YidC from Escherichia coli Forms an Ion-Conducting Pore upon Activation by Ribosomes

The universally conserved protein YidC aids in the insertion and folding of transmembrane polypeptides. Supposedly, a charged arginine faces its hydrophobic lipid core, facilitating polypeptide sliding along YidC's surface. How the membrane barrier to other molecules may be maintained is unclear. Here, we show that the purified and reconstituted E. coli YidC forms an ion-conducting transmembrane pore upon ribosome or ribosome-nascent chain complex (RNC) binding. In contrast to monomeric YidC structures, an AlphaFold parallel YidC dimer model harbors a pore. Experimental evidence for a dimeric assembly comes from our BN-PAGE analysis of native vesicles, fluorescence correlation spectroscopy studies, single-molecule fluorescence photobleaching observations, and crosslinking experiments. In the dimeric model, the conserved arginine and other residues interacting with nascent chains point into the putative pore. This result suggests the possibility of a YidC-assisted insertion mode alternative to the insertase mechanism.

Entropic barrier of water permeation through single-file channels

Facilitated water permeation through narrow biological channels is fundamental for all forms of life. Despite its significance in health and disease as well as for biotechnological applications, the energetics of water permeation are still elusive. Gibbs free energy of activation is composed of an enthalpic and an entropic component. Whereas the enthalpic contribution is readily accessible via temperature dependent water permeability measurements, estimation of the entropic contribution requires information on the temperature dependence of the rate of water permeation. Here, we estimate, by means of accurate activation energy measurements of water permeation through Aquaporin-1 and by determining the accurate single channel permeability, the entropic barrier of water permeation through a narrow biological channel. Thereby the calculated value for [Formula: see text] = 2.01 ± 0.82 J/(mol·K) links the activation energy of 3.75 ± 0.16 kcal/mol with its efficient water conduction rate of ~10¹⁰ water molecules/second. This is a first step in understanding the energetic contributions in various biological and artificial channels exhibiting vastly different pore geometries.

Trapped Pore Waters in the Open Proton Channel HV 1

The voltage-gated proton channel, H_V 1, is crucial for innate immune responses. According to alternative hypotheses, protons either hop on top of an uninterrupted water wire or bypass titratable amino acids, interrupting the water wire halfway across the membrane. To distinguish between both hypotheses, the water mobility for the putative case of an uninterrupted wire is estimated. The predicted single-channel water permeability 2.3 × 10^-12 cm³ s^-1 reflects the permeability-governing number of hydrogen bonds between water molecules in single-file configuration and pore residues. However, the measured unitary water permeability does not confirm the predicted value. Osmotic deflation of reconstituted lipid vesicles reveals negligible water permeability of the H_V 1 wild-type channel and the D174A mutant open at 0 mV. The conductance of 1400 H⁺ s^-1 per wild-type channel agrees with the calculated diffusion limit for a ≈2 Å capture radius for protons. Removal of a charged amino acid (D174) at the pore mouth decreases H⁺ conductance by reducing the capture radius. At least one intervening amino acid contributes to H⁺ conductance while interrupting the water wire across the membrane.

The Hidden Intricacies of Aquaporins: Remarkable Details in a Common Structural Scaffold

Evolution turned aquaporins (AQPs) into the most efficient facilitators of passive water flow through cell membranes at no expense of solute discrimination. In spite of a plethora of solved AQP structures, many structural details remain hidden. Here, by combining extensive sequence- and structural-based analysis of a unique set of 20 non-redundant high-resolution structures and molecular dynamics simulations of four representatives, key aspects of AQP stability, gating, selectivity, pore geometry, and oligomerization, with a potential impact on channel functionality, are identified. The general view of AQPs possessing a continuous open water pore is challenged and it is depicted that AQPs' selectivity is not exclusively shaped by pore-lining residues but also by the relative arrangement of transmembrane helices. Moreover, this analysis reveals that hydrophobic interactions constitute the main determinant of protein thermal stability. Finally, a numbering scheme of the conserved AQP scaffold is established, facilitating direct comparison of, for example, disease-causing mutations and prediction of potential structural consequences. Additionally, the results pave the way for the design of optimized AQP water channels to be utilized in biotechnological applications.

Scattering versus fluorescence self-quenching: more than a question of faith for the quantification of water flux in large unilamellar vesicles?

The endeavors to understand the determinants of water permeation through membrane channels, the effect of the lipid or polymer membrane on channel function, the development of specific water flow inhibitors, the design of artificial water channels and aquaporins for the use in industrial water filtration applications all rely on accurate ways to quantify water permeabilities (P f). A commonly used method is to reconstitute membrane channels into large unilamellar vesicles (LUVs) and to subject these vesicles to an osmotic gradient in a stopped-flow device. Fast recordings of either scattered light intensity or fluorescence self-quenching signals are taken as a readout for vesicle volume change, which in turn can be recalculated to accurate P f values. By means of computational and experimental data, we discuss the pros and cons of using scattering versus self-quenching experiments or subjecting vesicles to hypo- or hyperosmotic conditions. In addition, we explicate for the first time the influence of the LUVs size distribution, channel distribution between vesicles and remaining detergent after protein reconstitution on P f values. We point out that results such as the single channel water permeability (p f) depend on the membrane matrix or on the direction of the applied osmotic gradient may be direct results of the measurement and analysis procedure.

Interaction of the motor protein SecA and the bacterial protein translocation channel SecYEG in the absence of ATP

Translocation of many secretory proteins through the bacterial plasma membrane is facilitated by a complex of the SecYEG channel with the motor protein SecA. The ATP-free complex is unstable in detergent, raising the question how SecA may perform several rounds of ATP hydrolysis without being released from the membrane embedded SecYEG. Here we show that dual recognition of (i) SecYEG and (ii) vicinal acidic lipids confers an apparent nanomolar affinity. High-speed atomic force microscopy visualizes the complexes between monomeric SecA and SecYEG as being stable for tens of seconds. These long-lasting events and complementary shorter ones both give rise to single ion channel openings of equal duration. Furthermore, luminescence resonance energy transfer reveals two conformations of the SecYEG-SecA complex that differ in the protrusion depth of SecA's two-helix finger into SecYEG's aqueous channel. Such movement of the finger is in line with the power stroke mechanism of protein translocation.

Micropipette Aspiration-Based Assessment of Single Channel Water Permeability

Measurements of the unitary hydraulic conductivity of membrane channels, p_f , may be hampered by difficulties in producing sufficient quantities of purified and reconstituted proteins. Low yield expression, the purely empiric choice of detergents, as well as protein aggregation and misfolding during reconstitution may result in an average of less than one reconstituted channel per large unilamellar vesicle. This limits their applicability for p_f measurements, independent of whether light scattering or fluorescence quenching of encapsulated dyes is monitored. Here the micropipette aspiration technique is adopted because its superb sensitivity allows resolving p_f values for one order of magnitude smaller protein densities in sphingomyelin and cholesterol rich giant unilamellar vesicles (GUVs). Protein density is derived from intensity fluctuations that fluorescently labeled channels in the aspirated GUV induce by diffusing through the diffraction limited spot. A perfusion system minimizes unstirred layers in the immediate membrane vicinity as demonstrated by the distribution of both encapsulated and extravesicular aqueous dyes. p_f amounted to 2.4 ± 0.1 × 10^-13 cm³ s^-1 for aquaporin-1 that served as a test case. The new assay paves the way for directly monitoring the effect that interaction of aquaporins with other proteins or inhibitors may have on p_f on a single sample.

Author Correction: Quantification of Water Flux in Vesicular Systems

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Voltage Sensing in Bacterial Protein Translocation

The bacterial channel SecYEG efficiently translocates both hydrophobic and hydrophilic proteins across the plasma membrane. Translocating polypeptide chains may dislodge the plug, a half helix that blocks the permeation of small molecules, from its position in the middle of the aqueous translocation channel. Instead of the plug, six isoleucines in the middle of the membrane supposedly seal the channel, by forming a gasket around the translocating polypeptide. However, this hypothesis does not explain how the tightness of the gasket may depend on membrane potential. Here, we demonstrate voltage-dependent closings of the purified and reconstituted channel in the presence of ligands, suggesting that voltage sensitivity may be conferred by motor protein SecA, ribosomes, signal peptides, and/or translocating peptides. Yet, the presence of a voltage sensor intrinsic to SecYEG was indicated by voltage driven closure of pores that were forced-open either by crosslinking the plug to SecE or by plug deletion. We tested the involvement of SecY's half-helix 2b (TM2b) in voltage sensing, since clearly identifiable gating charges are missing. The mutation L80D accelerated voltage driven closings by reversing TM2b's dipolar orientation. In contrast, the L80K mutation decelerated voltage induced closings by increasing TM2b's dipole moment. The observations suggest that TM2b is part of a larger voltage sensor. By partly aligning the combined dipole of this sensor with the orientation of the membrane-spanning electric field, voltage may drive channel closure.

Quantification of Water Flux in Vesicular Systems

Water transport across lipid membranes is fundamental to all forms of life and plays a major role in health and disease. However, not only typical water facilitators like aquaporins facilitate water flux, but also transporters, ion channels or receptors represent potent water pathways. The efforts directed towards a mechanistic understanding of water conductivity determinants in transmembrane proteins, the development of water flow inhibitors, and the creation of biomimetic membranes with incorporated membrane proteins or artificial water channels depend on reliable and accurate ways of quantifying water permeabilities Pf. A conventional method is to subject vesicles to an osmotic gradient in a stopped-flow device: Fast recordings of scattered light intensity are converted into the time course of vesicle volume change. Even though an analytical solution accurately acquiring Pf from scattered light intensities exists, approximations potentially misjudging Pf by orders of magnitude are used. By means of computational and experimental data we point out that erroneous results such as that the single channel water permeability pf depends on the osmotic gradient are direct results of such approximations. Finally, we propose an empirical solution of which calculated permeability values closely match those calculated with the analytical solution in the relevant range of parameters.

Positively charged residues at the channel mouth boost single-file water flow

Positively charged residues in the vicinity of the channel entrance or exit accelerate single-file water flow.

Water molecules lose two of their four bulk neighbours when entering single-file channels. This process may be sensitive to the presence of positive and negative charges at the channel mouth, since the costs for dehydrating cations and anions differ by a large margin. However, it is not known whether entrance charges affect the single channel water permeability (p_f). So far, p_f is only known to be governed by H-bond formation between permeating water molecules and wall-lining residues. Here we compare the p_f values of five different aquaporin species (AQP1, AQPZ, AQP4 wild type, and two phosphorylation mimicking AQP4 mutants) that offer the same number of hydrogen bond donating and receiving residues in their single-file region but display different entrance charges. The p_f measurements were performed with reconstituted lipid vesicles. We assessed (i) the osmotically induced vesicle deflation from the light scattering intensity in a stopped-flow device and (ii) the aquaporin abundance by fluorescence correlation spectroscopy. Substitution of serine at positions 111 and 180 in AQP4 for aspartic acid showed only a marginal effect on p_f, suggesting that negative entrance charges are of minor importance. In contrast, the total number of positively charged amino acid side chains at entrances and exits correlates with p_f: a total of three, four and seven charges of AQP4, AQPZ, and AQP1 translate into p_f values of 1.1, 1.8, and 3.2 × 10^–13 cm³ s^–1, respectively. Thus, positive interfacial charges boost the p_f value of AQP1 to three times the value of AQP4. Nevertheless, the number of hydrogen bond donating and receiving residues in the single-file region remains the major determinant of p_f. Their effect on p_f may be a hundredfold larger than that of interfacial charges.

YidC and SecYEG form a heterotetrameric protein translocation channel

The heterotrimeric SecYEG complex cooperates with YidC to facilitate membrane protein insertion by an unknown mechanism. Here we show that YidC contacts the interior of the SecY channel resulting in a ligand-activated and voltage-dependent complex with distinct ion channel characteristics. The SecYEG pore diameter decreases from 8 Å to only 5 Å for the YidC-SecYEG pore, indicating a reduction in channel cross-section by YidC intercalation. In the presence of a substrate, YidC relocates to the rim of the pore as indicated by increased pore diameter and loss of YidC crosslinks to the channel interior. Changing the surface charge of the pore by incorporating YidC into the channel wall increases the anion selectivity, and the accompanying change in wall hydrophobicity is liable to alter the partition of helices from the pore into the membrane. This could explain how the exit of transmembrane domains from the SecY channel is facilitated by YidC.

Tuning membrane protein mobility by confinement into nanodomains

High-speed atomic force microscopy (HS-AFM) can be used to visualize function-related conformational changes of single soluble proteins. Similar studies of single membrane proteins are, however, hampered by a lack of suitable flat, non-interacting membrane supports and by high protein mobility. Here we show that streptavidin crystals grown on mica-supported lipid bilayers can be used as porous supports for membranes containing biotinylated lipids. Using SecYEG (protein translocation channel) and GlpF (aquaglyceroporin), we demonstrate that the platform can be used to tune the lateral mobility of transmembrane proteins to any value within the dynamic range accessible to HS-AFM imaging through glutaraldehyde-cross-linking of the streptavidin. This allows HS-AFM to study the conformation or docking of spatially confined proteins, which we illustrate by imaging GlpF at sub-molecular resolution and by observing the motor protein SecA binding to SecYEG.

The Sodium Glucose Cotransporter SGLT1 Is an Extremely Efficient Facilitator of Passive Water Transport

The small intestine is void of aquaporins adept at facilitating vectorial water transport, and yet it reabsorbs ∼8 liters of fluid daily. Implications of the sodium glucose cotransporter SGLT1 in either pumping water or passively channeling water contrast with its reported water transporting capacity, which lags behind that of aquaporin-1 by 3 orders of magnitude. Here we overexpressed SGLT1 in MDCK cell monolayers and reconstituted the purified transporter into proteoliposomes. We observed the rate of osmotic proteoliposome deflation by light scattering. Fluorescence correlation spectroscopy served to assess (i) SGLT1 abundance in both vesicles and plasma membranes and (ii) flow-mediated dilution of an aqueous dye adjacent to the cell monolayer. Calculation of the unitary water channel permeability, pf, yielded similar values for cell and proteoliposome experiments. Neither the absence of glucose or Na(+), nor the lack of membrane voltage in vesicles, nor the directionality of water flow grossly altered pf Such weak dependence on protein conformation indicates that a water-impermeable occluded state (glucose and Na(+) in their binding pockets) lasts for only a minor fraction of the transport cycle or, alternatively, that occlusion of the substrate does not render the transporter water-impermeable as was suggested by computational studies of the bacterial homologue vSGLT. Although the similarity between the pf values of SGLT1 and aquaporin-1 makes a transcellular pathway plausible, it renders water pumping physiologically negligible because the passive flux would be orders of magnitude larger.

The mobility of single-file water molecules is governed by the number of H-bonds they may form with channel-lining residues

Channel geometry governs the unitary osmotic water channel permeability, p_f, according to classical hydrodynamics. Yet, p_f varies by several orders of magnitude for membrane channels with a constriction zone that is one water molecule in width and four to eight molecules in length. We show that both the p_f of those channels and the diffusion coefficient of the single-file waters within them are determined by the number N_H of residues in the channel wall that may form a hydrogen bond with the single-file waters. The logarithmic dependence of water diffusivity on N_H is in line with the multiplicity of binding options at higher N_H densities. We obtained high-precision p_f values by (i) having measured the abundance of the reconstituted aquaporins in the vesicular membrane via fluorescence correlation spectroscopy and via high-speed atomic force microscopy, and (ii) having acquired the vesicular water efflux from scattered light intensities via our new adaptation of the Rayleigh-Gans-Debye equation.

High-speed AFM images of thermal motion provide stiffness map of interfacial membrane protein moieties

The flexibilities of extracellular loops determine ligand binding and activation of membrane receptors. Arising from fluctuations in inter- and intraproteinaceous interactions, flexibility manifests in thermal motion. Here we demonstrate that quantitative flexibility values can be extracted from directly imaging the thermal motion of membrane protein moieties using high-speed atomic force microscopy (HS-AFM). Stiffness maps of the main periplasmic loops of single reconstituted water channels (AqpZ, GlpF) revealed the spatial and temporal organization of loop-stabilizing intraproteinaceous H-bonds and salt bridges.

Ion conductivity of the bacterial translocation channel SecYEG engaged in translocation

While engaged in protein transport, the bacterial translocon SecYEG must maintain the membrane barrier to small ions. The preservation of the proton motif force was attributed to (i) cation exclusion, (ii) engulfment of the nascent chain by the hydrophobic pore ring, and (iii) a half-helix partly plugging the channel. In contrast, we show here that preservation of the proton motif force is due to a voltage-driven conformational change. Preprotein or signal peptide binding to the purified and reconstituted SecYEG results in large cation and anion conductivities only when the membrane potential is small. Physiological values of membrane potential close the activated channel. This voltage-dependent closure is not dependent on the presence of the plug domain and is not affected by mutation of 3 of the 6 constriction residues to glycines. Cellular ion homeostasis is not challenged by the small remaining leak conductance.

The bacterial translocon SecYEG opens upon ribosome binding

In co-translational translocation, the ribosome funnel and the channel of the protein translocation complex SecYEG are aligned. For the nascent chain to enter the channel immediately after synthesis, a yet unidentified signal triggers displacement of the SecYEG sealing plug from the pore. Here, we show that ribosome binding to the resting SecYEG channel triggers this conformational transition. The purified and reconstituted SecYEG channel opens to form a large ion-conducting channel, which has the conductivity of the plug deletion mutant. The number of ion-conducting channels inserted into the planar bilayer per fusion event roughly equals the number of SecYEG channels counted by fluorescence correlation spectroscopy in a single proteoliposome. Thus, the open probability of the channel must be close to unity. To prevent the otherwise lethal proton leak, a closed post-translational conformation of the SecYEG complex bound to a ribosome must exist.

Musculin and TCF21 coordinate the maintenance of myogenic regulatory factor expression levels during mouse craniofacial development

The specification of the skeletal muscle lineage during craniofacial development is dependent on the activity of MYF5 and MYOD, two members of the myogenic regulatory factor family. In the absence of MYF5 or MYOD there is not an overt muscle phenotype, whereas in the double Myf5;MyoD knockout branchiomeric myogenic precursors fail to be specified and skeletal muscle is not formed. The transcriptional regulation of Myf5 is controlled by a multitude of regulatory elements acting at different times and anatomical locations, with at least five operating in the branchial arches. By contrast, only two enhancers have been implicated in the regulation of MyoD. In this work, we characterize an enhancer element that drives Myf5 expression in the branchial arches from 9.5 days post-coitum and show that its activity in the context of the entire locus is dependent on two highly conserved E-boxes. These binding sites are required in a subset of Myf5-expressing cells including both progenitors and those which have entered the myogenic pathway. The correct levels of expression of Myf5 and MyoD result from activation by musculin and TCF21 through direct binding to specific enhancers. Consistent with this, we show that in the absence of musculin the timing of activation of Myf5 and MyoD is not affected but the expression levels are significantly reduced. Importantly, normal levels of Myf5 expression are restored at later stages, which might explain the absence of particular muscles in the Msc;Tcf21 double-knockout mice.

Members of the TEAD family of transcription factors regulate the expression of Myf5 in ventral somitic compartments

The transcriptional regulation of the Mrf4/Myf5 locus depends on a multitude of enhancers that, in equilibria with transcription balancing sequences and the promoters, regulate the expression of the two genes throughout embryonic development and in the adult. Transcription in a particular set of muscle progenitors can be driven by the combined outputs of several enhancers that are not able to recapitulate the entire expression pattern in isolation, or by the action of a single enhancer the activity of which in isolation is equivalent to that within the context of the locus. We identified a new enhancer element of this second class, ECR111, which is highly conserved in all vertebrate species and is necessary and sufficient to drive Myf5 expression in ventro-caudal and ventro-rostral somitic compartments in the mouse embryo. EMSA analyses and data obtained from binding-site mutations in transgenic embryos show that a binding site for a TEA Domain (TEAD) transcription factor is essential for the function of this new enhancer, while ChIP assays show that at least two members of the family of transcription factors bind to it in vivo.

EWS-FLI1 in Ewing's sarcoma: real targets and collateral damage

Ewing's sarcoma family of tumors (ESFT) are a clinically and scientifically very demanding group of tumors in children and young adults with still unknown histogenesis. The rate-limiting oncogenic mutation in this disease has been identified as a chromosomal translocation, t(11;22)(q24;q12), that leads to the expression of a chimeric transcription factor, EWS-FLI1. We have studied the downstream pathway of EWS-FLI1 by a dual strategy including the isolation of direct target genes from ESFT chromatin and the monitoring of transcriptomic changes after silencing of EWS-FLI1 by RNA interference. This study has lead to the identification of several directly EWS-FLI1-regulated genes and the characterization of their genomic distribution. By comparing several ESFT cell lines, not only variation in overall gene expression patterns downstream of EWS-FLIl was observed, but also differential regulation of directly EWS-FLI1-bound genes. Interestingly, there was variation between members of the same functional gene families. Studies on CD99, another diagnostic hallmark of ESFT, in relation to EWS-FLI1 provided additional evidence for context dependence of fusion protein function. Together, our study represents a first approach to the separation of essential molecular consequences from noise generated by the EWS-FLI1 gene rearrangement in ESFT.

EWS-FLI1 target genes recovered from Ewing's sarcoma chromatin

In all, 85% of Ewing's sarcoma family tumors (ESFT), a neoplasm of unknown histogenesis, express EWS-FLI1 transcription factor gene fusions. To characterize direct target genes avoiding artificial model systems, we cloned genomic DNA from ESFT chromatin precipitating with EWS-FLI1. We now present a comprehensive list of 99 putative transcription factor targets identified, for the first time, by a hypothesis-free approach based on physical interaction. Gene-derived chromatin fragments co-precipitating with EWS-FLI1 were nonrandomly distributed over the human genome and localized predominantly to the upstream region and the first two introns of the genes. At least 20% of putative direct EWS-FLI1 targets were neural genes. One-third of genes recovered showed a significant ESFT-specific expression pattern and were found to be altered upon RNAi-mediated knockdown of EWS-FLI1. Among them, MK-STYX, encoding a MAP kinase phosphatase-like protein, was consistently expressed in ESFT. EWS-FLI1 was found to drive MK-STYX expression by binding to a single ETS binding motif within the first gene intron. MK-STYX serves as precedence for successful recovery of direct EWS-FLI1 targets from the authentic ESFT cellular context, the most relevant system to study oncogenic mechanisms for the discovery of new therapeutic targets in this disease.

Constitutive and DNA damage inducible activation of pig3 and MDM2 genes by tumor-derived p53 mutant C277Y

The p53 gene is compromised in most human cancers by point mutation. Evidence is accumulating that these alterations frequently do not result in a complete loss of the sequence-specific transcriptional regulatory function of p53. Here, we describe the transcriptional activity of the p53 mutant C277Y isolated from a Ewing's sarcoma with high constitutive pig3 expression. Transient transfection of this mutant into a p53 null cell line resulted in activation not only of the pig3 but also of the MDM2 gene compatible with the presence of constitutively expressed MDM2 transcripts initiated from the P2 promoter in the p53-C277Y hemizygous Ewing's sarcoma cell line. Expression of endogenous pig3 and MDM2 genes was further enhanced on irradiation of this cell line. Here, suppression of p53-C277Y by RNAi reduced pig3 promoter activity, RNA, and protein expression. Reporter gene assays revealed that the potential of p53-C277Y to up-regulate MDM2 expression was similar to wild-type p53, whereas activation of the pig3 promoter was at least 5-fold increased over wild-type p53. The pentanucleotide microsatellite sequence present in exon 1 of the pig3 gene was found to be responsible for p53-C277Y-mediated activation. In concordance with a role of PIG3 protein for cell death, we showed residual apoptotic activity of p53-C277Y to which the described Ewing's sarcoma cell line was found to be resistant. p53-C277Y has previously been reported to bind to DNA with altered sequence specificity and to be unable to activate generic p53 target genes in yeast-based functional assays. Our results, therefore, show that a p53 mutant may behave differently when tested in its authentic cellular context.

Constitutive and DNA Damage Inducible Activation of pig3 and MDM2 Genes by Tumor-Derived p53 Mutant C277Y

The p53 gene is compromised in most human cancers by point mutation. Evidence is accumulating that these alterations frequently do not result in a complete loss of the sequence-specific transcriptional regulatory function of p53. Here, we describe the transcriptional activity of the p53 mutant C277Y isolated from a Ewing's sarcoma with high constitutive pig3 expression. Transient transfection of this mutant into a p53 null cell line resulted in activation not only of the pig3 but also of the MDM2 gene compatible with the presence of constitutively expressed MDM2 transcripts initiated from the P2 promoter in the p53-C277Y hemizygous Ewing's sarcoma cell line. Expression of endogenous pig3 and MDM2 genes was further enhanced on irradiation of this cell line. Here, suppression of p53-C277Y by RNAi reduced pig3 promoter activity, RNA, and protein expression. Reporter gene assays revealed that the potential of p53-C277Y to up-regulate MDM2 expression was similar to wild-type p53, whereas activation of the pig3 promoter was at least 5-fold increased over wild-type p53. The pentanucleotide microsatellite sequence present in exon 1 of the pig3 gene was found to be responsible for p53-C277Y-mediated activation. In concordance with a role of PIG3 protein for cell death, we showed residual apoptotic activity of p53-C277Y to which the described Ewing's sarcoma cell line was found to be resistant. p53-C277Y has previously been reported to bind to DNA with altered sequence specificity and to be unable to activate generic p53 target genes in yeast-based functional assays. Our results, therefore, show that a p53 mutant may behave differently when tested in its authentic cellular context.

Homotypic and heterotypic interactions of EWS, FLI1 and their oncogenic fusion protein

In Ewing's sarcoma family tumors, the ets transcription factor gene FLI1 is rearranged with one EWS allele resulting in coexpression of germline EWS and chimeric EWS-FLI1 proteins. Here, we investigated the potential of germline EWS, FLI1 and EWS-FLI1 to oligomerize. In two functional in vivo tests, fluorescence resonance energy transfer (FRET) and the mammalian two-hybrid (MTH) assay, self-association of EWS and EWS-FLI1, but not of FLI1 was detected. In addition, interaction of EWS-FLI1 with EWS and FLI1 was observed. GST pull-down assays and immunoprecipitation experiments largely confirmed these results. The EWS N-terminal domain present in both EWS and EWS-FLI1 was found to contribute to homotypic and heterotypic interactions of these proteins. However, in the context of germline EWS, the presence of the whole or part of the C-terminal RNA-binding domain greatly supported the self-association potential of the protein. Involvement of an RNA component in EWS oligomerization was confirmed by sensitivity of the corresponding GST pull-down assay to RNaseA treatment. In contrast, EWS-FLI1 was able to self-associate and also bind to FLI1 via its C-terminal domain, which comprises the FLI1 DNA-binding motif. Accordingly, the EWS-FLI1 interaction was not disrupted by RNaseA treatment. Despite its potential to oligomerize, EWS-FLI1 bound to a tandem ets-binding site of the TGFbeta type II receptor promoter as a monomer. Therefore, the functional consequences of homo- and hetero-oligomerization of EWS and EWS-FLI1 proteins remain to be elucidated.

Interaction of the EWS NH2 terminus with BARD1 links the Ewing's sarcoma gene to a common tumor suppressor pathway

In 85% of Ewing family tumors, the NH2 terminus of EWS is fused to the DNA-binding domain of FLI1, an ets transcription factor. The resulting chimeric protein is a strong transcriptional activator with transforming activity. We report that EWS and EWS-FLI1 interact via their common NH2 terminus with the COOH terminus of BARD1, a putative tumor suppressor, in vitro and in vivo. Because BARD1 associates via its NH2-terminal RING domain with the breast cancer susceptibility gene BRCA1 that provides a platform for interactions with proteins involved in DNA repair and checkpoint control, our results provide a link between the Ewing's sarcoma gene product and the genome surveillance complex.

SIMKK, a mitogen-activated protein kinase (MAPK) kinase, is a specific activator of the salt stress-induced MAPK, SIMK

In eukaryotes, mitogen-activated protein kinases (MAPKs) play key roles in the transmission of external signals, such as mitogens, hormones, and different stresses. MAPKs are activated by MAPK kinases through phosphorylation of MAPKs at both the threonine and tyrosine residues of the conserved TXY activation motif. In plants, several MAPKs are involved in signaling of hormones, stresses, cell cycle, and developmental cues. Recently, we showed that salt stress-induced MAPK (SIMK) is activated when alfalfa cells are exposed to hyperosmotic conditions. Here, we report the isolation and characterization of the alfalfa MAPK kinase SIMKK (SIMK kinase). SIMKK encodes an active protein kinase that interacts specifically with SIMK, but not with three other MAPKs, in the yeast two-hybrid system. Recombinant SIMKK specifically activates SIMK by phosphorylating both the threonine and tyrosine residues in the activation loop of SIMK. SIMKK contains a putative MAPK docking site at the N terminus that is conserved in mammalian MAPK kinases, transcription factors, and phosphatases. Removal of the MAPK docking site of SIMKK partially compromises but does not completely abolish interaction with SIMK, suggesting that other domains of SIMKK also are involved in MAPK binding. In transient expression assays, SIMKK specifically activates SIMK but not two other MAPKs. Moreover, SIMKK enhances the salt-induced activation of SIMK. These data suggest that the salt-induced activation of SIMK is mediated by the dual-specificity protein kinase SIMKK.