Assessment of machine-learning predictions for the Mediator complex subunit MED25 ACID domain interactions with transactivation domains

The human Mediator complex subunit MED25 binds transactivation domains (TADs) present in various cellular and viral proteins using two binding interfaces, named H1 and H2, which are found on opposite sides of its ACID domain. Here, we use and compare deep learning methods to characterize human MED25-TAD interfaces and assess the predicted models to published experimental data. For the H1 interface, AlphaFold produces predictions with high-reliability scores that agree well with experimental data, while the H2 interface predictions appear inconsistent, preventing reliable binding modes. Despite these limitations, we experimentally assess the validity of MED25 interface predictions with the viral transcriptional activators Lana-1 and IE62. AlphaFold predictions also suggest the existence of a unique hydrophobic pocket for the Arabidopsis MED25 ACID domain.

NMR Spectroscopy of the Main Protease of SARS-CoV-2 and Fragment-Based Screening Identify Three Protein Hotspots and an Antiviral Fragment

The main protease (3CLp) of the SARS-CoV-2, the causative agent for the COVID-19 pandemic, is one of the main targets for drug development. To be active, 3CLp relies on a complex interplay between dimerization, active site flexibility, and allosteric regulation. The deciphering of these mechanisms is a crucial step to enable the search for inhibitors. In this context, using NMR spectroscopy, we studied the conformation of dimeric 3CLp from the SARS-CoV-2 and monitored ligand binding, based on NMR signal assignments. We performed a fragment-based screening that led to the identification of 38 fragment hits. Their binding sites showed three hotspots on 3CLp, two in the substrate binding pocket and one at the dimer interface. F01 is a non-covalent inhibitor of the 3CLp and has antiviral activity in SARS-CoV-2 infected cells. This study sheds light on the complex structure-function relationships of 3CLp and constitutes a strong basis to assist in developing potent 3CLp inhibitors.

Crystal structure of human Mediator subunit MED23

The Mediator complex transduces regulatory information from enhancers to promoters and performs essential roles in the initiation of transcription in eukaryotes. Human Mediator comprises 26 subunits forming three modules termed Head, Middle and Tail. Here we present the 2.8 Å crystal structure of MED23, the largest subunit from the human Tail module. The structure identifies 25 HEAT repeats-like motifs organized into 5 α-solenoids. MED23 adopts an arch-shaped conformation, with an N-terminal domain (Nter) protruding from a large core region. In the core four solenoids, motifs wrap on themselves, creating triangular-shaped structural motifs on both faces of the arch, with extended grooves propagating through the interfaces between the solenoid motifs. MED23 is known to interact with several specific transcription activators and is involved in splicing, elongation, and post-transcriptional events. The structure rationalizes previous biochemical observations and paves the way for improved understanding of the cross-talk between Mediator and transcriptional activators.

Mediator is a large multi-subunits complex essential to the regulation of transcription by RNA pol II. Here the authors report the crystal structure of MED23—one of the largest subunits of the complex together with MED1 and MED14—revealing a complex architecture and filling an important gap in the structural characterization of Mediator.

Solution Structure of the N-Terminal Domain of Mediator Subunit MED26 and Molecular Characterization of Its Interaction with EAF1 and TAF7

MED26 is a subunit of Mediator, a large complex central to the regulation of gene transcription by RNA Polymerase II. MED26 plays a role in the switch between the initiation and elongation phases of RNA Polymerase II-mediated transcription process. Regulation of these steps requires successive binding of MED26 N-terminal domain (NTD) to TATA-binding protein-associated factor 7 (TAF7) and Eleven-nineteen lysine-rich in leukemia-Associated Factor 1 (EAF1). In order to investigate the mechanism of regulation by MED26, MED26-NTD structure was solved by NMR, revealing a 4-helix bundle. EAF1 (239-268) and TAF7 (205-235) peptide interactions were both mapped to the same groove formed by H3 and H4 helices of MED26-NTD. Both interactions are characterized by dissociation constants in the 10-μM range. Further experiments revealed a folding-upon-binding mechanism that leads to the formation of EAF1 (N247-S260) and TAF7 (L214-S227) helices. Chemical shift perturbations and nuclear Overhauser enhancement contacts support the involvement of residues I222/F223 in anchoring TAF7 helix to a hydrophobic pocket of MED26-NTD, including residues L48, W80 and I84. In addition, Ala mutations of charged residues located in the C-terminal disordered part of TAF7 and EAF1 peptides affected the binding, with a loss of affinity characterized by a 10-time increase of dissociation constants. A structural model of MED26-NTD/TAF7 complex shows bi-partite components, combining ordered and disordered segments, as well as hydrophobic and electrostatic contributions to the binding. This study provides molecular detail that will help to decipher the mechanistic basis for the initiation to elongation switch-function mediated by MED26-NTD.

1H, 15N and 13C assignments of the N-terminal domain of the Mediator complex subunit MED26

MED26 is a subunit of the Mediator, a very large complex involved in regulation of gene transcription by RNA Polymerase II. MED26 regulates the switch between initiation and elongation phases of the transcription. This function requires interaction of its N-terminal domain (NTD) with several protein partners implicated in transcriptional regulation. Molecular details of the structure and interaction mode of MED26 NTD would improve understanding of this complex regulation. As a first step towards structural characterization, sequence specific (1)H, (13)C and (15)N assignments for MED26 NTD was performed based on Nuclear Magnetic Resonance spectroscopy. TALOS+ analysis of the chemical shifts data revealed a domain solely composed of helices. Assignments will be further used to solve NMR structure and dynamics of MED26 NTD and investigate the molecular details of its interaction with protein partners.

Structure of UBE2Z Enzyme Provides Functional Insight into Specificity in the FAT10 Protein Conjugation Machinery

FAT10 conjugation, a post-translational modification analogous to ubiquitination, specifically requires UBA6 and UBE2Z as its activating (E1) and conjugating (E2) enzymes. Interestingly, these enzymes can also function in ubiquitination. We have determined the crystal structure of UBE2Z and report how the different domains of this E2 enzyme are organized. We further combine our structural data with mutational analyses to understand how specificity is achieved in the FAT10 conjugation pathway. We show that specificity toward UBA6 and UBE2Z lies within the C-terminal CYCI tetrapeptide in FAT10. We also demonstrate that this motif slows down transfer rates for FAT10 from UBA6 onto UBE2Z.

Characterization of ERM transactivation domain binding to the ACID/PTOV domain of the Mediator subunit MED25

The N-terminal acidic transactivation domain (TAD) of ERM/ETV5 (ERM_38–68), a PEA3 group member of Ets-related transcription factors, directly interacts with the ACID/PTOV domain of the Mediator complex subunit MED25. Molecular details of this interaction were investigated using nuclear magnetic resonance (NMR) spectroscopy. The TAD is disordered in solution but has a propensity to adopt local transient secondary structure. We show that it folds upon binding to MED25 and that the resulting ERM–MED25 complex displays characteristics of a fuzzy complex. Mutational analysis further reveals that two aromatic residues in the ERM TAD (F47 and W57) are involved in the binding to MED25 and participate in the ability of ERM TAD to activate transcription. Mutation of a key residue Q451 in the VP16 H1 binding pocket of MED25 affects the binding of ERM. Furthermore, competition experiments show that ERM and VP16 H1 share a common binding interface on MED25. NMR data confirms the occupancy of this binding pocket by ERM TAD. Based on these experimental data, a structural model of a functional interaction is proposed. This study provides mechanistic insights into the Mediator–transactivator interactions.

A Salmonella type three secretion effector/chaperone complex adopts a hexameric ring-like structure

Many bacterial pathogens use type three secretion systems (T3SS) to inject virulence factors, named effectors, directly into the cytoplasm of target eukaryotic cells. Most of the T3SS components are conserved among plant and animal pathogens, suggesting a common mechanism of recognition and secretion of effectors. However, no common motif has yet been identified for effectors allowing T3SS recognition. In this work, we performed a biochemical and structural characterization of the Salmonella SopB/SigE chaperone/effector complex by small-angle X-ray scattering (SAXS). Our results showed that the SopB/SigE complex is assembled in dynamic homohexameric-ring-shaped structures with an internal tunnel. In this ring, the chaperone maintains a disordered N-terminal end of SopB molecules, in a good position to be reached and processed by the T3SS. This ring dimensionally fits the ring-organized molecules of the injectisome, including ATPase hexameric rings; this organization suggests that this structural feature is important for ATPase recognition by T3SS. Our work constitutes the first evidence of the oligomerization of an effector, analogous to the organization of the secretion machinery, obtained in solution. As effectors share neither sequence nor structural identity, the quaternary oligomeric structure could constitute a strategy evolved to promote the specificity and efficiency of T3SS recognition.

Structure of the secretion domain of HxuA from Haemophilus influenzae

Haemophilus influenzae HxuA is a cell-surface protein with haem-haemopexin binding activity which is key to haem acquisition from haemopexin and thus is one of the potential sources of haem for this microorganism. HxuA is secreted by its specific transporter HxuB. HxuA/HxuB belongs to the so-called two-partner secretion systems (TPSs) that are characterized by a conserved N-terminal domain in the secreted protein which is essential for secretion. Here, the 1.5 Å resolution structure of the secretion domain of HxuA, HxuA301, is reported. The structure reveals that HxuA301 folds into a β-helix domain with two extra-helical motifs, a four-stranded β-sheet and an N-terminal cap. Comparisons with other structures of TpsA secretion domains are reported. They reveal that despite limited sequence identity, strong structural similarities are found between the β-helix motifs, consistent with the idea that the TPS domain plays a role not only in the interaction with the specific TpsB partners but also as the scaffold initiating progressive folding of the TpsA proteins at the bacterial surface.

The Mediator complex subunit MED25 is targeted by the N-terminal transactivation domain of the PEA3 group members

PEA3, ERM and ER81 belong to the PEA3 subfamily of Ets transcription factors and play important roles in a number of tissue-specific processes. Transcriptional activation by PEA3 subfamily factors requires their characteristic amino-terminal acidic transactivation domain (TAD). However, the cellular targets of this domain remain largely unknown. Using ERM as a prototype, we show that the minimal N-terminal TAD activates transcription by contacting the activator interacting domain (ACID)/Prostate tumor overexpressed protein 1 (PTOV) domain of the Mediator complex subunit MED25. We further show that depletion of MED25 disrupts the association of ERM with the Mediator in vitro. Small interfering RNA-mediated knockdown of MED25 as well as the overexpression of MED25-ACID and MED25-VWA domains efficiently inhibit the transcriptional activity of ERM. Moreover, mutations of amino acid residues that prevent binding of MED25 to ERM strongly reduce transactivation by ERM. Finally we show that siRNA depletion of MED25 diminishes PEA3-driven expression of MMP-1 and Mediator recruitment. In conclusion, this study identifies the PEA3 group members as the first human transcriptional factors that interact with the MED25 ACID/PTOV domain and establishes MED25 as a crucial transducer of their transactivation potential.

Structural insights into ChpT, an essential dimeric histidine phosphotransferase regulating the cell cycle in Caulobacter crescentus

The cell-cycle regulator ChpT of C. crescentus is a dimeric histidine phosphotransferase that resembles the typical structure of histidine kinases.

Two-component and phosphorelay signal-transduction proteins are crucial for bacterial cell-cycle regulation in Caulobacter crescentus. ChpT is an essential histidine phosphotransferase that controls the activity of the master cell-cycle regulator CtrA by phosphorylation. Here, the 2.2 Å resolution crystal structure of ChpT is reported. ChpT is a homodimer and adopts the domain architecture of the intracellular part of class I histidine kinases. Each subunit consists of two distinct domains: an N-terminal helical hairpin domain and a C-terminal α/β domain. The two N-terminal domains are adjacent within the dimer, forming a four-helix bundle. The ChpT C-terminal domain adopts an atypical Bergerat ATP-binding fold.

Solution structure of the N-terminal transactivation domain of ERM modified by SUMO-1

ERM is a member of the PEA3 group of the Ets transcription factor family that plays important roles in development and tumorigenesis. The PEA3s share an N-terminal transactivation domain (TADn) whose activity is inhibited by small ubiquitin-like modifier (SUMO). However, the consequences of sumoylation and its underlying molecular mechanism remain unclear. The domain structure of ERM TADn alone or modified by SUMO-1 was analyzed using small-angle X-ray scattering (SAXS). Low resolution shapes determined ab initio from the scattering data indicated an elongated shape and an unstructured conformation of TADn in solution. Covalent attachment of SUMO-1 does not perturb the structure of TADn as indicated by the linear arrangement of the SUMO moiety with respect to TADn. Thus, ERM belongs to the growing family of proteins that contain intrinsically unstructured regions. The flexible nature of TADn may be instrumental for ERM recognition and binding to diverse molecular partners.

Expression and purification in high yield of a functionally active recombinant human Type I inositol(1,4,5)P3 5-phosphatase

Inositol polyphosphates are the most widespread second messenger molecules in eukaryotic cells. Human Type I inositol 1,4,5-triphosphate (Ins(1,4,5)P(3)) 5-phosphatase removes the D-5 position phosphate from soluble Ins(1,4,5)P(3,) a key event in cell signaling particularly in Ca(2+) homeostasis. In this study, the cDNA encoding human Type I Ins(1,4,5)P(3) 5-phosphatase was subcloned into a modified pMAL expression vector. This plasmid produces a recombinant protein in fusion with affinity tags located at its N-terminus, consisting in a maltose binding protein (MPB) and an octa-histidine stretch. The construction was transformed into Escherichia coli BL21 (DE3) expression strain. This dual tag strategy allows the purification of milligrams of highly purified protein. The recombinant human Type I Ins(1,4,5)P(3) 5-phosphatase is active and can thus be used for functional and structural studies.

Characterization of the Arabidopsis thaliana Arath;CDC25 dual-specificity tyrosine phosphatase

CDC25 enzymes are dual-specificity phosphatases involved in the regulation of the cell cycle. No CDC25 enzymes have been described in higher plant organisms. We report here the characterization of an Arabidopsis thaliana CDC25 enzyme, constituted by a sole catalytic domain and devoid of the N-terminal regulatory region found in the human CDC25. We describe the recombinant expression in Escherichia coli of the Arath;CDC25 and its purification for activity assay and structure determination by NMR. The recombinant enzyme has a tyrosine phosphatase activity towards an artificial substrate, a NMR characterization equally concludes to its correct folding. The secondary structure of the protein was predicted on the basis of the assigned chemical shift of (1)H, (15)N, and (13)C backbone atoms of the protein. The presence of a metal ion in the C-terminus of this new protein points to a zinc finger, and sequence homology indicates that this new structural element might be conserved in related plant homologs.

A small CDC25 dual-specificity tyrosine-phosphatase isoform in Arabidopsis thaliana

The dual-specificity CDC25 phosphatases are critical positive regulators of cyclin-dependent kinases (CDKs). Even though an antagonistic Arabidopsis thaliana WEE1 kinase has been cloned and tyrosine phosphorylation of its CDKs has been demonstrated, no valid candidate for a CDC25 protein has been reported in higher plants. We identify a CDC25-related protein (Arath;CDC25) of A. thaliana, constituted by a sole catalytic domain. The protein has a tyrosine-phosphatase activity and stimulates the kinase activity of Arabidopsis CDKs. Its tertiary structure was obtained by NMR spectroscopy and confirms that Arath;CDC25 belongs structurally to the classical CDC25 superfamily with a central five-stranded beta-sheet surrounded by helices. A particular feature of the protein, however, is the presence of an additional zinc-binding loop in the C-terminal part. NMR mapping studies revealed the interaction with phosphorylated peptidic models derived from the conserved CDK loop containing the phosphothreonine-14 and phosphotyrosine-15. We conclude that despite sequence divergence, Arath;CDC25 is structurally and functionally an isoform of the CDC25 superfamily, which is conserved in yeast and in plants, including Arabidopsis and rice.

Visualization of TGN to endosome trafficking through fluorescently labeled MPR and AP-1 in living cells

We have stably expressed in HeLa cells a chimeric protein made of the green fluorescent protein (GFP) fused to the transmembrane and cytoplasmic domains of the mannose 6-phosphate/insulin like growth factor II receptor in order to study its dynamics in living cells. At steady state, the bulk of this chimeric protein (GFP-CI-MPR) localizes to the trans-Golgi network (TGN), but significant amounts are also detected in peripheral, tubulo-vesicular structures and early endosomes as well as at the plasma membrane. Time-lapse videomicroscopy shows that the GFP-CI-MPR is ubiquitously detected in tubular elements that detach from the TGN and move toward the cell periphery, sometimes breaking into smaller tubular fragments. The formation of the TGN-derived tubules is temperature dependent, requires the presence of intact microtubule and actin networks, and is regulated by the ARF-1 GTPase. The TGN-derived tubules fuse with peripheral, tubulo-vesicular structures also containing the GFP-CI-MPR. These structures are highly dynamic, fusing with each other as well as with early endosomes. Time-lapse videomicroscopy performed on HeLa cells coexpressing the CFP-CI-MPR and the AP-1 complex whose gamma-subunit was fused to YFP shows that AP-1 is present not only on the TGN and peripheral CFP-CI-MPR containing structures but also on TGN-derived tubules containing the CFP-CI-MPR. The data support the notion that tubular elements can mediate MPR transport from the TGN to a peripheral, tubulo-vesicular network dynamically connected with the endocytic pathway and that the AP-1 coat may facilitate MPR sorting in the TGN and endosomes.

The AP-3-dependent targeting of the melanosomal glycoprotein QNR-71 requires a di-leucine-based sorting signal

The Quail Neuroretina clone 71 gene (QNR-71) is expressed during the differentiation of retinal pigmented epithelia and the epidermis. It encodes a type I transmembrane glycoprotein that shares significant sequence homologies with several melanosomal proteins. We have studied its intracellular traffic in both pigmented and non-pigmented cells. We report that a di-leucine-based sorting signal (ExxPLL) present in the cytoplasmic domain of QNR-71 is necessary and sufficient for its proper targeting to the endosomal/premelanosomal compartments of both pigmented and non-pigmented cells. The intracellular transport of QNR-71 to these compartments is mediated by the AP-3 assembly proteins. As previously observed for the lysosomal glycoproteins LampI and LimpII, overexpression of QNR-71 increases the amount of AP-3 associated with membranes, and inhibition of AP-3 synthesis increases the routing of QNR-71 towards the cell surface. In addition, expression of QNR-71 induces a misrouting of endogenous LampI to the cell surface. Thus, the targeting of QNR-71 might be similar to that of the lysosomal integral membrane glycoproteins LampI and LimpII. This suggests that sorting to melanosomes and lysosomes requires similar sorting signals and transport machineries.

Rabenosyn-5, a novel Rab5 effector, is complexed with hVPS45 and recruited to endosomes through a FYVE finger domain

Rab5 regulates endocytic membrane traffic by specifically recruiting cytosolic effector proteins to their site of action on early endosomal membranes. We have characterized a new Rab5 effector complex involved in endosomal fusion events. This complex includes a novel protein, Rabenosyn-5, which, like the previously characterized Rab5 effector early endosome antigen 1 (EEA1), contains an FYVE finger domain and is recruited in a phosphatidylinositol-3-kinase-dependent fashion to early endosomes. Rabenosyn-5 is complexed to the Sec1-like protein hVPS45. hVPS45 does not interact directly with Rab5, therefore Rabenosyn-5 serves as a molecular link between hVPS45 and the Rab5 GTPase. This property suggests that Rabenosyn-5 is a closer mammalian functional homologue of yeast Vac1p than EEA1. Furthermore, although both EEA1 and Rabenosyn-5 are required for early endosomal fusion, only overexpression of Rabenosyn-5 inhibits cathepsin D processing, suggesting that the two proteins play distinct roles in endosomal trafficking. We propose that Rab5-dependent formation of membrane domains enriched in phosphatidylinositol-3-phosphate has evolved as a mechanism for the recruitment of multiple effector proteins to mammalian early endosomes, and that these domains are multifunctional, depending on the differing activities of the effector proteins recruited.

The Ets family member Erg gene is expressed in mesodermal tissues and neural crests at fundamental steps during mouse embryogenesis

The Erg gene belongs to the Ets family encoding a class of transcription factors. To gain new insight on the in vivo functional specificity of the Erg gene within the wide Ets family, we used in situ hybridization to determine its expression pattern during murine embryogenesis. We found that the Erg gene expression predominates in mesodermal tissues, including the endothelial, precartilaginous and urogenital areas. A specific Erg gene expression was also identified in migrating neural crest cells. A comparison with Fli-1, the most closely Erg-related gene, revealed that both gene expressions partially overlap, suggesting that they may contribute to related functions in these tissues. Like other Ets family genes, Erg seems involved in several fundamental developmental steps in murine embryogenesis, including epithelio-mesenchymal transition, cell migration, settlement and differentiation.

Characterization of the human and mouse ETV1/ER81 transcription factor genes: role of the two alternatively spliced isoforms in the human

The Ets transcription factors of the PEA3 group--E1AF/PEA3, ETV1/ER81 and ERM--are almost identical in the ETS DNA-binding and the transcriptional acidic domains. To accelerate our understanding of the molecular basis of putative diseases linked to ETV1 such as Ewing's sarcoma we characterized the human ETV1 and the mouse ER81 genes. We showed that these genes are both encoded by 13 exons in more than 90 kbp genomic DNA, and that the classical acceptor and donor splicing sites are present in each junction except for the 5' donor site of intron 9 where GT is replaced by TT. The genomic organization of the ETS and acidic domains in the human ETV1 and mouse ER81 (localized to chromosome 12) genes is similar to that observed in human ERM and human E1AF/PEA3 genes. Moreover, as in human ERM and human E1AF/PEA3 genes, a first untranslated exon is upstream from the first methionine, and the mouse ER81 gene transcription is regulated by a 1.8 kbp of genomic DNA upstream from this exon. In human, the alternative splicing of the ETV1 gene leads to the presence (ETV1 alpha) or the absence (ETV1 beta) of exon 5 encoding the C-terminal part of the transcriptional acidic domain, but without affecting the alpha helix previously described as crucial for transactivation. We demonstrated here that the truncated isoform (human ETV1 beta) and the full-length isoform (human ETV1 alpha) bind similarly specific DNA Ets binding sites. Moreover, they both activate transcription similarly through the PKA-transduction pathway, so suggesting that this alternative splicing is not crucial for the function of this protein as a transcription factor. The comparison of human ETV1 alpha and human ETV1 beta expression in the same tissues, such as the adrenal gland or the bladder, showed no clear-cut differences. Altogether, these data open a new avenue of investigation leading to a better understanding of the functional role of this transcription factor.

Genomic organization of the human ERM (ETV5) gene, a PEA3 group member of ETS transcription factors

The ERM protein belongs to the family of Ets transcription factors. We show here that the human ERM gene is organized into 14 exons distributed along 65 kb of genomic DNA on chromosome 3. The two main functional domains of ERM, the acidic domain and the DNA-binding ETS domain, are overlapped by three different exons each. The 3'-untranslated region of ERM is 2.1 kb, whereas the 5'-untranslated region is about 0.3 kb; this allows the transcription of ERM transcripts of approximately 4 kb. The human ERM gene is localized to the q27-q29 region of chromosome 3.

Two functionally distinct domains responsible for transactivation by the Ets family member ERM

The recently cloned human Ets transcription factor ERM is closely related to the ER81 and PEA3 genes. Here, we report the functional analysis of the DNA-binding and transactivation properties of ERM. Specific DNA-binding by ERM requires the ETS domain, conserved in all members of the Ets family and is inhibited by an 84 residue long central region and the carboxy-terminal tail. Two fragments of ERM are transferrable activation domains: alpha, which sits in the 72 first residues and encompasses the acidic domain conserved between ERM, ER81 and PEA3, and the carboxy-terminal tail which also bears a DNA-binding inhibition function. Deletion of alpha strongly reduces transactivation by ERM. Moreover, alpha and the carboxy-terminal tail exhibit functional synergism, suggesting that they activate transcription through different mechanisms. In support of this idea, we demonstrate that VP16 squelches transactivation by alpha but not by the carboxy-terminal tail. This result also indicates that alpha and VP16 may share common limiting cofactors. alpha and the carboxy-terminal tail do not seem to be conserved within the whole Ets family, indicating that the specificity of ERM may rely on interactions with distinct cofactors.

Mesodermal expression of the chicken erg gene associated with precartilaginous condensation and cartilage differentiation

The ets gene superfamily encodes a class of transcription factors that bind to a purine rich sequence through a 85 amino-acid ETS domain. Among them, the human erg gene has been found to be involved in Ewing's sarcoma, primitive neurectodermal tumour of childhood and acute myeloid leukaemia. Nevertheless, little is known about human erg expression. Northern blot analyses have shown a human erg expression restricted to few cell lines and thymus, but the status concerning expression during development remains unknown probably because no homologue of this gene has yet been isolated and studied in other vertebrates. We thus choose to clone the chicken erg gene (ck-erg) and to study its expression during chicken development. We obtained a bona fide clone of ck-erg and defined the transcriptional modulating properties of its product. The ck-Erg protein acts as a transcriptional activator through a conventional consensus ETS binding site. Northern blot studies on various chicken tissues, in situ analyses and comparison with the well-characterised c-ets-1 expression show that ck-erg is expressed in mesoderm- and, to a lesser extent, in ectoderm-derived tissues. During chicken development, two salient features could be observed. From stage E1 to E3.5, ck-erg expression was widely distributed in mesodermal derivatives and neural crest, resembling c-ets-1 expression. However, by E6, the expression of ck-erg exhibited, unlike c-ets-1, a drastically new and strong signal in precartilaginous condensation zones and cartilaginous skeletal primordia. These stages are the first steps of bone formation during skeletal elaboration. Our results show for the first time a possible specific involvement of ck-erg in cartilage morphogenesis.

A model for gene evolution of the ets-1/ets-2 transcription factors based on structural and functional homologies

The chicken c-ets-1 locus encodes two transcription factors, p54c-ets-1 and p68c-ets-1 that differ in their N-termini, encoded respectively by the I54 and alpha beta exons. p68c-ets-1 equivalents are only found in birds and reptiles while p54c-ets-1 is widely conserved in vertebrates, from amphibians to mammals. Thus, the classical view concerning the evolution of the c-ets-1 gene has been to consider that I54 is of ancient origin whereas alpha and beta, which provide an additional activating domain in p68c-ets-1, would have been acquired much more recently. Sequencing the alpha and beta exons in various species pinpointed a highly conserved region of 13 amino acids which is rich in acidic and hydrophobic residues, a feature of some other transactivating domains. Strikingly, this subdomain is also present in the otherwise unrelated N-terminal activating region of p58c-ets-2 and was thus named BEC for Ets-1-beta/Ets-2-Conserved sequence. Moreover, the two N-termini share the BEC sequence at a homologous position in their highly similar genomic organization indicating a common origin. This structural homology underlies a functional similarity since fusion of the heterologous GAL4 DNA-binding domain with either of the two isolated domains demonstrates that BEC is essential in both cases for the transactivating activity. The function of the alpha beta domain in the context of p68c-ets-1 also strictly depends on the presence of the BEC sequence. Finally, the whole N-terminus of p58c-ets-2 can functionally substitute for its counterpart in p68c-ets-1 further demonstrating that p68c-ets-1 and p58c-ets-2 are structurally and functionally more closely related than previously thought. Besides, we also found BEC in the N-terminus of the Drosophila pointed gene which may be considered as closely related to the uncommitted 'ets1/2' common ancestor. These data demonstrate that the alpha and beta exons are not a recent and specific acquisition but stem, like the p58c-ets-2 N-terminus, from the invertebrate unduplicated 'ets 1/2' gene. This work unravels a new model for the ets-1/ets-2 gene's evolution, based for the first time on both structural and functional evidences. Accordingly, p68c-ets-1 and p58c-ets-2 are the direct descendants of the ancestral 'ets1/2' gene whereas I54 may have been acquired as a second promoter in the c-ets-1 gene after the duplication. Indeed, I54 is not found in the Drosophila pointed gene. The high degree of similarity, and hence of functional redundancy, between p68c-ets-1 and p58c-ets-2 may have led to the rapid divergence (and even loss in mammals) of alpha and beta during evolution whereas I54, which provided a novel function unique to c-ets-1, was maintained within the presently widespread p54c-ets-1 version.