current knowledge of S-acylated proteins in plants, the interaction of different lipid modifications, and the general effects of S-acylation on cellular function.

The Arabidopsis mediator complex subunits MED16, MED14, and MED2 regulate mediator and RNA polymerase II recruitment to CBF-responsive cold-regulated genes

The Mediator16 (MED16; formerly termed SENSITIVE TO FREEZING6 [SFR6]) subunit of the plant Mediator transcriptional coactivator complex regulates cold-responsive gene expression in Arabidopsis thaliana, acting downstream of the C-repeat binding factor (CBF) transcription factors to recruit the core Mediator complex to cold-regulated genes. Here, we use loss-of-function mutants to show that RNA polymerase II recruitment to CBF-responsive cold-regulated genes requires MED16, MED2, and MED14 subunits. Transcription of genes known to be regulated via CBFs binding to the C-repeat motif/drought-responsive element promoter motif requires all three Mediator subunits, as does cold acclimation-induced freezing tolerance. In addition, these three subunits are required for low temperature-induced expression of some other, but not all, cold-responsive genes, including genes that are not known targets of CBFs. Genes inducible by darkness also required MED16 but required a different combination of Mediator subunits for their expression than the genes induced by cold. Together, our data illustrate that plants control transcription of specific genes through the action of subsets of Mediator subunits; the specific combination defined by the nature of the stimulus but also by the identity of the gene induced.

Modelling and experimental analysis of the role of interacting cytosolic and vacuolar pools in shaping low temperature calcium signatures in plant cells

A major challenge to understanding low temperature calcium signatures in plants is defining how these signatures emerge from the interactions of different molecular components that are stored in different subcellular pools of a plant cell. Here we develop an integrative model that incorporates the interactions of Ca²⁺, H⁺, K⁺, Cl⁻ and ATP in both cytosolic and vacuolar pools. Our analysis reveals how these four major ions along with ATP forms a complex network to relate the emergence of calcium signatures to other responses (e.g. pH response). Modelling results are in agreement with experimental observations for both cytosolic free calcium concentration ([Ca²⁺](c)) and pH. The model is further validated by experimentally measuring the response of [Ca²⁺](c) to six fluctuating (rather than constant) temperature profiles. We found that modelling results are in reasonable agreement with experimental observations, in particular, if the rate of reducing temperature is relatively high. In addition, we show that both calcium-induced calcium release (CICR) at the vacuolar membrane and transport of ions from the cytosolic pool to the vacuolar membrane play important roles in the interaction between cytosolic and vacuolar pools. In combination they control the amount and timing of calcium release from the vacuolar to cytosolic pool, shaping the specific calcium signature. The methodology and principles developed here establish an integrative view on the role of cytosolic and vacuolar pools in shaping calcium signatures in general, and they are universally applicable to study of the interactions of multiple subcellular pools.

A toolset of aequorin expression vectors for in planta studies of subcellular calcium concentrations in Arabidopsis thaliana

Calcium has long been acknowledged as one of the most important signalling components in plants. Many abiotic and biotic stimuli are transduced into a cellular response by temporal and spatial changes in cellular calcium concentration and the calcium-sensitive protein aequorin has been exploited as a genetically encoded calcium indicator for the measurement of calcium in planta. The objective of this work was to generate a compatible set of aequorin expression plasmids for the generation of transgenic plant lines to measure changes in calcium levels in different cellular subcompartments. Aequorin was fused to different targeting peptides or organellar proteins as a means to localize it to the cytosol, the nucleus, the plasma membrane, and the mitochondria. Furthermore, constructs were designed to localize aequorin in the stroma as well as the inner and outer surface of the chloroplast envelope membranes. The modular set-up of the plasmids also allows the easy replacement of targeting sequences to include other compartments. An additional YFP-fusion was included to verify the correct subcellular localization of all constructs by laser scanning confocal microscopy. For each construct, pBin19-based binary expression vectors driven by the 35S or UBI10 promoter were made for Agrobacterium-mediated transformation. Stable Arabidopsis lines were generated and initial tests of several lines confirmed their feasibility to measure calcium signals in vivo.

TCDD-mediated oxidative stress in male rat pups following perinatal exposure

2,3,7,8-tetrachlorododibenzo-p-dioxin (TCDD) is a highly persistent trace environmental contaminant and is one of the most potent toxicants known. Exposure to TCDD has been shown to cause oxidative stress in a variety of animal models. In this study, pregnant Long Evans rats were dosed with 1 microg TCDD/kg on gestational day (GD) 15 so as to investigate oxidative stress in the liver of male pups following gestational exposure to TCDD. Lipid peroxidation (TBARS), production of reactive oxygen species (ROS), and total glutathione (GSH) were assayed to identify changes in oxidative stress parameters in the pup liver at GD 21 and postnatal days (PND) 4, 25, 32, 49, and 63. Mean ROS levels in pups were elevated at all time points tested with a significant elevation at PND 4 and PND 25. However, pup hepatic lipid peroxidation was unchanged throughout the time course. In addition, hepatic total GSH levels were not significantly changed although the means for the TCDD-treated groups were less than those of the controls at all time points except PND 49. The results indicate that although the levels of ROS are increased following gestational/lactational exposure, this increase does not translate to direct oxidative damage or significant changes to endogenous antioxidant defense mechanisms. Further investigation into the effect of gestational/lactational exposure in pups should include additional endpoints for further characterization of the time course of the response, the effect upon extrahepatic tissues, and investigation of differences between male and female offspring.

Tissue disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in maternal and developing long-evans rats following subchronic exposure

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces alterations in the reproductive system of the developing pups. The objective of this study was to determine the disposition of TCDD in maternal and fetal Long-Evans (LE) rats following subchronic exposure, since the adverse reproductive and developmental effects have been extensively characterized in this strain of rat. LE rats were dosed by gavage with 1, 10, or 30 ng [(3)H]TCDD/kg in corn oil, 5 days/week for 13 weeks. At the end of 13 weeks, females were mated and dosing continued every day throughout gestation. Dams were sacrificed on gestation day (GD) 9, GD16, GD21, and post-natal day 4 and analyzed for [(3)H]TCDD-derived activity in maternal and fetal tissues. Maternal body burdens were equivalent at different time points, indicating that the dams were at steady state. Maternal body burdens were approximately 19, 120, and 300 ng TCDD/kg following doses of 1, 10, and 30 ng TCDD/kg, respectively. Individual embryo concentrations on GD9 were 1.6, 7, and 16 pg TCDD/g after maternal exposure of 1, 10, and 30 ng/kg/d, respectively. On GD 16, fetal liver, urogenital tract, head, and body concentrations were similar and averaged 1.4, 7.8, and 16.4 pg TCDD/g after administration of 1, 10, or 30 ng TCDD/kg/d, respectively, indicating no preferential sequestration within the different fetal tissues. These concentrations of TCDD within fetal tissues after subchronic exposure are comparable to those seen after a single dose of 50, 200, or 1000 ng TCDD/kg administered on GD15, a critical period of gestation.

Acute administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in pregnant Long Evans rats: association of measured tissue concentrations with developmental effects

Prenatal exposure to TCDD interferes with fetal development at doses lower than those causing overt toxicity in adult animals. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups- delayed puberty and reduced sperm counts in males and malformations in the external genitalia of females. The objectives of this study were to determine maternal and fetal tissue concentrations of TCDD after acute exposure and whether these tissue concentrations can be used to estimate the intensity of the developmental abnormalities reported by other laboratories. Pregnant Long Evans rats received a single, oral dose of 0.05, 0.20, 0.80, or 1.0 microg [3H]-TCDD/kg on gestation day (GD) 15, and maternal and fetal tissue concentrations of TCDD were measured on GD16 and GD21. On GD16, maternal liver contained the greatest amount of TCDD (30-47% administered dose). One day after administration of 0.20 microg TCDD/kg on GD15, there were 13.2 pg TCDD/g present in an individual fetus. This concentration is associated with delayed puberty and decreased epididymal sperm counts in male pups as well as malformations in the external genitalia of females. For the responses studied, tissue concentration measured during a critical period of gestation adequately predicts the intensity of the response. In addition, there was a strong correlation between fetal body burden and maternal body burden on GD16. A dose of 0.05 microg TCDD/kg resulted in maternal body burdens of 30.6+/-3.1 and 26.6+/-3.1 ng TCDD/kg on GD16 and GD21, respectively. In conclusion, low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups and that tissue concentration measured during a critical period is the appropriate dose metric to predict adverse reproductive and developmental effects.

2,3,7,8-Tetrachlorodibenzo-p-dioxin in pregnant Long Evans rats: disposition to maternal and embryo/fetal tissues

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) interferes with fetal development at doses lower than those causing overt toxicity in adult animals. In a multigeneration study (Murray et al., 1979), female rats that were administered 0.01 microgram TCDD/kg/day in their diet did not experience reduced fertility; however, reduced fertility was seen in the F1 and F2 generations. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups, such as delayed puberty and reduced sperm counts in males (Mably et al., 1992a; Gray et al., 1995) and malformations in the external genitalia of females (Gray and Ostby, 1995). Therefore, the objectives of this study were to determine maternal and fetal tissue concentrations of TCDD that are associated with the adverse reproductive effects seen by Gray and co-workers. Pregnant Long Evans rats received a single oral dose of 1.15 micrograms [3H]TCDD/kg on Gestation Day (GD) 8 and maternal as well as fetal tissue concentrations of TCDD were measured on GD9, GD16, and GD21. On GD9, the highest level of TCDD localized in the maternal liver (25.1% dose). In addition, the amount reaching all the embryos on GD9 was 0.01% of the administered dose, which resulted in a concentration of 0.02% dose/g. The amount of TCDD reaching the fetal compartment (fetuses + placentas) increased to 0.12% dose/tissue on GD16 and 0.71% by GD21. The concentration of TCDD within the fetal compartment (0.01% dose/g) on GD16 was comparable to that found in the maternal blood and spleen. Concentrations of TCDD in a single embryo/fetus were 39.6, 18.1, and 22.1 pg/g on GD9, GD16, and GD21, respectively. Estimates of hepatic half-life of elimination in pregnant rats suggested that TCDD may be eliminated faster in pregnant LE rats. Therefore, measurements of biliary elimination were made in pregnant and nonpregnant LE rats to compare rates of metabolism; however, biliary elimination of TCDD is not affected by pregnancy. In conclusion, this dose administered during a critical period of organogenesis causes adverse effects on the developing reproductive system of rodents. This dose produced a body burden of 22.1 pg TCDD/g within a single fetus on GD21. This indicates that low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups.

Pathways of hepatic oxalate synthesis and their regulation

Important features of hepatic oxalate synthesis remain uncertain despite its clinical significance. To clarify the terminal steps of the biosynthetic pathway and their modulation, we have examined oxalate and glyoxylate synthesis in vitro using isolated guinea pig peroxisomes and purified lactate dehydrogenase (LDH). Glycolate was rapidly oxidized to glyoxylate by isolated peroxisomes followed by a slower conversion of glyoxylate to oxalate. The glycolate oxidase (GO)-catalyzed conversion of glyoxylate to oxalate was strongly inhibited by physiological concentrations of glycolate and lactate. In contrast, the LDH-catalyzed conversion of glyoxylate to oxalate was only marginally affected by physiological concentrations of lactate and unaffected by physiological glycolate concentrations. This inhibition pattern suggests that LDH, not GO, catalyzes this conversion in vivo. Alanine inhibited oxalate synthesis by converting the bulk of the glyoxylate to glycine. On exposure to high alanine concentrations, however, inhibition was not complete and peroxisomes were able to convert sufficient glycolate to oxalate to account for daily endogenous oxalate production. NADH was a potent inhibitor of oxalate production by LDH by increasing glycolate formation from glyoxylate. Glycine was an ineffective source of glyoxylate, and an alkaline pH, a high-glycine concentration, and a prolonged incubation time were required to obtain a detectable synthesis. These results suggest that oxalate synthesis will be modulated by the metabolic state of the liver and resultant changes in NADH, lactate, and alanine levels.

Glucagon increases urinary oxalate excretion in the guinea pig

Factors that influence hepatic oxalate synthesis are poorly defined. Hormones are important regulators of hepatic metabolism and could potentially be involved. The effects of hyperglucagonemia were examined in guinea pigs injected with either saline or pharmacological doses of glucagon for 4 days. Glucagon treatment increased mean urinary oxalate excretion by 77% in male and 34% in female animals. The levels of hepatic peroxisomal enzymes involved in oxalate synthesis declined with glucagon treatment, but experiments with isolated peroxisomes indicated that oxalate synthesis in vitro was unaffected. Glucagon decreased hepatic alanine levels by 66%, lactate by 69%, and pyruvate by 73%, but glycolate and glyoxylate levels were unaffected. This decrease in alanine would substantially lower the activity of alanine-to-glyoxylate aminotransferase activity in vivo and make more glyoxylate available for oxalate synthesis. The decrease in lactate and pyruvate concentrations would stimulate the enzymatic conversion of glyoxylate to oxalate and may account for the increase in oxalate synthesis without an increase in glyoxylate concentration. These results demonstrate that hepatic oxalate synthesis is influenced by metabolic changes and that alterations in hepatic alanine, lactate, and pyruvate concentrations may be important elements.