Year of publication
- 2009 (5) (remove)
- English (5) (remove)
- Structural rearrangements and subunit interactions in P2X receptors (2009)
- P2X receptors represent the third superfamily of ligand gated ion channels with ATP as their natural ligand. Most of the mammalian P2X receptors are non-selective cation channels, which upon activation, mediate membrane depolarization and have physiological roles ranging from fast excitatory synaptic transmission, modulation of pain-sensation, LTP to apoptosis etc. In spite of them being an attractive drug target, their potential as a drug target is limited by the lack of basic understanding of the structure-function relationship of these receptors. In my thesis, I have investigated the behavior of homomeric P2X receptor subunits with the help of photolabeling and fluorescence techniques coupled to electrophysiological measurements using Xenopus laevis oocytes heterologous expression system. Concurrent photolabeling by BzATP and current recordings from the same set of receptors in real time has revealed that the gating process in homomeric P2X receptors is contributed individually by each subunit in an additive manner. Our study for the first time describes the agonist potency of Alexa-ATP (a fluorescent ATP analog) on P2X1 receptors. The use of Alexa-ATP in our experiments elucidated that receptor subunits are not independent but interacting with each other in a cooperative manner. The type of cooperativity, however, depended on the type and concentrations of allosteric/competing ligands. Based on our results, in my thesis we propose an allosteric model for ligand-receptor interactions in P2X receptors. When simulated, the model could replicate our experimental findings thus, further validating our model. Further, correlation between occupancy of P2X1 receptors (determined using binding curve for Alexa-ATP) with the steady-state desensitization suggests that binding of three agonist molecules per receptor are required to desensitize P2X1 receptors. We further extended the approach of fluorescence with electrophysiological measurement to assign the role for different domains in P2X1 receptors with the help of environmental sensitive, cysteine reactive fluorophore (TMRM). Cysteine rich domain-1 of P2X1 receptors (C117-C165) was found to be involved in structural rearrangements after agonist and antagonist binding. In contrast to the present understanding, that the binding of an antagonist cannot induce desensitization in P2X1 receptors and the receptors need to open first before undergoing desensitization, we propose based on our results that a competitive antagonist can also induce desensitization in P2X1 receptors by bypassing the open state. We have attempted to answer few intriguing questions in the field of P2X receptor research and we think that our answers provide many avenues to the basic understanding of functioning of P2X receptors.
- Crystallization and structural characterization of protein complexes involved in the energy metabolism of Yarrowia lipolytica (2009)
- 1. Fab co-complexes of proton pumping NADH:ubiquinone oxidoreductase (complex I) Fab fragments suitable for co-crystallization with complex I were generated using an immobilized papainbased protocol. The binding of the antibody fragments to complex I was verified using Surface Plasmon Resonance and size exclusion chromatography. The binding constants of the antibodies and their respective Fab fragments were found to be in the nanomolar range. This work presents the first report on successful crystallization of complex I (proton pumping NADH:ubiquinone oxidoreductase) from Yarrowia lipolytica with proteolytic Fab fragments. The quality of the crystals was significantly improved when compared to the initial experiments and the best crystals diffracted X-rays to a resolution of ~7 Å. The activity of complex I remained uninfluenced by antibody fragment binding. The initial diffraction data suggest that the complex I/Fab co-complex crystals represent a space group different to the one observed for the native protein. Ongoing experiments are aimed at further enhancements of the diffraction quality of the crystals. Providing a different space group the CI/Fab co-complexes may become a very useful approach for structure determination of the enzyme. Moreover, the bound Fab offers an additional possibility to generate phase information. The antibody-mediated crystallization represents a valuable tool in structural characterization of the NADH:oxidoreductase subcomplexes or even single subunits. 2. UDP-glucose pyrophosphorylase UDP-glucose pyrophosphorylase from Yarrowia lipolytica displays affinity towards Ni2+ NTA and was first detected in a contaminated sample of complex I. Following, separation from complex I, Ugp1p was purified using anion exchange chromatography. Sequence similarity studies revealed high identity to other known pyrophosphorylases. As indicated by laser-based mass spectrometry method (LILBID) Ugp1p from Y. lipolytica builds octamers similarly to the enzyme from Saccharomyces cerevisiae. The initial crystals grew as thin needles favorably in sitting drop setups. The size of the crystals was increased by employment of a micro batch technique. The improved crystals diffracted X-rays to a resolution of 3.2 Å at the synchrotron beamline. Structural characterization is under way using a molecular replacement approach based on the published structure of baker’s yeast UGPase.
- Targeted cell entry of lentiviral vectors (2009)
- Lentiviral vectors mediate gene transfer into dividing and most non-dividing cells. Thereby, they stably integrate the transgene into the host cell genome. For this reason, lentiviral vectors are a promising tool for gene therapy. However, safety and efficiency of lentiviral mediated gene transfer still needs to be optimised. Ideally, cell entry should be restricted to the cell population relevant for a particular therapeutic application. Furthermore, lentiviral vectors able to transduce quiescent lymphocytes are desirable. Although many approaches were followed to engineer retroviral envelope proteins, an effective and universally applicable system for retargeting of lentiviral cell entry is still not available. Just before the experimental work of this thesis was started, retargeting of measles virus (MV) cell entry was achieved. This virus has two types of envelope glycoproteins, the hemagglutinin (H) protein responsible for receptor recognition and the fusion (F) protein mediating membrane fusion. For retargeting, the H protein was mutated in its interaction sites for the native MV receptors and a ligand or a single-chain antibody (scAb) was fused to its ectodomain. It was hypothesised that the retargeting system of MV can be transferred to lentiviral vectors by pseudotyping human immunodeficiency virus-1 (HIV-1) derived vector particles with the MV glycoproteins. As the unmodified MV glycoproteins did not pseudotype HIV vectors, two F and 15 H protein variants carrying stepwise truncations or amino acid (aa) exchanges in their cytoplasmic tails were screened for their ability to form MV-HIV pseudotypes. The combinations Hcd18/Fcd30, Hcd19/Fcd30 and Hcd24+4A/Fcd30 led to most efficient pseudotype formation with titers above 10exp6 transducing units /ml, using concentrated particles. The F cytoplasmic tail was truncated by 30 aa and the H cytoplasmic tail was truncated by 18, 19 or 24 residues with four added alanines after the start methionine in the latter case. Western blot analysis indicated that particle incorporation of the MV glycoproteins was enhanced upon truncation of their cytoplasmic tails. With the MV-HIV vectors high titers on different cell lines expressing one or both MV receptors were obtained, whereas MV receptor-negative cells remained untransduced. Titers were enhanced using an optimal H to F plasmid ratio (1:7) during vector particle production. Based on the described pseudotyping with the MV glycoprotein variants, HIV vectors retargeted to the epidermal growth factor receptor (EGFR) or the B cell surface marker CD20 were generated. For the production of the retargeted vectors MVaEGFR-HIV and MVaCD20-HIV, Fcd30 together with a native receptor blind Hcd18 protein, displaying at its ectodomain either the ligand EGF or a scAb directed against CD20 were used. With these vectors, gene transfer into target receptor-positive cells was several orders of magnitude more efficient than into control cells. The almost complete absence of background transduction of non-target cells was e.g. demonstrated in mixed cell populations, where the CD20-targeting vector selectively eliminated CD20-positive cells upon suicide gene transfer. Remarkably, transduction of activated primary human CD20-positive B cells was much more efficient with the MVaCD20-HIV vector than with the standard pseudotype vector VSV-G-HIV. Even more surprisingly, MVaCD20-HIV vectors were able to transduce quiescent primary human B cells, which until then had been resistant towards lentiviral gene transfer. The most critical step during the production of MV-HIV pseudotypes was the identification of H cytoplasmic tail mutants that allowed pseudotyping while retaining the fusion helper function. In contrast to previously inefficient targeting strategies, the reason for the success of this novel targeting system must be based on the separation of the receptor recognition and fusion functions onto two different proteins. Furthermore, with the CD20-targeting vector transduction of quiescent B cells was demonstrated for the first time. Own data and literature data suggest that CD20 binding and hyper-cross-linking by the vector particles results in calcium influx and thus activation of quiescent B cells. Alternatively this feature may be based on a residual binding activity of the MV glycoproteins to the native MV receptors that is insufficient for entry but induces cytoskeleton rearrangements dissolving the post-entry block of HIV vectors. Hence, in this thesis efficient retargeting of lentiviral vectors and transduction of quiescent cells was combined. This novel targeting strategy should be easily adaptable to many other target molecules by extending the modified MV H protein with appropriate specific domains or scAbs. It should now be possible to tailor lentiviral vectors for highly selective gene transfer into any desired target cell population with an unprecedented degree of efficiency.
- Mechanisms of energy transfer and conversion in plant light-harvesting complex II (2009)
- The light-harvesting complex of photosystem II (LHC-II) is the major antenna complex in plant photosynthesis. It accounts for roughly 30% of the total protein in plant chloroplasts, which makes it arguably the most abundant membrane protein on Earth, and binds about half of plant chlorophyll (Chl). The complex assembles as a trimer in the thylakoid membrane and binds a total of 54 pigment molecules, including 24 Chl a, 18 Chl b, 6 lutein (Lut), 3 neoxanthin (Neo) and 3 violaxanthin (Vio). LHC-II has five key roles in plant photosynthesis. It: (1) harvests sunlight and transmits excitation energy to the reaction centres of photosystems II and I, (2) regulates the amount of excitation energy reaching each of the two photosystems, (3) has a structural role in the architecture of the photosynthetic supercomplexes, (4) contributes to the tight appression of thylakoid membranes in chloroplast grana, and (5) protects the photosynthetic apparatus from photo damage by non photochemical quenching (NPQ). A major fraction of NPQ is accounted for its energy-dependent component qE. Despite being critical for plant survival and having been studied for decades, the exact details of how excess absorbed light energy is dissipated under qE conditions remain enigmatic. Today it is accepted that qE is regulated by the magnitude of the pH gradient (ΔpH) across the thylakoid membrane. It is also well documented that the drop in pH in the thylakoid lumen during high-light conditions activates the enzyme violaxanthin de-epoxidase (VDE), which converts the carotenoid Vio into zeaxanthin (Zea) as part of the xanthophyll cycle. Additionally, studies with Arabidopsis mutants revealed that the photosystem II subunit PsbS is necessary for qE. How these physiological responses switch LHC-II from the active, energy transmitting to the quenched, energy-dissipating state, in which the solar energy is not transmitted to the photosystems but instead dissipated as heat, remains unclear and is the subject of this thesis. From the results obtained during this doctoral work, five main conclusions can be drawn concerning the mechanism of qE: 1. Substitution of Vio by Zea in LHC-II is not sufficient for efficient dissipation of excess excitation energy. 2. Aggregation quenching of LHC-II does not require Vio, Neo nor a specific Chl pair. 3. With one exception, the pigment structure in LHC-II is rigid. 4. The two X-ray structures of LHC-II show the same energy transmitting state of the complex. 5. Crystalline LHC-II resembles the complex in the thylakoid membrane. Models of the aggregation quenching mechanism in vitro and the qE mechanism in vivo are presented as a corollary of this doctoral work. LHC-II aggregation quenching in vitro is attributed to the formation of energy sinks on the periphery of LHC-II through random interaction with other trimers, free pigments or impurities. A similar but unrelated process is proposed to occur in the thylakoid membrane, by which excess excitation energy is dissipated upon specific interaction between LHC-II and a PsbS monomer carrying Zea. At the end of this thesis, an innovative experimental model for the analysis of all key aspects of qE is proposed in order to finally solve the qE enigma, one of the last unresolved problems in photosynthesis research.
- Thermodynamic and kinetic characterisation of the interaction between mitochondrial cytochrome bc 1 complex and cytochrome c and implications for transient binding and electron transfer (2009)
- In mitochondrial respiration, the soluble protein cytochrome c accepts an electron from the membrane bound cytochrome bc1. The interaction between cytochrome bc1 and cytochrome c is highly transient in nature, enabling turnover numbers greater than 160 s-1. Yeast cytochrome bc1 has been successfully crystallised with bound cytochrome c with the help of an antibody fragment (Lange and Hunte 2002; Solmaz and Hunte 2008). In all crystal structures of the complex, the homodimeric cytochrome bc1 binds only one cytochrome c, with the binding site located on subunit cytochrome c1. Univalent cytochrome c binding is correlated with conformational changes of the Rieske protein head domain and subunit QCR6p. The interface of the complex is small. The haem moieties are centrally located in a mainly non-polar contact site that includes a cation–! interaction and is surrounded by complementary charged residues. The crystal structure is in agreement with the general architecture of the interfaces of transient redox complexes and also reveals several interesting features unique to the cytochrome bc1. On the basis of the crystal structures, an extensive thermodynamic and kinetic characterisation of the interaction was carried out in this work to challenge the static snapshot of the bound proteins in the crystal structure as the relevant physiological electron transfer. The thermodynamic parameters of the interaction between the redox partners were determined using isothermal titration calorimetry (ITC). The association constant for cytochrome bc1 and cytochrome c in oxidised state under physiological ionic strength of 120 mM at 25 °C, was determined to be 5 " 103 M-1 by direct ITC titration. So, the partners interact with an affinity of 200 #M. In spite of the low affinity the complex has a life time ($ = 1/koff) of 5 #second, sufficiently long to enable the theoretically calculated electron transfer rates of 1.0 " 106 to 2.6 " 107 s%1 with a lifetime ($ = 1/rate) of 1-0.04 μseconds and experimentally determined rate of 7.7 " 104 s%1 with a lifetime of 13 μseconds. The low affinity makes it difficult to ascertain the stoichiometry of binding. The enthalpy of the interaction is endothermic, which is consistent with the nature of an interface where hydrophobic interactions are dominant. The enthalpy and entropy is 3.6 kJmol-1 and 83 kJmol-1K-1, respectively. The importance of key interface residues was also investigated. The role of the interface residue G89 of cytochrome c which might have a role in the dissociation of the complex has been probed by site-directed mutagenesis. The interface contains a cation-! interaction between F230 of cytochrome bc1 and R19 of cytochrome c, which is thought to provide the specificity to the interaction between the otherwise promiscuous partners. To analyse the role of this interaction pair in electron transfer, F230L and F230W mutants were used to measure direct electron transfer rates by flash photolysis and steady state kinetics. The findings indicate that another ! system can work as functional substitution of F230, while deleting the ! system has a deleterious effect on the complex formation. The inability of F230L to achieve the transient and steady state turnover rates as wild type protein indicates a scenario where the variant achieves an altered bound state with inefficient electron transfer pathways and higher edge-to-edge distance. The role of supernumerary subunit QCR6p in complex formation was investigated by steady state kinetics measurements. Subunit QCR6p does not interact directly with cytochrome c but is positioned in such a way that it could electrostatically steer cytochrome c in a reactive ensemble. The highly acidic and disordered N-terminus of QCR6p could interact with a patch of conserved lysine residues on cytochrome c. The role of subunit QCR6p has been assessed using QCR6p deleted cytochrome bc1 and a lysine variant of cytochrome c. The results show that QCR6p not only affects the kinetics of the interaction but is also important for the stability of cytochrome bc1. The kinetic and thermodynamic data obtained during this study provide evidence for the functional importance of non-catalytic cytochrome bc1 subunit QCR6p, show that the entropy driven interaction is indeed of low affinity and highly transient in nature and indicate that the interface is well suited to ensure the high turnover of the electron transfer chain where cytochrome c interacts with multiple partners using overlapping interfaces. The suggested role of the cation-! interaction as a highly specific interaction has been validated.