Potential distribution and ecological impacts of Acmella radicans (Jacquin) R.K. Jansen (a new Yunnan invasive species record) in China.

BACKGROUND ACMELLA RADICANS: (Jacquin) R.K. Jansen is a new invasive species record for Yunnan Province, China. Native to Central America, it has also been recently recorded invading other parts of Asia. To prevent this weed from becoming a serious issue, an assessment of its ecological impacts and potential distribution is needed. We predicted the potential distribution of A. radicans in China using the MaxEnt model and its ecological impacts on local plant communities and soil nutrients were explored. RESULTS: Simulated training using model parameters produced an area under curve value of 0.974, providing a high degree of confidence in model predictions. Environmental variables with the greatest predictive power were precipitation of wettest month, isothermality, topsoil TEB (total exchangeable bases), and precipitation seasonality, with a cumulative contribution of more than 72.70% and a cumulative permutation importance of more than 69.20%. The predicted potential suitable area of A. radicans in China is concentrated in the southern region. Projected areas of A. radicans ranked as high and moderately suitable comprised 5425 and 26,338 km2, accounting for 0.06 and 0.27% of the Chinese mainland area, respectively. Over the 5 years of monitoring, the population density of A. radicans increased while at the same time the population density and importance values of most other plant species declined markedly. Community species richness, diversity, and evenness values significantly declined. Soil organic matter, total N, total P, available N, and available P concentrations decreased significantly with increasing plant cover of A. radicans, whereas pH, total K and available K increased. CONCLUSION: Our study was the first to show that A. radicans is predicted to expand its range in China and may profoundly affect plant communities, species diversity, and the soil environment. Early warning and monitoring of A. radicans must be pursued with greater vigilance in southern China to prevent its further spread.

Sweet potato (Ipomoea batatas) and hyacinth bean (Lablab purpureus) in combination provide greater suppression of mile-a-minute (Mikania micrantha) than either crop alone.

Introduction: In natural systems, diverse plant communities tend to prevent a single species from dominating. Similarly, management of invasive alien plants may be achieved through various combinations of competing species. Methods: We used a de Wit replacement series to compare different combinations of sweet potato (Ipomoea batatas (L.) Lam), hyacinth bean (Lablab purpureus (L.) Sweet) and mile-a-minute (Mikania micrantha Kunth) through measures of photosynthesis, plant growth, nutrient levels in plant tissue and soil, and competitive ability. Results: Cultured alone sweet potato and hyacinth beans exhibited higher total biomass, leafstalk length, and leaf area than mile-a-minute. In mixed culture, either sweet potato or hyacinth bean or both together significantly suppressed the mile-a-minute parameters, i.e., plant height, branch, leaf, adventitious root, and biomass (P<0.05). Based on a significantly lower than 1.0 relative yield of the three plant species in mixed culture, we showed intraspecific competition to be less than interspecific competition. Calculated indices (relative yield, relative yield total, competitive balance index, and change in contribution) demonstrated a higher competitive ability and higher influence of either crop compared to mile-a-minute. The presence of sweet potato and hyacinth bean, especially with both species in combination, significantly reduced (P<0.05) mile-a-minute's net photosynthetic rate (Pn), antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase, and malondialdehyde), chlorophyll content, and nutrient content (N, P, and K). In soil with mile-a-minute in monoculture soil organic matter, total and available N, total and available K, and available P were significantly greater (P<0.05) than in soil with sweet potato grown in monoculture, but less than in soil with hyacinth bean grown in monoculture soil. Nutrient soil content was comparatively reduced for plant mixtures. Plant height, leaf, biomass, Pn, antioxidant enzyme activities, and plant and soil nutrient contents of sweet potato and hyacinth bean tended to be much greater when grown with two crops compared to in mixture with just sweet potato or hyacinth bean. Discussion: Our results suggest that the competitive abilities of both sweet potato and hyacinth bean were greater than that of mile-a-minute, and also that mile-a-minute suppression was significantly improved via a combination of the two crops compared to either sweet potato or hyacinth bean alone.

Regulation of TFEB nuclear localization by HSP90AA1 promotes autophagy and longevity.

TFEB (transcription factor EB) regulates multiple genes involved in the process of macroautophagy/autophagy and plays a critical role in lifespan determination. However, the detailed mechanisms that regulate TFEB activity are not fully clear. In this study, we identified a role for HSP90AA1 in modulating TFEB. HSP90AA1 was phosphorylated by CDK5 at Ser 595 under basal condition. This phosphorylation inhibited HSP90AA1, disrupted its binding to TFEB, and impeded TFEB's nuclear localization and subsequent autophagy induction. Pro-autophagy signaling attenuated CDK5 activity and enhanced TFEB function in an HSP90AA1-dependent manner. Inhibition of HSP90AA1 function or decrease in its expression significantly attenuated TFEB's nuclear localization and transcriptional function following autophagy induction. HSP90AA1-mediated regulation of a TFEB ortholog was involved in the extended lifespan of Caenorhabditis elegans in the absence of its food source bacteria. Collectively, these findings reveal that this regulatory process plays an important role in modulation of TFEB, autophagy, and longevity.Abbreviations : AL: autolysosome; AP: autophagosome; ATG: autophagy related; BafA1: bafilomycin A1; CDK5: cyclin-dependent kinase 5; CDK5R1: cyclin dependent kinase 5 regulatory subunit 1; CR: calorie restriction; FUDR: 5-fluorodeoxyuridine; HSP90AA1: heat shock protein 90 alpha family class A member 1; MAP1LC3: microtubule associated protein 1 light chain 3; NB: novobiocin sodium; SQSTM1: sequestosome 1; TFEB: transcription factor EB; WT: wild type.

Allelochemicals Identified From Sweet Potato (Ipomoea batatas) and Their Allelopathic Effects on Invasive Alien Plants.

Sweet potato [Ipomoea batatas (L.) Lam] is grown as important cash and food crop worldwide and has been shown to exhibit allelopathic effects on other plants. However, its metabolome has not been studied extensively, particularly with respect to the production of phytotoxic bioactive secondary products. In this study, the chemical composition of petroleum ether extract of sweet potato was characterized, and the morphological and physiological effects of some individual components against four invasive alien weeds Bidens pilosa L., Galinsoga parviflora Cav., Lolium multiflorum Lam., and Phalaris minor Retz. were determined. Twenty-one components were identified by GS-MS, constituting 96.08% of petroleum ether extract in sweet potato. The major components were palmitic acid (PA) (17.48%), ethyl linoleate (EL) (13.19%), linoleic acid (LA) (12.55%), ethyl palmitate (EP) (11.77%), ethyl linolenate (ELL) (8.29%) oleic acid (5.82%), ethyl stearate (4.19%), and 3-methylphenol acetate (3.19%). The five most abundant compounds exhibited strong inhibition activity against the four invasive weeds tested. The highest inhibition rates were seen for LA, followed by PA and EP, respectively. Catalase (CAT), malondialdehyde (MDA), and peroxidase (POD) content of L. multiflorum were increased by the three allelochemicals, i.e., LA, PA and EP, but superoxide dismutase (SOD), chlorophyll-a and chlorophyll-b levels declined. Overall, the combined impact of all five compounds could be quite effective in suppressing the invasive weeds of concern.

Regulation of IFN-Is by MEF2D Promotes Inflammatory Homeostasis in Microglia.

BACKGROUND: Microglia play an essential role in the central nervous system immune response. The transcription factor myocyte enhancer factor-2 D (MEF2D) is known to participate in stress regulation in various cell types and is easily activated in microglia. MEF2D has been shown to transcriptionally regulate several cytokine genes in immune cells and directly regulates the inflammatory response, suggesting that MEF2D may act as a key stimulus response regulator of microglia and is involved in the regulation of brain microhomeostasis. To uncover the molecular mechanism of MEF2D in the inflammatory system, in the present study, we investigated the global effect of MEF2D in activated microglia and explored its potential regulatory network. METHODS: Experiments with a recombinant lentiviral vector containing either shRNA or overexpressing MEF2D were performed in the murine microglial BV2 cell line. Transcriptome sequencing and global gene expression patterns were analysed in lipopolysaccharide-stimulated shMEF2D BV2 cells. Pro- and anti-inflammatory factors were assessed by Western blot, qPCR or ELISA, and microglial activity was assessed by phagocytosis and morphologic analysis. The direct binding of MEF2D to the promoter region of interferon regulatory factor 7 (IRF7) was tested by ChIP-qPCR. The interferon-stimulated genes (ISGs) were tested by qPCR. RESULTS: MEF2D actively participated in the inflammatory response of BV2 microglial cells. Stably expressed RNAi-induced silencing of MEF2D disrupted the microglial immune balance in two ways: (1) the expression of proinflammatory factors, such as NLRP3, IL-1ß, and iNOS was promoted; and (2) the type-I interferon signalling pathway was markedly inhibited by directly modulating IRF7 transcription. In contrast, overexpression of MEF2D significantly reduced the expression of NLRP3 and iNOS under LPS stimulation and alleviated the level of immune stress in microglia. CONCLUSION: These findings demonstrate that MEF2D plays an important role in regulating inflammatory homeostasis partly through transcriptional regulation of the type-I interferon signalling pathway.

Adventitious roots support population expansion of the invasive plant Mikania micrantha Kunth.

Mikania micrantha Kunth is a serious invasive alien plant characterized by the formation of an adventitious root system in its prostrate growth form. Unlike the initial roots from seed germination, adventitious roots gradually appear above the stem and branch nodes. Little is known about adventitious roots play on plant growth and population expansion of M. micrantha. We hypothesized that adventitious roots provide an advantage for plant growth and nutrient availability. To test this hypothesis, plant growth, physiology, and nutrition characteristics of M. micrantha were measured under four soil surface conditions allowing various plant parts to touch the soil to stimulate variable adventitious root formation. The results showed that the biomass, stem length, branch number, and adventitious root biomass of M. micrantha were significantly increased (P < 0.05) with increasing nodes bearing adventitious roots. As the number of nodes with adventitious roots increased, the net photosynthetic rate, antioxidant enzyme activities like superoxide dismutase, catalase, peroxidase, and malondialdehyde, chlorophyll content, and plant nutrient contents (N, P, and K) of M. micrantha were increased (P < 0.05), with higher values in main stem leaves than in those of branch leaves. The concentrations of soil organic matter, total N, total P, total K, available N, available P, and available K were greater (P < 0.05) in initial soil (CK) than in treatment soil (with M. micrantha) and were significantly reduced by adventitious roots. Our study was the first to show that plant growth, physiology and nutrition status of M. micrantha were strongly promoted by adventitious roots in the prostrate growth form.

Potential use of Helianthus tuberosus to suppress the invasive alien plant Ageratina adenophora under different shade levels.

BACKGROUND: An ecological approach for managing biological invasions in agroecosystems is the selection of alternative crop species to manage the infestation of invasive alien plants through competition. In the current study, plant growth, photosynthesis, and competitive ability of the crop Helianthus tuberosus L. (Jerusalem artichoke) and the invasive alien plant Ageratina adenophora (Spreng.) R. M. King and H. Rob were compared under varying shade levels by utilizing a de Wit replacement series method. We hypothesized that H. tuberosus had higher competitive ability than A. adenophora even under shaded conditions. RESULTS: The results showed the main stem, leafstalk length, leaf area, underground biomass, and aboveground biomass of A. adenophora were significantly lower compared to H. tuberosus in monoculture although A. adenophora had a greater number of branches that were longer on average. Under full sunlight, the total shoot length (stem + branch length), main stem length and branch length of A. adenophora were significantly suppressed (P < 0.05) by increasing proportions of H. tuberosus, and the same morphological variables of H. tuberosus were significantly higher with decreasing proportions of H. tuberosus. With increasing shade rates and plant ratios, the plant height, branch, leaf, and biomass of both plants were significantly suppressed, but to a greater degree in the case of A. adenophora. The net photosynthetic rate (Pn) of H. tuberosus and A. adenophora increased gradually from July to September, then decreased in October. The Pn of H. tuberosus was consistently higher than that of A. adenophora. Although the Pn for both species was significantly reduced with increasing shade rates and plant ratios, A. adenophora experienced greater inhibition than H. tuberosus. The relative yield (RY) of A. adenophora was significantly less than 1.0 (P < 0.05) in mixed culture under all shade levels, indicating that the intraspecific competition was less than interspecific competition. The RY of H. tuberosus was significantly less than 1.0 under 40-60% shade and greater than 1.0 (P < 0.05) under 0-20% shade in mixed culture, respectively, showing that intraspecific competition was higher than interspecific competition under low shade, but the converse was true under high shade. The relative yield total (RYT) of A. adenophora and H. tuberosus was less than 1.0 in mixed culture, indicating that there was competition between the two plants. The fact that the competitive balance (CB) index of H. tuberosus was greater than zero demonstrated a higher competitive ability than A. adenophora even at the highest shade level (60%). CONCLUSIONS: Our results suggest that H. tuberosus is a promising replacement control candidate for managing infestations of A. adenophora, and could be widely used in various habitats where A. adenophora invades.

Fabrication of Ag-nanosheets-built micro/nanostructured arrays via in situ conversion on Cu2O-coated Si nanocone platform and their highly structurally-enhanced SERS effect.

A controllable and flexible route is presented for the fabrication of Ag-nanosheets-built micro/nanostructural ordered arrays via in situ conversion on the Cu2O-coated silicon nanocone (SNC) platform in the AgNO3-contained solution. The platform is pre-prepared by the reactive ion etching of the organic colloidal monolayer-covered silicon wafer, Cu sputtering deposition and in situ oxidation. The obtained Ag micro/nanostructured array consists of nearly spherical and micro-sized particles, which are hexagonally arranged on the substrate. The spherical particles are built of the vertically standing and cross-linked nanosheets with about 30 nm in thickness. This Ag-nanosheets-built array shows high number density of the edges and nanogaps as well as the robust and homogeneous structure. Its formation is attributed to the in situ conversion reaction on the Cu2O-coated SNC platform and the preferentially-oriented connection of Ag nanoparticles. Such Ag array has shown significantly higher surface enhanced Raman scattering (SERS) activity than the Ag nanoparticles' film-covered SNC array, with the enhancement factor up to 107 and the detection limitation down to ∼1 ppt level to the test molecules 4-aminothiophenol, as well as the good reproducibility in measurements. This study not only presents a controllable and flexible fabrication route to the plasmonic micro/nanostructured arrays but also provides the highly efficient and the practical chips for the SERS-based devices.

Controllable corrosion-assisted fabrication of Au-Ag alloyed hollow nanocrystals for highly efficient and environmentally-stable SERS substrates.

Surface enhanced Raman scattering (SERS) substrates with both high activity and long term chemical-stability have been expected in the practical application of the SERS-based detection. In this paper, Au-Ag bimetal nanocrystals are fabricated based on the template-etching reaction in the Ag nanocubes-contained cetylpyridinium chloride (CPC) aqueous solution via adding the HAuCl4 solution. The obtained nanocrystals are Au-Ag alloyed and hollow in structure. Further, it has been found that with the increasing Au/Ag molar ratio, the shape of the alloyed nanocrystals evolve from the truncated nanocubes to the hollow boxes and then nanocages, showing the ever red-shifting surface plasmon resonance from the visible to the infrared region. The formation of the alloyed hollow nanocrystals is attributed to the preferential dissolution of the Ag nanocubes induced by CPC selective adsorption and the three to one galvanic replacement reaction between Ag and Au atoms. Importantly, such Au-Ag alloyed hollow nanocrystals, especially the ones with a low Au/Ag atomic ratio, show both high SERS activity and long term environmental stability compared with pure Ag or Au nanocrystals, and are the ideal candidate for the SERS substrate with practical application value. This work not only demonstrates the nanofabrication route to the alloyed hollow nanocrystals with controllable shapes and tunable optical properties in a large region, but also presents highly active and chemically-stable SERS substrates for the practical SERS-based detection.

Loss of Drosha underlies dopaminergic neuron toxicity in models of Parkinson's disease.

MiRNAs, a group of powerful modulator of gene expression, participate in multiple cellular processes under physiological and pathological conditions. Emerging evidence shows that Drosha, which controls the initial step in canonical miRNA biogenesis, is involved in modulating cell survival and death in models of several diseases. However, the role of Drosha in Parkinson's disease (PD) has not been well established. Here, we show that the level of Drosha decreases in 6-OHDA-induced cellular and animal models of PD. 6-OHDA induced a p38 MAPK-dependent phosphorylation of Drosha. This triggered Drosha degradation. Enhancing the level of Drosha protected the dopaminergic (DA) neurons from 6-OHDA-induced toxicity in both in vitro and in vivo models of PD and alleviated the motor deficits of PD mice. These findings reveal that Drosha plays a critical role in the survival of DA neurons and suggest that stress-induced destabilization of Drosha may be part of the pathological process in PD.

Transcription Factors: Potential Cell Death Markers in Parkinson's Disease.

Parkinson's disease (PD) is a neurodegenerative disease with a long preclinical phase. The continuous loss of dopaminergic (DA) neurons is one of the pathogenic hallmarks of PD. Diagnosis largely depends on clinical observation, but motor dysfunctions do not emerge until 70%-80% of the nigrostriatal nerve terminals have been destroyed. Therefore, a biomarker that indicates the degeneration of DA neurons is urgently needed. Transcription factors are sequence-specific DNA-binding proteins that regulate RNA synthesis from a DNA template. The precise control of gene expression plays a critical role in the development, maintenance, and survival of cells, including DA neurons. Deficiency of certain transcription factors has been associated with DA neuron loss and PD. In this review, we focus on some transcription factors and discuss their structure, function, mechanisms of neuroprotection, and their potential for use as biomarkers indicating the degeneration of DA neurons.

Endoplasmic reticulum stress mediates distinct impacts of sevoflurane on different subfields of immature hippocampus.

Sevoflurane, a typical inhaled anesthetic, is widely used in patients of all ages during surgery. The negative effects, such as inducing cell death and damaging spatial memory, of sevoflurane on neurodevelopment have raised increasing concerns in recent years. However, the molecular mechanism remains unclear. This study focused on the crucial role of endoplasmic reticulum (ER) stress in sevoflurane-induced hippocampal injury. Three-week-old rats were exposed to sevoflurane or control air for 5 h with or without ER stress inhibitor (4-phenylbutyric acid, 4-PBA) injection. The hippocampus was harvested to measure the ER stress sensors by western immunoblotting. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling staining was used to detect cell apoptosis and electrophysiology was used to measure the intrinsic excitability of neurons in hippocampus. We measured learning and memory ability by Morris water maze tests 5 weeks after sevoflurane exposure. Interestingly, persistent sevoflurane exposure significantly increased the levels of ER stress sensors in hippocampus. But it resulted in different effects in CA1 and dentate gyrus. Greatly increased caspase-12-mediated apoptotic cells, which were proved to be the neural stem cells, were detected in the dentate gyrus. Meanwhile, CA1 pyramidal neurons exhibited significantly reduced intrinsic excitability. Furthermore, the administration of ER stress inhibitor attenuated the above mentioned detrimental effects evidently and prevented the following relevant learning and memory deficits. In conclusion, sevoflurane-mediated ER stress performs distinct effects on the different subfields of the immature hippocampus and inhibiting ER stress during sevoflurane anesthesia will be a potential method to prevent the following learning and memory deficits in adulthood.

Regulation of ER stress-induced autophagy by GSK3ß-TIP60-ULK1 pathway.

Endoplasmic reticulum (ER) stress is involved in many cellular processes. Emerging evidence suggests that ER stress can trigger autophagy; however, the mechanisms by which ER stress regulates autophagy and its role in this condition are not fully understood. HIV Tat-interactive protein, 60 kDa (TIP60) is a newly discovered acetyltransferase that can modulate autophagy flux by activating ULK1 upon growth factor deprivation. In this study, we investigated the mechanisms by which ER stress induces autophagy. We showed that ER stress activates glycogen synthase kinase-3ß (GSK3ß). This led to a GSK3ß-dependent phosphorylation of TIP60, triggering a TIP60-mediated acetylation of ULK1 and activation of autophagy. Inhibition of either GSK3ß or TIP60 acetylation activities significantly attenuated ER stress-induced autophagy. Moreover, enhancing the level of TIP60 attenuated the level of CHOP after ER stress, and reduced the ER stress-induced cell death. In contrast, expression of TIP60 mutant that could not be phosphorylated by GSK3ß exacerbated the generation of CHOP and increased the ER stress-induced cell death. These findings reveal that ER stress engages the GSK3ß-TIP60-ULK1 pathway to increase autophagy. Attenuation of this pathway renders cells more sensitive to and increases the toxicity of ER stress.

Transcription factor myocyte enhancer factor 2D regulates interleukin-10 production in microglia to protect neuronal cells from inflammation-induced death.

BACKGROUND: Neuroinflammatory responses have been recognized as an important aspect in the pathogenesis of Parkinson's disease (PD). Transcriptional regulation plays a critical role in the process of inflammation. Transcription factor myocyte enhancer factor 2D (MEF2D) is identified as a central factor in transmission of extracellular signals and activation of the genetic programs in response to a wide range of stimuli in several cell types, including neurons. But its presence and function in microglia have not been reported. We therefore investigated the effect of MEF2D in activated microglia on the progress of neuroinflammation and the survival of neurons. METHODS: BV2 cells and primary cultured glial cells were stimulated with lipopolysaccharide (LPS). Samples from cells were examined for MEF2D expression, interleukin-10 (IL-10), and tumor necrosis factor alpha (TNF-α) by immunoblotting, quantitative real-time PCR (qPCR) or enzyme-linked immunosorbent assay (ELISA). The activity of MEF2D was examined by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP). Recombinant lentivirus expressing shRNA specific to MEF2D was used to silence MEF2D expression in BV2 cells. The role of IL-10 transcriptionally induced by MEF2D on neuronal survival was assessed by anti-IL-10 neutralizing antibody. The survival of neurons was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining. Male C57bl/6 mice were used to establish an acute PD model. Brain sections and cell slides were tested by immunofluorescence. RESULTS: We demonstrated that MEF2D was present in microglia. Activation of microglia was associated with an increase in MEF2D level and activity in response to different stimuli in vivo and in vitro. MEF2D bound to a MEF2 consensus site in the promoter region of IL-10 gene and stimulated IL-10 transcription. Silencing MEF2D decreased the level of IL-10, increased the TNF-α mRNA, and promoted inflammation-induced cytotoxicity, consistent with the result of inhibiting IL-10 activity with an anti-IL-10 neutralizing antibody. CONCLUSIONS: Our study identifies MEF2D as a critical regulator of IL-10 gene expression that negatively controls microglia inflammation response and prevents inflammation-mediated cytotoxicity.

An ARC/Mediator subunit required for SREBP control of cholesterol and lipid homeostasis.

The sterol regulatory element binding protein (SREBP) family of transcription activators are critical regulators of cholesterol and fatty acid homeostasis. We previously demonstrated that human SREBPs bind the CREB-binding protein (CBP)/p300 acetyltransferase KIX domain and recruit activator-recruited co-factor (ARC)/Mediator co-activator complexes through unknown mechanisms. Here we show that SREBPs use the evolutionarily conserved ARC105 (also called MED15) subunit to activate target genes. Structural analysis of the SREBP-binding domain in ARC105 by NMR revealed a three-helix bundle with marked similarity to the CBP/p300 KIX domain. In contrast to SREBPs, the CREB and c-Myb activators do not bind the ARC105 KIX domain, although they interact with the CBP KIX domain, revealing a surprising specificity among structurally related activator-binding domains. The Caenorhabditis elegans SREBP homologue SBP-1 promotes fatty acid homeostasis by regulating the expression of lipogenic enzymes. We found that, like SBP-1, the C. elegans ARC105 homologue MDT-15 is required for fatty acid homeostasis, and show that both SBP-1 and MDT-15 control transcription of genes governing desaturation of stearic acid to oleic acid. Notably, dietary addition of oleic acid significantly rescued various defects of nematodes targeted with RNA interference against sbp-1 and mdt-15, including impaired intestinal fat storage, infertility, decreased size and slow locomotion, suggesting that regulation of oleic acid levels represents a physiologically critical function of SBP-1 and MDT-15. Taken together, our findings demonstrate that ARC105 is a key effector of SREBP-dependent gene regulation and control of lipid homeostasis in metazoans.

Structural analysis of autoinhibition in the Ras activator Son of sevenless.

The classical model for the activation of the nucleotide exchange factor Son of sevenless (SOS) involves its recruitment to the membrane, where it engages Ras. The recent discovery that Ras*GTP is an allosteric activator of SOS indicated that the regulation of SOS is more complex than originally envisaged. We now present crystallographic and biochemical analyses of a construct of SOS that contains the Dbl homology-pleckstrin homology (DH-PH) and catalytic domains and show that the DH-PH unit blocks the allosteric binding site for Ras and suppresses the activity of SOS. SOS is dependent on Ras binding to the allosteric site for both a lower level of activity, which is a result of Ras*GDP binding, and maximal activity, which requires Ras*GTP. The action of the DH-PH unit gates a reciprocal interaction between Ras and SOS, in which Ras converts SOS from low to high activity forms as Ras*GDP is converted to Ras*GTP by SOS.

Autoinhibition of Sos by intramolecular interactions.

Sos proteins function as activators of Ras signaling by catalyzing guanine nucleotide exchange on Ras. Sos regulation was initially thought to be accomplished primarily through its growth factor-dependent recruitment to the plasma membrane. More recent data has indicated that while membrane association is an indispensable means of Sos regulation, additional mechanisms involving intramolecular interactions function to control Sos activity towards Ras. This review will examine the experimental evidence for Sos intramolecular interactions and their contribution to Sos regulation.