Identification and characterization two isoforms of NADH:ubiquinone oxidoreductase from the hyperthermophilic eubacterium Aquifex aeolicus.

The NADH:ubiquinone oxidoreductase (complex I) is the first enzyme of the respiratory chain and the entry point for most electrons. Generally, the bacterial complex I consists of 14 core subunits, homologues of which are also found in complex I of mitochondria. In complex I preparations from the hyperthermophilic bacterium Aquifex aeolicus we have identified 20 partially homologous subunits by combining MALDI-TOF and LILBID mass spectrometry methods. The subunits could be assigned to two different complex I isoforms, named NQOR1 and NQOR2. NQOR1 consists of subunits NuoA2, NuoB, NuoD2, NuoE, NuoF, NuoG, NuoI1, NuoH1, NuoJ1, NuoK1, NuoL1, NuoM1 and NuoN1, with an entire mass of 504.17 kDa. NQOR2 comprises subunits NuoA1, NuoB, NuoD1, NuoE, NuoF, NuoG, NuoH2, NuoI2, NuoJ1, NuoK1, NuoL2, NuoM2 and NuoN2, with a total mass of 523.99 kDa. Three Fe-S clusters could be identified by EPR spectroscopy in a preparation containing predominantly NQOR1. These were tentatively assigned to a binuclear center N1, and two tetranuclear centers, N2 and N4. The redox midpoint potentials of N1 and N2 are -273 mV and -184 mV, respectively. Specific activity assays indicated that NQOR1 from cells grown under low concentrations of oxygen was the more active form. Increasing the concentration of oxygen in the bacterial cultures induced formation of NQOR2 showing the lower specific activity.

Stoichiometry and deletion analyses of subunits in the heterotrimeric F-ATP synthase c ring from the acetogenic bacterium Acetobacterium woodii.

The ion-translocating c ring of the Na(+) F1 Fo ATP synthase of the anaerobic bacterium Acetobacterium woodii is the first heteromeric c ring found in nature that contains one V- (c1 ) and two F-type-like c subunits (c2 /c3 ), the latter of identical amino acid sequence. To address whether they are of equal or different importance for function, they were deleted in combination or individually. Deletion of c1 was compensated by incorporation of two c2 /c3 subunits but the enzyme was unstable and largely impaired in Na(+) transport. Deletion of c2 was compensated by incorporation of c3 but also led to a reduction of Na(+) transport. Deletion of c3 had no effect. In contrast, deletion of both c2 and c3 led to a complete loss of ATPase activity at the cytoplasmic membrane. Mass spectrometric analysis of c2 +1 Ala and c2 +2 Ala variants revealed a copy number of 8 : 1 for c2 /c3 which is consistent with the biochemical characteristics of the variants. These data indicate a role of c1 in assembly and a function of c2 as the predominant c ring constituent.

Functional production of the Na+ F1F(O) ATP synthase from Acetobacterium woodii in Escherichia coli requires the native AtpI.

The Na(+) F(1)F(O) ATP synthase of the anaerobic, acetogenic bacterium Acetobacterium woodii has a unique F(O)V(O) hybrid rotor that contains nine copies of a F(O)-like c subunit and one copy of a V(O)-like c(1) subunit with one ion binding site in four transmembrane helices whose cellular function is obscure. Since a genetic system to address the role of different c subunits is not available for this bacterium, we aimed at a heterologous expression system. Therefore, we cloned and expressed its Na(+) F(1)F(O) ATP synthase operon in Escherichia coli. A Δatp mutant of E. coli produced a functional, membrane-bound Na(+) F(1)F(O) ATP synthase that was purified in a single step after inserting a His(6)-tag to its ß subunit. The purified enzyme was competent in Na(+) transport and contained the F(O)V(O) hybrid rotor in the same stoichiometry as in A. woodii. Deletion of the atpI gene from the A. woodii operon resulted in a loss of the c ring and a mis-assembled Na(+) F(1)F(O) ATP synthase. AtpI from E. coli could not substitute AtpI from A. woodii. These data demonstrate for the first time a functional production of a F(O)V(O) hybrid rotor in E. coli and revealed that the native AtpI is required for assembly of the hybrid rotor.

Recognition of the let-7g miRNA precursor by human Lin28B.

Mammalian homologs of lin28: Lin28 and Lin28B block the post-transcriptional processing of the let-7 family of miRNAs. We report that in vitro the terminal stem-loop region of the let-7g miRNA precursor (pre-let-7g) required to bind Lin28B is restricted to 24 nucleotides (nt) including the 3' GGAG motif. Additionally, full length Lin28B is required for efficient binding to pre-let-7g and the stoichiometry of the complex is 1:1. Molecular dynamics (MD) simulations reveal the interactions of the pre-let-7g stem-loop and the GGAG motif in the stem region to the cold shock domain (CSD) and to the zinc knuckle domain (ZKD) of Lin28B, respectively.

Structural study on the architecture of the bacterial ATP synthase Fo motor.

We purified the F(o) complex from the Ilyobacter tartaricus Na(+)-translocating F(1)F(o)-ATP synthase and performed a biochemical and structural study. Laser-induced liquid bead ion desorption MS analysis demonstrates that all three subunits of the isolated F(o) complex were present and in native stoichiometry (ab(2)c(11)). Cryoelectron microscopy of 2D crystals yielded a projection map at a resolution of 7.0 Å showing electron densities from the c(11) rotor ring and up to seven adjacent helices. A bundle of four helices belongs to the stator a-subunit and is in contact with c(11). A fifth helix adjacent to the four-helix bundle interacts very closely with a c-subunit helix, which slightly shifts its position toward the ring center. Atomic force microscopy confirms the presence of the F(o) stator, and a height profile reveals that it protrudes less from the membrane than c(11). The data limit the dimensions of the subunit a/c-ring interface: Three helices from the stator region are in contact with three c(11) helices. The location and distances of the stator helices impose spatial restrictions on the bacterial F(o) complex.

Heme-copper terminal oxidase using both cytochrome c and ubiquinol as electron donors.

The cytochrome c oxidase Cox2 has been purified from native membranes of the hyperthermophilic eubacterium Aquifex aeolicus. It is a cytochrome ba(3) oxidase belonging to the family B of the heme-copper containing terminal oxidases. It consists of three subunits, subunit I (CoxA2, 63.9 kDa), subunit II (CoxB2, 16.8 kDa), and an additional subunit IIa of 5.2 kDa. Surprisingly it is able to oxidize both reduced cytochrome c and ubiquinol in a cyanide sensitive manner. Cox2 is part of a respiratory chain supercomplex. This supercomplex contains the fully assembled cytochrome bc(1) complex and Cox2. Although direct ubiquinol oxidation by Cox2 conserves less energy than ubiquinol oxidation by the cytochrome bc(1) complex followed by cytochrome c oxidation by a cytochrome c oxidase, ubiquinol oxidation by Cox2 is of advantage when all ubiquinone would be completely reduced to ubiquinol, e.g., by the sulfidequinone oxidoreductase, because the cytochrome bc(1) complex requires the presence of ubiquinone to function according to the Q-cycle mechanism. In the case that all ubiquinone has been reduced to ubiquinol its reoxidation by Cox2 will enable the cytochrome bc(1) complex to resume working.

Analysis of AcrB and AcrB/DARPin ligand complexes by LILBID MS.

The AcrA/AcrB/TolC complex is responsible for intrinsic multidrug resistance (MDR) in Escherichia coli. Together with the periplasmic adaptor protein AcrA and the outer membrane channel TolC, the inner membrane component AcrB forms an efflux complex that spans both the inner and outer membrane and bridges the periplasm of the Gram-negative cell. Within the entire tripartite complex, homotrimeric AcrB plays a central role in energy transduction and substrate selection. In vitro selected designed ankyrin repeat proteins (DARPin) that specifically bind to the periplasmic domain of AcrB were shown to ameliorate diffraction resolution of AcrB/DARPin protein co-crystals (G. Sennhauser, P. Amstutz, C. Briand, O. Storchenegger, M.G. Grutter, Drug export pathway of multidrug exporter AcrB revealed by DARPin inhibitors, PLoS Biol 5 (2007) e7). Structural analysis by X-ray crystallography revealed that 2 DARPin molecules were bound to the trimeric AcrB wildtype protein in the crystal, whereas the V612F and G616N AcrB variant crystal structures show 3 DARPin molecules bound to the trimer. These specific stoichiometric differences were analyzed in solution via densitometry after microchannel electrophoresis, analytical ultracentrifugation and via laser-induced liquid bead ion desorption mass spectrometry (LILBID-MS). Using the latter technology, we investigated the gradual disassembly of the AcrB trimer and bound DARPin ligands in dependence on laser intensity in solution. At low laser intensity, the release of the detergent molecule micelle from the AcrB/DARPin complex was observed. By increasing laser intensity, dimeric and monomeric AcrB species with bound DARPin molecules were detected showing the high affinity binding of DARPin to monomeric AcrB species. High laser intensity LILBID MS experiments indicated a spectral shift of the monomeric AcrB peak of 3.1kDa, representing a low molecular weight ligand in all detergent-solubilized AcrB samples and in the AcrB crystal. The identity of this ligand was further investigated using phospholipid analysis of purified AcrB and AcrB variant samples, and indicated the presence of phosphatidylethanolamine and possibly cardiolipin, both constituents of the Escherichia coli membrane.

Mass spectrometric characterization of oligomers in Pseudomonas aeruginosa azurin solutions.

We have employed laser-induced liquid bead ion desorption mass spectroscopy (LILBID MS) to study the solution behavior of Pseudomonas aeruginosa azurin as well as two mutants and corresponding Re-labeled derivatives containing a Re(CO)(3)(4,7-dimethyl-1,10-phenanthroline)(+) chromophore appended to a surface histidine. LILBID spectra show broad oligomer distributions whose particular patterns depend on the solution composition (pure H(2)O, 20-30 mM NaCl, 20 and 50 mM NaP(i) or NH(4)P(i) at pH = 7). The distribution maximum shifts to smaller oligomers upon decreasing the azurin concentration and increasing the buffer concentration. Oligomerization is less extensive for native azurin than its mutants. The oligomerization propensities of unlabeled and Re-labeled proteins are generally comparable, and only Re126 shows some preference for the dimer that persists even in highly diluted solutions. Peak shifts to higher masses and broadening in 20-50 mM NaP(i) confirm strong azurin association with buffer ions and solvation. We have found that LILBID MS reveals the solution behavior of weakly bound nonspecific protein oligomers, clearly distinguishing individual components of the oligomer distribution. Independently, average data on oligomerization and the dependence on solution composition were obtained by time-resolved anisotropy of the Re-label photoluminescence that confirmed relatively long rotation correlation times, 6-30 ns, depending on Re-azurin and solution composition. Labeling proteins with Re-chromophores that have long-lived phosphorescence extends the time scale of anisotropy measurements to hundreds of nanoseconds, thereby opening the way for investigations of large oligomers with long rotation times.

Fold and function of the InlB B-repeat.

Host cell invasion by the facultative intracellular pathogen Listeria monocytogenes requires the invasion protein InlB in many cell types. InlB consists of an N-terminal internalin domain that binds the host cell receptor tyrosine kinase Met and C-terminal GW domains that bind to glycosaminoglycans (GAGs). Met binding and activation is required for host cell invasion, while the interaction between GW domains and GAGs enhances this effect. Soluble InlB elicits the same cellular phenotypes as the natural Met ligand hepatocyte growth factor/scatter factor (HGF/SF), e.g. cell scatter. So far, little is known about the central part of InlB, the B-repeat. Here we present a structural and functional characterization of the InlB B-repeat. The crystal structure reveals a variation of the ß-grasp fold that is most similar to small ubiquitin-like modifiers (SUMOs). However, structural similarity also suggests a potential evolutionary relation to bacterial mucin-binding proteins. The B-repeat defines the prototype structure of a hitherto uncharacterized domain present in over a thousand bacterial proteins. Generally, this domain probably acts as a spacer or a receptor-binding domain in extracellular multi-domain proteins. In cellular assays the B-repeat acts synergistically with the internalin domain conferring to it the ability to stimulate cell motility. Thus, the B-repeat probably binds a further host cell receptor and thereby enhances signaling downstream of Met.

DNA damage in oocytes induces a switch of the quality control factor TAp63α from dimer to tetramer.

TAp63α, a homolog of the p53 tumor suppressor, is a quality control factor in the female germline. Remarkably, already undamaged oocytes express high levels of the protein, suggesting that TAp63α's activity is under tight control of an inhibitory mechanism. Biochemical studies have proposed that inhibition requires the C-terminal transactivation inhibitory domain. However, the structural mechanism of TAp63α inhibition remains unknown. Here, we show that TAp63α is kept in an inactive dimeric state. We reveal that relief of inhibition leads to tetramer formation with ∼20-fold higher DNA affinity. In vivo, phosphorylation-triggered tetramerization of TAp63α is not reversible by dephosphorylation. Furthermore, we show that a helix in the oligomerization domain of p63 is crucial for tetramer stabilization and competes with the transactivation domain for the same binding site. Our results demonstrate how TAp63α is inhibited by complex domain-domain interactions that provide the basis for regulating quality control in oocytes.

Asymmetric ATP hydrolysis cycle of the heterodimeric multidrug ABC transport complex TmrAB from Thermus thermophilus.

ATP-binding cassette (ABC) systems translocate a wide range of solutes across cellular membranes. The thermophilic gram-negative eubacterium Thermus thermophilus, a model organism for structural genomics and systems biology, discloses ∼46 ABC proteins, which are largely uncharacterized. Here, we functionally analyzed the first two and only ABC half-transporters of the hyperthermophilic bacterium, TmrA and TmrB. The ABC system mediates uptake of the drug Hoechst 33342 in inside-out oriented vesicles that is inhibited by verapamil. TmrA and TmrB form a stable heterodimeric complex hydrolyzing ATP with a K(m) of 0.9 mm and k(cat) of 9 s(-1) at 68 °C. Two nucleotides can be trapped in the heterodimeric ABC complex either by vanadate or by mutation inhibiting ATP hydrolysis. Nucleotide trapping requires permissive temperatures, at which a conformational ATP switch is possible. We further demonstrate that the canonic glutamate 523 of TmrA is essential for rapid conversion of the ATP/ATP-bound complex into its ADP/ATP state, whereas the corresponding aspartate in TmrB (Asp-500) has only a regulatory role. Notably, exchange of this single noncanonic residue into a catalytic glutamate cannot rescue the function of the E523Q/D500E complex, implicating a built-in asymmetry of the complex. However, slow ATP hydrolysis in the newly generated canonic site (D500E) strictly depends on the formation of a posthydrolysis state in the consensus site, indicating an allosteric coupling of both active sites.

The archaeal Lsm protein binds to small RNAs.

Proteins of the Lsm family, including eukaryotic Sm proteins and bacterial Hfq, are key players in RNA metabolism. Little is known about the archaeal homologues of these proteins. Therefore, we characterized the Lsm protein from the haloarchaeon Haloferax volcanii using in vitro and in vivo approaches. H. volcanii encodes a single Lsm protein, which belongs to the Lsm1 subfamily. The lsm gene is co-transcribed and overlaps with the gene for the ribosomal protein L37e. Northern blot analysis shows that the lsm gene is differentially transcribed. The Lsm protein forms homoheptameric complexes and has a copy number of 4000 molecules/cell. In vitro analyses using electrophoretic mobility shift assays and ultrasoft mass spectrometry (laser-induced liquid bead ion desorption) showed a complex formation of the recombinant Lsm protein with oligo(U)-RNA, tRNAs, and an small RNA. Co-immunoprecipitation with a FLAG-tagged Lsm protein produced in vivo confirmed that the protein binds to small RNAs. Furthermore, the co-immunoprecipitation revealed several protein interaction partners, suggesting its involvement in different cellular pathways. The deletion of the lsm gene is viable, resulting in a pleiotropic phenotype, indicating that the haloarchaeal Lsm is involved in many cellular processes, which is in congruence with the number of protein interaction partners.

ATP synthases: cellular nanomotors characterized by LILBID mass spectrometry.

Mass spectrometry of membrane protein complexes is still a methodological challenge due to hydrophobic and hydrophilic parts of the species and the fact that all subunits are bound non-covalently together. The present study with the novel laser induced liquid bead ion desorption mass spectrometry (LILBID-MS) reports on the determination of the subunit composition of the F(1)F(o)-ATP synthase from Bacillus pseudofirmus OF4, that of both bovine heart and, for the first time, of human heart mitochondrial F(1)F(o)-ATP synthases. Under selected buffer conditions the mass of the intact F(1)F(o)-ATP synthase of B. pseudofirmus OF4 could be measured, allowing the analysis of complex subunit stoichiometry. The agreement with theoretical masses derived from sequence databases is very good. A comparison of the ATP synthase subunit composition of 5 different ATPases reveals differences in the complexity of eukaryotic and bacterial ATP synthases. However, whereas the overall construction of eukaryotic enzymes is more complex than the bacterial ones, functionally important subunits are conserved among all ATPases.

Studying the stoichiometries of membrane proteins by mass spectrometry: microbial rhodopsins and a potassium ion channel.

In the present work we demonstrate the advantages of LILBID mass spectrometry in the mass analysis of membrane proteins with emphasis on ion-pumps and channels. Due to their hydrophobic nature, membrane proteins have to be solubilized by detergents. However, these molecules tend to complicate the analysis by mass spectrometry. In LILBID, detergent molecules are readily tolerated which allows for the study of solution phase quaternary structures of membrane proteins. This is shown for the proton-pump bacteriorhodospin and the potassium channel KcsA where in both cases the stoichiometries found by LILBID reflect the known structures from 2D or 3D crystals. With proteorhodopsin we demonstrate a preliminary detergent screening showing different structures in different detergents and the implications for the functionality of this protein. We show that Triton-X 100 prevents the formation of the pentamer of proteorhodopsin. Furthermore, the quaternary structures of proteorhodopsin cloned without the signal peptide and of the cation channel channelrhodopsin-2 were studied. The intrinsic properties of channelrhodopsin-2 allow for mass spectrometric analysis in very high salt concentrations up to 100 mM of NaCl. In summary we demonstrate that LILBID is an alternative mass spectrometric method for the analysis of membrane proteins from solution phase.

Probing the limits of liquid droplet laser desorption mass spectrometry in the analysis of oligonucleotides and nucleic acids.

In the present work we demonstrate the advantages of LILBID mass spectrometry (laser-induced liquid bead ion desorption) in the analysis of nucleic acids and large oligonucleotides. For established methods like matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), the mass analysis of oligonucleotides or of noncovalent oligonucleotide-protein complexes, in particular of very large ones, still represents a considerable challenge either due to the lack of native solutions or nonspecific adduct formation or due to a reduced salt tolerance or a high charge state of the ions. With LILBID, oligonucleotides, solvated in micro-droplets of aqueous buffer at certain pH and ion strength, are brought into the gas phase by laser ablation. We show that our method is able to detect single- and double-stranded oligonucleotides with high softness, demonstrated by the buffer dependence of the melting of a duplex. The absolute sensitivity is in the attomole range concomitant with a total analyte consumption in the femtomole region. The upper mass limit of oligonucleotides still detected with good signal-to-noise ratio with LILBID is the 1.66 MDa plasmid pUC19. With DNA ladders from short duplexes with sticky ends, we show that LILBID correctly reflects the relative thermodynamic stabilities of the ladders. Moreover, as an example for a specific DNA-protein complex we show that a NF-kappaB p50 homodimer binds sequence specifically to its match DNA. In summary we demonstrate that LILBID, although presently performed only with low mass resolution, due to these advantages, is an alternative mass spectrometric method for the analysis of oligonucleotides in general and of specific noncovalent nucleic acid-protein complexes in particular.

An intermediate step in the evolution of ATPases: a hybrid F(0)-V(0) rotor in a bacterial Na(+) F(1)F(0) ATP synthase.

The Na(+) F(1)F(0) ATP synthase operon of the anaerobic, acetogenic bacterium Acetobacterium woodii is unique because it encodes two types of c subunits, two identical 8 kDa bacterial F(0)-like c subunits (c(2) and c(3)), with two transmembrane helices, and a 18 kDa eukaryal V(0)-like (c(1)) c subunit, with four transmembrane helices but only one binding site. To determine whether both types of rotor subunits are present in the same c ring, we have isolated and studied the composition of the c ring. High-resolution atomic force microscopy of 2D crystals revealed 11 domains, each corresponding to two transmembrane helices. A projection map derived from electron micrographs, calculated to 5 A resolution, revealed that each c ring contains two concentric, slightly staggered, packed rings, each composed of 11 densities, representing 22 transmembrane helices. The inner and outer diameters of the rings, measured at the density borders, are approximately 17 and 50 A. Mass determination by laser-induced liquid beam ion desorption provided evidence that the c rings contain both types of c subunits. The stoichiometry for c(2)/c(3) : c(1) was 9 : 1. Furthermore, this stoichiometry was independent of the carbon source of the growth medium. These analyses clearly demonstrate, for the first time, an F(0)-V(0) hybrid motor in an ATP synthase.

Insights into the structure of cyclohexane from femtosecond degenerate four-wave mixing spectroscopy and ab initio calculations.

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order Møller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.