SummaryESX-5 is one of the five type VII secretion systems found in mycobacteria. These secretion systems are also known as ESAT-6-like secretion systems. Here, we have determined the secretome of ESX-5 by a proteomic approach in two different strains of Mycobacterium marinum. Comparison of the secretion profile of wild-type strains and their ESX-5 mutants showed that a number of PE_PGRS and PPE-MPTR proteins are dependent on ESX-5 for transport. The PE and PPE protein families are unique to mycobacteria, are highly expanded in several pathogenic species, such as Mycobacterium tuberculosis and M. marinum, and certain family members are cell surface antigens associated with virulence. Using a monoclonal antibody directed against the PGRS domain we showed that nearly all PE_PGRS proteins that are recognized by this antibody are missing in the supernatant of ESX-5 mutants. In addition to PE_PGRS and PPE proteins, the ESX-5 secretion system is responsible for the secretion of a ESAT-6-like proteins. Together, these data show that ESX-5 is probably a major secretion pathway for mycobacteria and that this system is responsible for the secretion of recently evolved PE_PGRS and PPE proteins.

Mycobacteria possess different type VII secretion (T7S) systems to secrete proteins across their unusual cell envelope. One of these systems, ESX-5, is only present in slow-growing mycobacteria and responsible for the secretion of multiple substrates. However, the role of ESX-5 substrates in growth and/or virulence is largely unknown. In this study, we show that esx-5 is essential for growth of both Mycobacterium marinum and Mycobacterium bovis. Remarkably, this essentiality can be rescued by increasing the permeability of the outer membrane, either by altering its lipid composition or by the introduction of the heterologous porin MspA. Mutagenesis of the first nucleotide-binding domain of the membrane ATPase EccC5 prevented both ESX-5-dependent secretion and bacterial growth, but did not affect ESX-5 complex assembly. This suggests that the rescuing effect is not due to pores formed by the ESX-5 membrane complex, but caused by ESX-5 activity. Subsequent proteomic analysis to identify crucial ESX-5 substrates confirmed that all detectable PE and PPE proteins in the cell surface and cell envelope fractions were routed through ESX-5. Additionally, saturated transposon-directed insertion-site sequencing (TraDIS) was applied to both wild-type M. marinum cells and cells expressing mspA to identify genes that are not essential anymore in the presence of MspA. This analysis confirmed the importance of esx-5, but we could not identify essential ESX-5 substrates, indicating that multiple of these substrates are together responsible for the essentiality. Finally, examination of phenotypes on defined carbon sources revealed that an esx-5 mutant is strongly impaired in the uptake and utilization of hydrophobic carbon sources. Based on these data, we propose a model in which the ESX-5 system is responsible for the transport of cell envelope proteins that are required for nutrient uptake. These proteins might in this way compensate for the lack of MspA-like porins in slow-growing mycobacteria.

Pseudomonas aeruginosa secretes two siderophores, pyoverdine and pyochelin, under iron-limiting conditions. These siderophores are recognized at the cell surface by specific outer membrane receptors, also known as TonB-dependent receptors. In addition, this bacterium is also able to incorporate many heterologous siderophores of bacterial or fungal origin, which is reflected by the presence of 32 additional genes encoding putative TonB-dependent receptors. In this work, we have used a proteomic approach to identify the inducing conditions for P. aeruginosa TonB-dependent receptors. In total, 11 of these receptors could be discerned under various conditions. Two of them are only produced in the presence of the hydroxamate siderophores ferrioxamine B and ferrichrome. Regulation of their synthesis is affected by both iron and the presence of a cognate siderophore. Analysis of the P. aeruginosa genome showed that both receptor genes are located next to a regulatory locus encoding an extracytoplasmic function sigma factor and a transmembrane sensor. The involvement of this putative regulatory locus in the specific induction of the ferrioxamine B and ferrichrome receptors has been demonstrated. These results show that P. aeruginosa has evolved multiple specific regulatory systems to allow the regulation of TonB-dependent receptors.In most environments, the amount of free iron (approximately 10 Ϫ9 M) is below the concentration required by most microorganisms for growth (49). To fulfill their requirements for iron, bacteria have developed several strategies, including (i) the reduction of ferric to ferrous ions, (ii) the direct acquisition of iron from host proteins, (iii) the secretion of highaffinity iron-chelating compounds, called siderophores, and (iv) the uptake of heterologous siderophores (23). The transport of (heterologous) siderophores and heme/hemophores through the outer membranes of gram-negative bacteria is an energy-dependent process that needs the function of the ExbBExbD-TonB system. This system transduces the proton-motive force of the cytoplasmic membrane to the high-affinity outer membrane receptors, which are therefore also known as TonBdependent receptors (48). The synthesis of siderophores and their transport systems is negatively regulated by the Fur repressor protein (18). In addition, some siderophore transport systems are also positively regulated by the presence of their cognate siderophores (12). This level of regulation often involves extracytoplasmic function (ECF) sigma factors (56). The ECF family of sigma factors constitutes a group of environmentally responsive transcriptional factors of the RpoD ( 70 ) family that control a wide range of bacterial functions (39). The ECF factors involved in the regulation of iron uptake systems have been classified as the iron starvation class of sigma factors (35). One well-studied example is ferric dicitrate uptake by Escherichia coli (10

SummaryPathogenic mycobacteria require type VII secretion (T7S) systems to transport virulence factors across their complex cell envelope. These bacteria have up to five of these systems, termed ESX-1 to ESX-5. Here, we show that ESX-5 of Mycobacterium tuberculosis mediates the secretion of EsxN, PPE and PE_PGRS proteins, indicating that ESX-5 is a major secretion pathway in this important pathogen. Using the ESX-5 system of Mycobacterium marinum and Mycobacterium bovis BCG as a model, we have purified and analysed the T7S membrane complex under native conditions. blue native-PAGE and immunoprecipitation experiments showed that the ESX-5 membrane complex of both species has a size of~1500 kDa and is composed of four conserved membrane proteins, i.e. EccB 5, EccC5, EccD5 and EccE5. Subsequent limited proteolysis suggests that EccC5 and EccE5 mostly reside on the periphery of the complex. We also observed that EccC5 and EccD5 expression is essential for the formation of a stable membrane complex. These are the first data on a T7S membrane complex and, given the high conservation of its components, our data can likely be generalized to most mycobacterial T7S systems.

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The imaging capabilities of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and MALDI-MS sample preparation methods were combined. We used this method, named matrix-enhanced (ME) SIMS, for direct molecular imaging of nervous tissue at micrometer spatial resolution. Cryosections of the cerebral ganglia of the freshwater snail Lymnaea stagnalis were placed on indium-tin-oxide (ITO)-coated conductive glass slides and covered with a thin layer of 2,5-dihydroxybenzoic acid by electrospray deposition. High-resolution molecular ion maps of cholesterol and the neuropeptide APGWamide were constructed. APGWamide was predominantly localized in the cluster of neurons that regulate male copulation behavior of Lymnaea. ME-SIMS imaging allows direct molecule-specific imaging from tissue sections without labeling and opens a complementary mass window (<2500 Da) to MALDI imaging mass spectrometry at an order of magnitude higher spatial resolution (<3 microm).

Mycobacterium tuberculosis requires a large number of secreted and exported proteins for its virulence, immune modulation and nutrient uptake. Most of these proteins are transported by the different type VII secretion systems. The most recently evolved type VII secretion system, ESX-5, secretes dozens of substrates belonging to the PE and PPE families, which are named for conserved proline and glutamic acid residues close to the amino terminus. However, the role of these proteins remains largely elusive . Here, we show that mutations of ppe38 completely block the secretion of two large subsets of ESX-5 substrates, that is, PPE-MPTR and PE_PGRS, together comprising>80 proteins. Importantly, hypervirulent clinical M. tuberculosis strains of the Beijing lineage have such a mutation and a concomitant loss of secretion . Restoration of PPE38-dependent secretion partially reverted the hypervirulence phenotype of a Beijing strain, and deletion of ppe38 in moderately virulent M. tuberculosis increased virulence. This indicates that these ESX-5 substrates have an important role in virulence attenuation. Phylogenetic analysis revealed that deletion of ppe38 occurred at the branching point of the 'modern' Beijing sublineage and is shared by Beijing outbreak strains worldwide, suggesting that this deletion may have contributed to their success and global distribution.

Estrogen receptor alpha (ERα) is involved in numerous physiological and pathological processes, including breast cancer. Breast cancer therapy is therefore currently directed at inhibiting the transcriptional potency of ERα, either by blocking estrogen production through aromatase inhibitors or antiestrogens that compete for hormone binding. Due to resistance, new treatment modalities are needed and as ERα dimerization is essential for its activity, interference with receptor dimerization offers a new opportunity to exploit in drug design. Here we describe a unique mechanism of how ERα dimerization is negatively controlled by interaction with 14-3-3 proteins at the extreme C terminus of the receptor. Moreover, the small-molecule fusicoccin (FC) stabilizes this ERα/14-3-3 interaction. Cocrystallization of the trimeric ERα/ 14-3-3/FC complex provides the structural basis for this stabilization and shows the importance of phosphorylation of the penultimate Threonine (ERα-T 594 ) for high-affinity interaction. We confirm that T 594 is a distinct ERα phosphorylation site in the breast cancer cell line MCF-7 using a phospho-T 594 -specific antibody and by mass spectrometry. In line with its ERα/14-3-3 interaction stabilizing effect, fusicoccin reduces the estradiol-stimulated ERα dimerization, inhibits ERα/chromatin interactions and downstream gene expression, resulting in decreased cell proliferation. Herewith, a unique functional phosphosite and an alternative regulation mechanism of ERα are provided, together with a small molecule that selectively targets this ERα/14-3-3 interface.T he estrogen receptor alpha (ERα) is a ligand-dependent transcription factor and the driving force of cell proliferation in 75% of all breast cancers. Current therapeutic strategies to treat these tumors rely on selective ER modulators (SERMs), like tamoxifen (TAM) (1) or aromatase inhibitors (AIs) that block estradiol synthesis (2). Although the benefits of treating hormone-sensitive breast cancers with SERMs and AIs are evident, resistance to treatment is commonly observed (3, 4). To overcome resistance, selective ERα down-regulators (SERDs) can for instance be applied that inhibit ERα signaling through receptor degradation (5, 6). Approaches that target the ERα/ DNA or ERα/cofactor interactions are explored as well (5, 7), but other essential steps in the ERα activation cascade are currently unexploited in drug design, also due to a lack of molecular understanding of the processes at hand.One such step that is crucial for many aspects of ERα functioning is ligand-driven receptor dimerization (8, 9). 17β-Estradiol (E2) association with the ERα ligand binding domain (LBD) drives large conformational changes (10) resulting in ERα dissociation from chaperones (11, 12), unmasking of domains for receptor dimerization, and DNA binding (13,14). Whereas the LBD contains the main dimerization domain (15), the extreme C-terminal domain of the receptor (F domain) imposes a restraint on dimerization (15, 16), although the regulation of this remain...