Molecular Characterization of the Na+/H+-Antiporter NhaA from Salmonella Typhimurium
Christopher J. Lentes
Syed H. Mir
- Na+/H+ antiporters are integral membrane proteins that are present in almost every cell and in every kingdom of life. They are essential for the regulation of intracellular pH-value, Na+-concentration and cell volume. These secondary active transporters exchange sodium ions against protons via an alternating access mechanism, which is not understood in full detail. Na+/H+ antiporters show distinct species-specific transport characteristics and regulatory properties that correlate with respective physiological functions. Here we present the characterization of the Na+/H+ antiporter NhaA from Salmonella enterica serovar Thyphimurium LT2, the causing agent of food-born human gastroenteritis and typhoid like infections. The recombinant antiporter was functional in vivo and in vitro. Expression of its gene complemented the Na+-sensitive phenotype of an E. coli strain that lacks the main Na+/H+ antiporters. Purified to homogeneity, the antiporter was a dimer in solution as accurately determined by size-exclusion chromatography combined with multi-angle laser-light scattering and refractive index monitoring. The purified antiporter was fully capable of electrogenic Na+(Li+)/H+-antiport when reconstituted in proteoliposomes and assayed by solid-supported membrane-based electrophysiological measurements. Transport activity was inhibited by 2-aminoperimidine. The recorded negative currents were in agreement with a 1Na+(Li+)/2H+ stoichiometry. Transport activity was low at pH 7 and up-regulation above this pH value was accompanied by a nearly 10-fold decrease of KmNa (16 mM at pH 8.5) supporting a competitive substrate binding mechanism. K+ does not affect Na+ affinity or transport of substrate cations, indicating that selectivity of the antiport arises from the substrate binding step. In contrast to homologous E. coli NhaA, transport activity remains high at pH values above 8.5. The antiporter from S. Typhimurium is a promising candidate for combined structural and functional studies to contribute to the elucidation of the mechanism of pH-dependent Na+/H+ antiporters and to provide insights in the molecular basis of species-specific growth and survival strategies.
Characterization of mouse NOA1 : subcellular localizaion, G-Quadruplex binding and proteolysis
- Mitochondria contain their own protein synthesis machinery with mitoribosomes that are similar to prokaryotic ribosomes. The thirteen proteins encoded in the mitochondrial genome are members of the respiratory chain complexes that generate a proton gradient, which is the electromotoric force for ATP synthesis.
NOA1 (Nitric Oxide Associated Protein-1) is a nuclear encoded GTPase that positively influences mitochondrial respiration and ATP production. Although a role in mitoribosome assembly was assigned to NOA1 the underlying molecular mechanism is poorly understood. This work shows that the multi-domain protein NOA1 serves multiple purposes for the function of mitochondria. NOA1 is a dual localized protein that makes a detour through the nucleus before mitochondrial import. The nuclear shuttling is mediated by a nuclear localization signal and the now identified nuclear export signal. SELEX (Systemic Evolution of Ligands by Exponential Enrichment) analysis revealed a G-quadruplex binding motif that characterizes NOA1 as ribonucleoprotein (RNP). G-quadruplex binding was coupled to the GTPase activity and increased the GTP hydrolysis rate. The sequence of localization events and the identification of NOA1 being a RNP lead to the discussion of an alternative import pathway for RNPs into mitochondria. The short-lived NOA1 contains ClpX recognition motifs and is specifically degraded by the mitochondrial matrix protease ClpXP. NOA1 is the first reported substrate of ClpXP in higher eukaryotes and augments the contribution of the ClpXP protease for mitochondrial metabolism. To assess the direct action of NOA1 on the mitoribosome co-sedimentation assays were performed. They showed that the interaction of NOA1 and the mitoribosome is dependent on the GTPase function and the nascent peptide chain. In vitro, NOA1 facilitated the membrane insertion of newly translated and isotope labeled mitochondrial translation products into inverted mitochondrial inner membrane vesicles. In conclusion, NOA1 is a G-quadruplex-RNP that acts as mitochondrial membrane insertion factor for mtDNA-encoded proteins.
This thesis provides a comprehensive model of the molecular function of NOA1 and is the basis for future research. The identification of NOA1 as ClpXP substrate is a major contribution to the field of mitochondrial research.
Cell-free expression of GPCRs: the endothelin system
- The human endothelin receptors, ETA and ETB, are two members of the G-protein coupled receptors family (GPCRs) and they are key players in cardiovascular regulation. The characterization of their functionality in vitro has been limited by the possibility to obtain high quality samples using conventional expression systems. The Cell-Free expression system is an alternative technique for the production of membrane protein as well as GPCRs and can overcome some of the limitations that are commonly encountered using an in vivo approach. Cell-Free expression protocols for the two receptors ETA and ETB have been optimized by implementing post- and co-translational association to lipid bilayers. The efficiency of the reconstitution or association to liposomes and nanodiscs has systematically been studied and the ligand binding properties of the two receptors have been analyzed using a set of different complementary techniques. In several different conditions a high affinity binding of the peptide ligand ET-1 to both endothelin receptors could be obtained and the highest activity values were detected in sample prepared using a co-translational approach in presence of nanodiscs. Furthermore, the characteristic differential binding pattern of selected agonists and antagonists to the two receptors was confirmed. In samples obtained from several Cell-Free expression conditions, two intrinsic properties of the functionally folded ETB receptor, such as the proteolytic processing based on conformational recognition as well as the formation of SDS-resistant complexes with the peptide ligand ET-1, were detected. ETA and ETB are able to induce in vivo the activation of hetrotrimeric G proteins upon stimulation with an agonist, leading to the dissociation of the heterotrimeric complex and the exchange of GDP to GTP in the Galpha subunit. The Cell-Free expression system was chosen for the production of two G alpha subunit, Galpha s and Galpha q. Soluble expression of the two proteins was achieved and the production of active Galpha s was confirmed using fluorescent as well as radioactive assays. In conclusion, the obtained results document a new process for the production of ligand binding competent endothelin receptors, as well as Galpha proteins, using a Cell-Free expression system. The combination of this expression system and the nanodiscs technology appears to be a promising tool for the further characterization of membrane proteins as well as GPCRs.
MALDI-MS characterization of the human 5-lipoxygenase protein
- 5-lipoxygenase (5-LO) is an enzyme with a substantial role in inflammatory processes. In vitro kinase assays using [32P]-ATP in combination with mutagenesis have revealed that serine residues 271, 523 and 663 can be phosphorylated by MK2, PKA and ERK2 kinases, respectively. A few available reports regarding 5-LO protein sequence have covered up to 30% of the sequence after amino acid sequencing including Ser663. In LCMS/MS analyses of 5-LO tryptic digests from different cellular sources different peptides have been detected; however, none of the three phosphorylations has been detected and only Ser663 was included in the covered sequence.
As there was no comprehensive mass spectrometric analysis of 5-LO, the purpose of this study was to optimize the experimental conditions under which detection of the aforementioned phosphorylation events, as well as other possible post-translational modifications (PTMs), would be feasible. Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS) was used for peptide analysis of 5-LO cleaved either by chemical reagents or by proteases. Sequence coverage of 5-LO could be enhanced to be close to completion by combination of results from digestions by trypsin, AspN and chymotrypsin. In-gel trypsin digestion followed by in-solution AspN digestion proved to be a useful sample treatment for reproducible detection of the Ser271-containing peptide.
Nevertheless, in none of the examined cleavage protocols the sequence around Ser523 was detected reproducibly or with acceptable signal intensity for subsequent peptide fragmentation. Propionic anhydride and sulfo-NHS-SS-biotin cross-linker (EZ-linkTM), were used for derivatization of lysine side chains and hindrance of lysine residue recognition by trypsin. Phosphopeptide enrichment became possible after tryptic digestion of these samples, not only due to formation of an individual Ser523-containing peptide, but also because TiO2-mediated enrichment, which is performed in acidic pH, was not impaired by positively charged free lysine side chains. Additionally, biotinylation of lysine residues was exploited for an intermediate enrichment step of the lysine containing peptides, prior to TiO2 phosphopeptide enrichment.
MALDI-MS analysis after in-vitro phosphorylation of 5-LO by the three kinases showed that Ser271 was phosphorylated in the MK2 and PKA kinase assays, while Ser523 was phosphorylated only in the PKA kinase assay. Surpisingly, no phosphopeptides were detected in the in-vitro kinase assays with ERK2, even though the unmodified counterpart of the Ser663-containing peptide was easily detected. The detection limit for each of the three phosphorylation sites was determined by the use of custom made phosphopeptides and an amount of 0.06 pmol of phosphopeptide in 1 μg 5-LO (representing 0.5% phosphorylation rate) was sufficient in all cases for successful enrichment and detection by MS.
In-vitro kinase assays with [32P]-ATP were performed for some kinases that were expected to phosphorylate 5-LO according to in-silico data. Three members of the Src tyrosine kinase family (Fgr, Hck and Yes) and the Ser/Thr specific kinase DNA-PK used 5-LO as their substrate and mainly residues at the N-terminal part of 5-LO were detected phosphorylated by MS (e.g. Y42, Y53). Additional in-vitro assays for recombinant 5-LO modification included incubation with glutathione or compound U73122, previously described as inhibitor of 5-LO.
Since in-vitro assays might have generated artifacts, a method for 5-LO purification from human cells was sought, in order to examine the modification state of the protein in the cellular context. ATP-agarose affinity purification and anti-5-LO immunoprecipitation proved inappropriate for sample purification for MALDI-MS analysis. Consequently, two human cell lines that are able to express 5-LO (Rec-1 Blymphocytes and MM6 monocytes) were transduced with a DNA cassette that contained recombinant human 5-LO sequence with an attached N-terminal FLAG-tag. Anti-FLAG immunoprecipitation was then performed effectively in cell lysates and the precipitated FLAG-5-LO was separated by SDS-PAGE before MALDI-MS analysis.
The examined cell stimuli were expected to result to phosphorylation of 5-LO at Ser523 by PKA in Rec-1 cells and to phosphorylation of Ser271 and/or Ser663 in MM6 cells by activated MK2 and ERK2, respectively. Additionally, under the conditions of MM6 cell stimulation, Fgr, Hck and Yes kinases, which phosphorylated 5-LO in vitro, were expected to be activated and the possibility of 5-LO phosphorylation on tyrosine was investigated. Although immunoblotting results indicated that all the aforementioned phosphorylation events existed in the examined samples, MALDI-MS analysis verified only phosphorylation on Ser271 in differentiated MM6 cells, interestingly regardless of cell stimulation.
Finally, the primary amine derivatization procedure by EZ-linkTM was utilized for MS analysis of lysine rich proteins. In the past, chemical propionylation of histones had been employed prior to trypsin digestion; however it was easily confused in MS with combinations of other PTMs (e.g. acetylation, methylation). Moreover, propionylation is a PTM for histone H3 and this information was lost. Consequently, the EZ-link reagent was more useful for analysis of histones, as unambiguous assignment of PTMs and detection of native propionylation on bovine H3 became possible.
Molecular Dynamics Simulation Study of Conformational Changes of Transcription Factor TFIIS during RNA Polymerase II Transcriptional Arrest and Reactivation
Juan Manuel Ortiz-Sánchez
J. Andrew McCammon
- Transcription factor IIS (TFIIS) is a protein known for catalyzing the cleavage reaction of the 3′-end of backtracked RNA transcript, allowing RNA polymerase II (Pol II) to reactivate the transcription process from the arrested state. Recent structural studies have provided a molecular basis of protein-protein interaction between TFIIS and Pol II. However, the detailed dynamic conformational changes of TFIIS upon binding to Pol II and the related thermodynamic information are largely unknown. Here we use computational approaches to investigate the conformational space of TFIIS in the Pol II-bound and Pol II-free (unbound) states. Our results reveal two distinct conformations of TFIIS: the closed and the open forms. The closed form is dominant in the Pol II-free (unbound) state of TFIIS, whereas the open form is favorable in the Pol II-bound state. Furthermore, we discuss the free energy difference involved in the conformational changes between the two forms in the presence or absence of Pol II. Additionally, our analysis indicates that hydrophobic interactions and the protein-protein interactions between TFIIS and Pol II are crucial for inducing the conformational changes of TFIIS. Our results provide novel insights into the functional interplay between Pol II and TFIIS as well as mechanism of reactivation of Pol II transcription by TFIIS.
Identification of residues required for stalled-ribosome rescue in the codon-independent release factor YaeJ
- The YaeJ protein is a codon-independent release factor with peptidyl-tRNA hydrolysis (PTH) activity, and functions as a stalled-ribosome rescue factor in Escherichia coli. To identify residues required for YaeJ function, we performed mutational analysis for in vitro PTH activity towards rescue of ribosomes stalled on a non-stop mRNA, and for ribosome-binding efficiency. We focused on residues conserved among bacterial YaeJ proteins. Additionally, we determined the solution structure of the GGQ domain of YaeJ from E. coli using nuclear magnetic resonance spectroscopy. YaeJ and a human homolog, ICT1, had similar levels of PTH activity, despite various differences in sequence and structure. While no YaeJ-specific residues important for PTH activity occur in the structured GGQ domain, Arg118, Leu119, Lys122, Lys129 and Arg132 in the following C-terminal extension were required for PTH activity. All of these residues are completely conserved among bacteria. The equivalent residues were also found in the C-terminal extension of ICT1, allowing an appropriate sequence alignment between YaeJ and ICT1 proteins from various species. Single amino acid substitutions for each of these residues significantly decreased ribosome-binding efficiency. These biochemical findings provide clues to understanding how YaeJ enters the A-site of stalled ribosomes.
Formation of PGE 2 in the tumor microenvironment and its impact on tumorigenesis
- Tumor development usually follows predictable paths where tumor cells acquire common characteristics and features known as the hallmarks of cancer. Recently, additional characteristics have been added to these hallmarks since solid tumors are composed of a very heterogeneous population of transformed, formerly normal tissue cells and stromal cells, e.g. immune cells and fibroblasts. Compelling evidence suggests that stromal cells and tumor cells maintain a symbiotic relationship to build up the tumor microenvironment and to fuel tumor growth. In cancer therapies, common features of tumors such as unrestricted cell growth, suppression of immunological responses, and the ability to form new blood vessels (angiogenesis) have emerged as the main targets of interest. The lipid mediator prostaglandin E2 (PGE2) is known to promote all these features and thus, is connected to cancer progression in general. Its synthesis is triggered in response to stress factors or during inflammation. Inducible PGE2 production relies on the enzymes cyclooxygenase 2 (COX-2) and microsomal prostanglandin E synthase 1 (mPGES-1), which are simultaneously expressed in response to a variety of different stimuli and are functionally coupled. Inhibition of COX-2 with non-steroidal antiinflammatory drugs (NSAIDs) for cancer treatment is, however, limited by cardiovascular risks, since selective COX-2 inhibition disrupts the prostacyclin/thromboxane balance. Therefore targeting mPGES-1 downstream of COX-2 for PGE2 inhibition was evaluated in this work in different steps of carcinogenesis. Knockdown of mPGES-1 in DU145 prostate cancer cells revealed that the mPGES-1 status did not affect growth of monolayer tumor cells, but significantly impaired 3D growth of multi-cellular tumor spheroids (MCTS). Spheroid formation induced COX-2 in DU145 and other prostate cancer spheroids. High levels of PGE2 were detected in supernatants of DU145 MCTS as opposed to monolayer DU145 cells. Pharmacological inhibition of COX-2 and mPGES-1 confirmed the pivotal role of PGE2 for DU145 MCTS growth. Besides promoting spheroid growth, MCTS-derived PGE2 also inhibited cytotoxic T lymphocyte (CTL) activation. When investigating the mechanisms of COX-2 induction during spheroid formation, the typical tumor microenvironmental factors such as glucose deprivation, hypoxia or tumor cell apoptosis failed to enhance COX-2. Interestingly, when interfering with apoptosis in DU145 spheroids, the pan-caspase inhibitor Z-VAD-FMK triggered a Summary 12 shift towards necrosis, thus enhancing COX-2 expression. Coculturing viable DU145 monolayer cells with isolated heat-shocked-treated necrotic DU145 cells, but not with necrotic cell supernatants, induced COX-2 and PGE2, confirming the impact of necrosis for MCTS growth and CTL inhibition. As mentioned, in vivo tumors are very heterogenous mixtures of tumor cells and stromal cells e.g. immune cells. Hence, the interaction of the immune system with tumors was investigated in further experiments. When coculturing MCF-7 breast cancer spheroids with human peripheral blood mononuclear cells (PBMCs), only low levels of PGE2 were detected, since MCF-7 cells did not upregulate COX-2 during spheroid formation and did not induce PGE2 production by PBMCs. Under inflammatory conditions, by adding the toll-like receptor 4 (TLR4) agonist lipopolysaccharide (LPS) to cocultures, PGE2 production was triggered, spheroid sizes were reduced, and numbers of high levels of granzyme B expressing (GrBhi) CTLs were increased, while CD80 expression by tumor-associated phagocytes was also elevated. Inhibition of CD80 but not CD86 diminished numbers of GrBhi CTLs and attenuated spheroid lysis. To determine the role of ctivation-induced PGE2 production, use of the COX-2 inhibitor celecoxib and the experimental mPGES-1 inhibitor C3 further increased CD80 expression. Addition of PGE2, the prostaglandin E2 (EP2) receptor agonist butaprost, and the phosphodiesterase 4 (PDE4) inhibitor rolipram reduced LPS/C3-triggered CD80 expression, confirming the impact of COX- 2/mPGES-1-derived PGE2 on shaping phagocyte phenotypes in an EP2/cAMP-dependent manner. In a spontaneous breast cancer model (MMTV-PyMT), mPGES-1-deficiency significantly delayed tumor growth in mice, confirming an overall protumorigenic role of mPGES-1 in breast cancer development in vivo. However in tumors of mPGES-1-/- mice, tumor-infiltrating phagocytes expressed low levels of CD80 similar to their wildtype counterparts. These data suggest that the immunosuppressive microenvironment does not allow for immunostimulatory effects by mPGES-1 inhibition without an activating stimulus. Evidences in this study recommend the application of mPGES-1 inhibitors for treating cancer diseases, since mPGES-1 promotes tumor growth in multiple steps of carcinogenesis, ranging from well-characterized effects of tumor cell growth to immune suppression of CTL activity and phagocyte polarization. Regarding the latter, blunting PGE2 during immune activation may limit the tumor-favoring features of inflammation and improve the efficiency of TLR4 based immune therapies.
Development and application of optogenetic methods to functionally characterize synaptic transmission and neural circuits in the nematode Caenorhabditis elegans
Structural determinants for substrate specificity of the promiscuous multidrug efflux pump AcrB
- Opportunistic Gram-negative pathogens such as Escherichia coli, Klebsiella pneumoniae, Acinetobacter Baumanii and Pseudomonas aeruginosa are becoming more and more multiresistant against many commonly available antibiotics [39, 40]. An important resistance mechanism of Gram-negative bacteria is the efflux of noxious compounds by tripartite systems [39, 41-44]. The best studied and most clinically relevant tripartite system is the AcrA-AcrB-TolC system of Escherichia coli, where substrate recognition and energy transduction takes place in the inner membrane protein AcrB. AcrB has a remarkably huge substrate spectrum and can recognize structurally diverse molecules, such as hexan in contrast to erythromycin, as its substrates . Therefore, overproduction of the tripartite system can render a Gram-negative pathogen resistant against multiple antibiotics at once. The mechanisms of how AcrB is able to recognize such an enormous spectrum of molecules as substrates, without compromising its specificity (e.g. by neglecting essential compounds like lipids or gluclose as its susbtates), remained puzzling. Structural insight into substrate specificity was so far limited to two co-crystal structures of AcrB, where minocycline and doxorubicin, respectively, were identified bound to an internal binding pocket of AcrB. This binding pocket is particularly deeply buried into internal parts of the T monomer of AcrB and was, therefore, denoted deep binding pocket (DBP). Analysis of several AcrB co-crystal structures with substrate molecules bound to the DBP [4, 23, 25] indicated that the substrate promiscuity involved multisite binding modes within the DBP. Multisite binding modes, where different substrate molecules can bind to slightly different positions and orientations to the same binding pocket, is a common feature of multidrug recognizing proteins such as QacR or BmrR [27-29]. Nevertheless, AcrB's substrate spectrum is much broader than substrate spectra of most other multidrug recognizing proteins. Therefore, it is likely that additional mechanisms are involved in mediating the observed high substrate promiscuity of AcrB. In our recently published high-resolution AcrB/doxorubicin co-crystal structure (pdb entry: 4DX7 ) we were able to identify two additional substrate binding pockets in the L monomer of AcrB: i) the access pocket (AP), with an opening towards the periplasm, and ii) a putative binding site in a groove between transmembrane helices 8 and 9 (TM8/TM9 groove), accessible from the lipid layer of the inner membrane. Both binding pockets are likely to be access sites for substrates towards AcrB. Furthermore, each of the binding pockets are possibly specialized to recognize a specific subset of the entire substrate spectrum of AcrB, i.e. highly hydrophobic substrates (e.g. n-dodecyl-ß-d-maltoside or sodium dodecylsulfate) might access AcrB towards the TM8/TM9 groove and water soluble substrates (e.g. berberine) might access AcrB towards the AP. Since substrates will accumulate in the membrane or the periplasm according to their hydrophilic or hydrophobic nature, substrates will be "pre-selected" by the medium, rather than by the protein itself, and guided to their appropriate access site. This process is proposed to be called "medium- mediated pre-selection". The AcrB/doxorubicin co-crystal structure (pdb entry: 4DX7 ) furthermore revealed that the AP and DBP are in next neighborhood to each other and are separated by a switch loop. This switch loop adopts distinct conformations in the L, T and O monomers. Specific switch loop conformations are strongly involved in coordinating the selective occupation of both binding pockets, the AP and the DBP. The conformation of the switch loop in the L monomer (L-switch loop) opens the AP and closes the DBP, whereas the conformation of the switch loop in the T monomer (T-switch Loop) opens the DBP and closes the AP. An analysis of all asymmetric AcrB structures indicated that the L-switch loop is able to adopt multiple distinct conformations, whereas the conformation of T-switch loop remained largely congruent in all crystal structures. Moreover, each distinct switch loop conformation, observed in co-crystal structures of AcrB with occupied AP [4, 23], was perfectly adapted to the bound substrate molecule. Therefore, the putatively flexible switch loop is likely to act as an adaptive module and mediates a high binding pocket plasticity without altering the global protein structure. This binding mode is called adaptor-mediated binding mechanism, where an flexible adaptive module (like the switch loop) is able to adapt the surface shape of an binding pocket to different substrate molecules. Furthermore, structural and biochemical analyses of an AcrB G616N variant, revealed the involvement of specific switch loop conformations in the substrate specificity of AcrB. A substitution of G616, located on the switch loop, to N616 was able to alter the conformation of the switch loop exclusively in the L monomers of AcrB, whereas the switch loop conformations in T and O monomers remained congruent to the conformations observed in crystal structures of wildtype AcrB. Moreover, cells producing the AcrB G616N and MexB, both bearing the G616N amino acid substitution, exhibited a reduced resistance against certain substrates, whereas the resistance against most other substrates remained on the level of wildtype AcrB. Correlations of the phenotypes with minimal projection areas, a novel 2-spatiodimensional parameter which approximates the size of a substrate molecule, revealed that AcrB variants with a G616N substitution have a reduced efflux activity for exclusively large substrate molecules. The rejection of large substrates is most likely connected with altered L-switch loop conformations....
Topographic embedding of MOR18-2 in the mouse olfactory bulb
- Poster presentation at 1st International Workshop on Odor Spaces.
Mice are exceptional in their ability to capture their chemical environment, mapping the olfactory world into a basic sensory representation with over one thousand different types of chemical sensors, that is, olfactory sensory neurons (OSNs). OSNs of each type converge in the olfactory bulb onto exclusive distinct physiological areas called glomeruli. The glomeruli constitute the first relay station of olfactory stimulus representation in the mouse brain. Thus, the stimulus induced glomerular input pattern spatially embodies an important part of the sensory representation in the olfactory bulb. Still, topographic organization principles (chemotopy, tunotopy) are under debate. One reason might be that investigation are, due to experimental limitations, only performed on stimuli sets in the size of one hundred odors. But this represents only a tiny snapshot of the vast amount of molecules in the olfactory world and topographic relationships might be disguised in the incomplete representation of molecular receptive ranges (MRR). Therefore we investigated the problem with the MOR18-2 glomerulus as point of reference: First we determined it's MRR. Then, based on a measurement set covering this MRR, we elucidated the topographic embedding. It shows that MOR18-2 is embedded in a hierarchy of patchy tunotopic domains.