The kinetic parameters and energy cost of the Hsp70 chaperone as a polypeptide unfoldase.

Hsp70-Hsp40-NEF and possibly Hsp100 are the only known molecular chaperones that can use the energy of ATP to convert stably pre-aggregated polypeptides into natively refolded proteins. However, the kinetic parameters and ATP costs have remained elusive because refolding reactions have only been successful with a molar excess of chaperones over their polypeptide substrates. Here we describe a stable, misfolded luciferase species that can be efficiently renatured by substoichiometric amounts of bacterial Hsp70-Hsp40-NEF. The reactivation rates increased with substrate concentration and followed saturation kinetics, thus allowing the determination of apparent V(max)' and K(m)' values for a chaperone-mediated renaturation reaction for the first time. Under the in vitro conditions used, one Hsp70 molecule consumed five ATPs to effectively unfold a single misfolded protein into an intermediate that, upon chaperone dissociation, spontaneously refolded to the native state, a process with an ATP cost a thousand times lower than expected for protein degradation and resynthesis.

Actin filament bundling and different nucleating effects of mouse Diaphanous-related formin FH2 domains on actin/ADF and actin/cofilin complexes.

Mouse Diaphanous-related formins (mDias) are members of the formin protein family that nucleate actin polymerization and subsequently promote filamentous actin (F-actin) elongation by monomer addition to fast-growing barbed ends. It has been suggested that mDias preferentially recruit actin complexed to profilin due to their proline-rich FH1 domains. During filament elongation, dimeric mDias remain attached to the barbed ends by their FH2 domains, which form an anti-parallel ring-like structure enclosing the filament barbed ends. Dimer formation of mDia-FH2 domains is dependent on their N-terminal lasso and linker subdomains (connector). Here, we investigated the effect of isolated FH2 domains on actin polymerization using mDia1-FH2 domain plus connector, as well as core mDia1, mDia2, and mDia3 missing the connector, by cosedimentation and electron microscopy after negative staining. Analytical ultracentrifugation showed that core FH2 domains of mDia1 and mDia2 exhibited a low degree of dimer formation, whereas mDia3-FH2 minus connector and mDia1-FH2 plus connector readily dimerized. Only core mDia3-FH2 was able to nucleate actin polymerization. However, all tested core FH2 domains decorated and bundled F-actin, as demonstrated by electron microscopy after negative staining. Bundling activity was highest for mDia3-FH2, decreased for mDia2-FH2, and further decreased for mDia1-FH2. The mDia1-FH2 domain plus connector induced actin polymerization also in the absence of profilin, but failed to induce F-actin deformation and bundling. We also tested whether mDia1-FH2 was able to repolymerize actin in complex with different proteins that stabilize globular actin. The data obtained demonstrated that mDia1-FH2 induced actin repolymerization only from the actin/cofilin-1 complex, but not when complexed to actin depolymerizing factor, gelsolin segment 1, vitamin D binding protein, or deoxyribonuclease I.

Laminin chain assembly is regulated by specific coiled-coil interactions.

Laminins are large heterotrimeric, multidomain proteins that play a central role in organising and establishing all basement membranes. Despite a total of 45 potential heterotrimeric chain combinations formed through the coiled-coil domain of the 11 identified laminin chains (alpha1-5, beta1-3, gamma1-3), to date only 15 different laminin isoforms have been reported. This observation raises the question whether laminin assembly is regulated by differential gene expression or specific chain recognition. To address this issue, we here perform a complete analysis of laminin chain assembly and specificity. Using biochemical and biophysical techniques, all possible heterotrimeric combinations from recombinant C-terminal coiled-coil fragments of all chains were analysed. Apart from laminin 323 (alpha3, beta2, gamma3), for which no biochemical evidence of its existence in vivo is available, these experiments confirmed all other known laminin isoforms and identified two novel potential chain combinations, laminins 312 (alpha3, beta1, gamma2) and 422 (alpha4, beta2, gamma4). Our findings contribute to the understanding of basement membrane structure, function and diversity.

Structural analysis of B-Box 2 from MuRF1: identification of a novel self-association pattern in a RING-like fold.

The B-box motif is the defining feature of the TRIM family of proteins, characterized by a RING finger-B-box-coiled coil tripartite fold. We have elucidated the crystal structure of B-box 2 (B2) from MuRF1, a TRIM protein that supports a wide variety of protein interactions in the sarcomere and regulates the trophic state of striated muscle tissue. MuRF1 B2 coordinates two zinc ions through a cross-brace alpha/beta-topology typical of members of the RING finger superfamily. However, it self-associates into dimers with high affinity. The dimerization pattern is mediated by the helical component of this fold and is unique among RING-like folds. This B2 reveals a long shallow groove that encircles the C-terminal metal binding site ZnII and appears as the defining protein-protein interaction feature of this domain. A cluster of conserved hydrophobic residues in this groove and, in particular, a highly conserved aromatic residue (Y133 in MuRF1 B2) is likely to be central to this role. We expect these findings to aid the future exploration of the cellular function and therapeutic potential of MuRF1.

Molecular insights into the self-assembly mechanism of dystrophia myotonica kinase.

Self-assembly via coiled-coil domains (CC) is crucial for the regulation of the dystrophia myotonica kinase (DMPK) -related family of kinases. These CC domains are thought to form dimeric arrangements and thus to mediate dimerization in these enzymes. Using size exclusion chromatography combined with multiangle static light scattering, we analyzed the oligomeric state of DMPK as well as that of a truncated variant lacking the CC fraction. Remarkably, both forms were found to assemble into robust dimers. In contrast, the CC domain in isolation yielded trimeric assemblies, indicating that the oligomerization properties of CC domains from this kinase family are more diversified than anticipated. The crystal structure of this CC has been elucidated to 1.6 angstroms resolution and its properties in solution established using sedimentation equilibrium and thermal denaturation. These data show that, contrary to expectations, the self-assembly of DMPK is not dictated by the association properties of its CC domain. Instead, it appears to be driven by sequence segments flanking both N and C termini of the catalytic kinase fraction, as suggested by models of head-to-head dimers based on small angle X-ray scattering data. Our findings support a shared pattern of assembly across DMPK, ROCKs, and MRCK members of this family.

Structure/function analysis of spinalin, a spine protein of Hydra nematocysts.

The nematocyst capsules of the cnidarians are specialized explosive organelles that withstand high osmotic pressures of approximately 15 MPa (150 bar). A tight disulfide network involving cysteine-rich capsule wall proteins, like minicollagens and nematocyst outer wall antigen, characterizes their molecular composition. Nematocyst discharge leads to the expulsion of a long inverted tubule that was coiled inside the capsule matrix before activation. Spinalin has been characterized as a glycine-rich, histidine-rich protein associated with spine structures on the surface of everted tubules. Here, we show that full-length Hydra spinalin can be expressed recombinantly in HEK293 cells and has the property to form disulfide-linked oligomers, reflecting its state in mature capsules. Furthermore, spinalin showed a high tendency to associate into dimers in vitro and in vivo. Our data, which show incomplete disulfide connectivity in recombinant spinalin, suggest a possible mechanism by which the spine structure may be linked to the overall capsule polymer.

Structure of a membrane-based steric chaperone in complex with its lipase substrate.

Secretion via the type II secretion pathway in Gram-negative bacteria often relies crucially on steric chaperones in the periplasm. Here, we report the crystal structure of the soluble form of a lipase-specific foldase (Lif) from Burkholderia glumae in complex with its cognate lipase. The structure reveals how Lif uses a novel alpha-helical scaffold to embrace lipase, thereby creating an unusually extensive folding platform.

Co-axial association of recombinant eye lens aquaporin-0 observed in loosely packed 3D crystals.

Aquaporin-0 (AQP0) is the major membrane protein in vertebrate eye lenses. It has been proposed that AQP0 tetramers mediate contact between membranes of adjacent lens fiber cells, which would be consistent with the extraordinarily narrow inter-cellular spacing. We have obtained 3D crystals of recombinant bovine AQP0 that diffract to 7.0 A resolution. The crystal packing was determined by molecular replacement and shows that, within the cubic lattice, AQP0 tetramers are associated head-to-head along their 4-fold axes. Oligomeric states larger than the tetramer were also observed in solution by native gel electrophoresis and analytical ultracentrifugation methods. In the crystals, there are no direct contacts between octamers, and it can thus be inferred that crystalline order is mediated solely by the detergent belts surrounding the membrane protein. Across the tetramer-tetramer interface, extracellular loops A and C interdigitate at the center and the perimeter of the octamer, respectively. The octamer structure is compared with that of the recently determined structure of truncated ovine AQP0 derived from electron diffraction of 2D crystals. Intriguingly, also in these crystals, octamers are observed, but with significantly different relative tetramer-tetramer orientations. The interactions observed in the loosely packed 3D crystals reported here may in fact represent an in vivo association mode between AQP0 tetramers from juxtaposed membranes in the eye lens.

Structure-based design of peptides that self-assemble into regular polyhedral nanoparticles.

Artificial particulate systems such as polymeric beads and liposomes are being applied in drug delivery, drug targeting, antigen display, vaccination, and other technologies. Here we used computer modeling to design a novel type of nanoparticles composed of peptides as building blocks. We verified the computer models via solid-phase peptide synthesis and biophysical analyses. We describe the structure-based design of a novel type of nanoparticles with regular polyhedral symmetry and a diameter of about 16 nm, which self-assembles from single polypeptide chains. Each peptide chain is composed of two coiled coil oligomerization domains with different oligomerization states joined by a short linker segment. In aqueous solution the peptides form nanoparticles of about 16 nm diameter. Such peptide nanoparticles are ideally suited for medical applications such as drug targeting and drug delivery systems, such as imaging devices, or they may be used for repetitive antigen display.

Three-dimensional structure of human monoamine oxidase A (MAO A): relation to the structures of rat MAO A and human MAO B.

The three-dimensional structure of recombinant human monoamine oxidase A (hMAO A) as its clorgyline-inhibited adduct is described. Although the chain-fold of hMAO A is similar to that of rat MAO A and human MAO B (hMAO B), hMAO A is unique in that it crystallizes as a monomer and exhibits the solution hydrodynamic behavior of a monomeric form rather than the dimeric form of hMAO B and rat MAO A. hMAO A's active site consists of a single hydrophobic cavity of approximately 550 A3, which is smaller than that determined from the structure of deprenyl-inhibited hMAO B (approximately 700 A3) but larger than that of rat MAO A (approximately 450 A3). An important component of the active site structure of hMAO A is the loop conformation of residues 210-216, which differs from that of hMAO B and rat MAO A. The origin of this structural alteration is suggested to result from long-range interactions in the monomeric form of the enzyme. In addition to serving as a basis for the development of hMAO A specific inhibitors, these data support the proposal that hMAO A involves a change from the dimeric to the monomeric form through a Glu-151 --> Lys mutation that is specific of hMAO A [Andrès, A. M., Soldevila, M., Navarro, A., Kidd, K. K., Oliva, B. & Bertranpetit, J. (2004) Hum. Genet. 115, 377-386]. These considerations put into question the use of MAO A from nonhuman sources in drug development for use in humans.

A conserved trimerization motif controls the topology of short coiled coils.

In recent years, short coiled coils have been used for applications ranging from biomaterial to medical sciences. For many of these applications knowledge of the factors that control the topology of the engineered protein systems is essential. Here, we demonstrate that trimerization of short coiled coils is determined by a distinct structural motif that encompasses specific networks of surface salt bridges and optimal hydrophobic packing interactions. The motif is conserved among intracellular, extracellular, viral, and synthetic proteins and defines a universal molecular determinant for trimer formation of short coiled coils. In addition to being of particular interest for the biotechnological production of candidate therapeutic proteins, these findings may be of interest for viral drug development strategies.

Escherichia coli dihydroxyacetone kinase controls gene expression by binding to transcription factor DhaR.

Dihydroxyacetone (Dha) kinases are a sequence-conserved family of enzymes, which utilize either ATP (in animals, plants, bacteria) or the bacterial phosphoenolpyruvate carbohydrate phosphotransferase system (PTS) as a source of high-energy phosphate. The PTS-dependent kinase of Escherichia coli consists of three subunits: DhaK contains the Dha binding site, DhaL contains ADP as cofactor for the double displacement of phosphate from DhaM to Dha, and DhaM provides a phospho-histidine relay between the PTS and DhaL::ADP. DhaR is a transcription activator belonging to the AAA+ family of enhancer binding proteins. It stimulates transcription of the dhaKLM operon from a sigma70 promoter and autorepresses dhaR transcription. Genetic and biochemical studies indicate that the enzyme subunits DhaL and DhaK act antagonistically as coactivator and corepressor of the transcription activator by mutually exclusive binding to the sensing domain of DhaR. In the presence of Dha, DhaL is dephosphorylated and DhaL::ADP displaces DhaK and stimulates DhaR activity. In the absence of Dha, DhaL::ADP is converted by the PTS to DhaL::ATP, which does not bind to DhaR.

On the molecular mass of Lumbricus erythrocruorin.

A critical examination of the published molecular mass of erythrocruorin (Ec) from Lumbricus and related earthworm species reveals that the results do cluster, not at one, but at two values of the molecular mass. One cluster corresponds to approximately 3.6 MDa as predicted from the Vinogradov model for the hexagonal bilayer (HBL) assembly of Lumbricus terrestris EC and as estimated from the crystal structure of HBL at 5.5 A resolution. The other cluster corresponds to approximately 4.4 MDa. However, in contrast to the controversy over the molecular mass, there is a consensus that the sedimentation coefficient of intact L. terrestris Ec is approximately 60S. Drawing on the occurrence of central subunits in Ec of Oenone fulgida and few other annelid species, we propose for the 4.4 MDa molecule a model of HBL supplemented by a central mass. The proposed model abides by D6 symmetry and is a suitable candidate to represent 60S Lumbricus terrestris Ec.

Isolation and Characterization of an Outer Membrane Protein of Chlorobium tepidum.

A protein was isolated from membranes of the green sulfur bacterium Chlorobium tepidum. This protein was characterized by gel electrophoresis, gel filtration, analytical ultracentrifugation and amino acid sequencing. The molecular weight of the purified protein was shown to be 26 kDa by SDS-PAGE. HPLC gelfiltration, SDS-PAGE and analytical ultracentrifugation are consistent with the presence of a homogenous protein in the preparations. Amino acid analysis was obtained from the isolated protein after fragmentation with Lys-C, trypsin and cyanogen bromide. The cleavage pattern resulting from these treatments combined with Edman sequencing yield a sequence allowing the identification of an integral membrane agglutinin in Chl. tepidum.

Dimerization of TonB is not essential for its binding to the outer membrane siderophore receptor FhuA of Escherichia coli.

FhuA belongs to a family of specific siderophore transport systems located in the outer membrane of Escherichia coli. The energy required for the transport process is provided by the proton motive force of the cytoplasmic membrane and is transmitted to FhuA by the protein TonB. Although the structure of full-length TonB is not known, the structure of the last 77 residues of a fragment composed of the 86 C-terminal amino acids was recently solved and shows an intertwined dimer (Chang, C., Mooser, A., Pluckthun, A., and Wlodawer, A. (2001) J. Biol. Chem. 276, 27535-27540). We analyzed the ability of truncated C-terminal TonB fragments of different lengths (77, 86, 96, 106, 116, and 126 amino acid residues, respectively) to bind to the receptor FhuA. Only the shortest TonB fragment, TonB-77, could not effectively interact with FhuA. We have also observed that the fragments TonB-77 and TonB-86 form homodimers in solution, whereas the longer fragments remain monomeric. TonB fragments that bind to FhuA in vitro also inhibit ferrichrome uptake via FhuA in vivo and protect cells against attack by bacteriophage Phi80.

On the oligomeric state of chloroplast chaperonin 10 and chaperonin 20.

Type I chaperonins are fundamental protein folding machineries that function in eubacteria, mitochondria and chloroplasts. Eubacteria and mitochondria contain chaperonin systems comprised of homo-oligomeric chaperonin 60 tetradecamers and co-chaperonin 10 heptamers. In contrast, the chloroplast chaperonins are heterooligomeric tetradecamers that are composed of two subunit types, alpha and beta. Additionally, chloroplasts contain two structurally distinct co-chaperonins. One, ch-cpn10, is probably similar to the mitochondrial and bacterial co-chaperonins, and is composed of 10 kDa subunits. The other, termed ch-cpn20 is composed of two cpn10-like domains that are held together by a short linker. While the oligomeric structure of ch-cpn10 remains to be elucidated, it was previously suggested that ch-cpn20 forms tetramers in solution, and that this is the functional oligomer. In the present study, we investigated the properties of purified ch-cpn10 and ch-cpn20. Using bifunctional cross-linking reagents, gel filtration chromatography and analytical ultracentrifugation, we show that ch-cpn10 is a heptamer in solution. In contrast, ch-cpn20 forms multiple oligomers that are in dynamic equilibrium with each other and cover a broad spectrum of molecular weights in a concentration-dependent manner. However, upon association with GroEL, only one type of co-chaperonin-GroEL complex is formed.

Towards a resolution of the long-standing controversy regarding the molecular mass of extracellular erythrocruorin of the earthworm Lumbricus terrestris.

The published molecular mass of erythrocruorin of Lumbricus terrestris and related earthworm species covers a bewildering range of 3.23-4.5 MDa. A critical reexamination reveals that some mass determinations were underestimated and the results do cluster, not at one, but at two values of the molecular mass. One cluster corresponds to approximately 3.6 MDa, as predicted for a stoichiometry of 144 globin and 36 linker chains-the Vinogradov model for the hexagonal bilayer (HBL) assembly of Lumbricus erythrocruorin-and as estimated from the crystal structure of HBL at 5.5 A resolution [Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 7107]. The other cluster corresponds to approximately 4.4 MDa. In addition, a molecular mass of 4.1 MDa, determined by multiangle laser light scattering (MALLS), stands apart of the two clusters, separated from the masses obtained by other methods of molecular mass determination. We propose a stoichiometry of 192 globin and 36 linker chains for the 4.4-MDa molecule. The 36 linkers and 144 out of 192 globin chains are identified with the HBL and the remaining 48 globins are allotted equally to the two halves of the axial cavity above and below the central torus of the structure. The proposed model is supported by the occurrence in some annelid species of erythrocruorin with centrally placed subunits [Biochim. Biophys. Acta 359 (1974) 210], and by the oxidation-dependent shedding of subunits in Lumbricus erythrocruorin. We propose further that the 4.1 MDa determination represents the weight average molecular mass of a population of molecules resulting from a partial dissociation of 4.4-MDa erythrocruorin. This interpretation seems reasonable on the background of the very low protein concentrations ( approximately 100 microg/ml and lower) prevailing at the MALLS experiment.

Type III protein translocase: HrcN is a peripheral ATPase that is activated by oligomerization.

Type III protein secretion (TTS) is catalyzed by translocases that span both membranes of Gram-negative bacteria. A hydrophilic TTS component homologous to F1/V1-ATPases is ubiquitous and essential for secretion. We show that hrcN encodes the putative TTS ATPase of Pseudomonas syringae pathovar phaseolicola and that HrcN is a peripheral protein that assembles in clusters at the membrane. A decahistidinyl HrcN derivative was overexpressed in Escherichia coli and purified to homogeneity in a folded state. Hydrodynamic analysis, cross-linking, and electron microscopy revealed four distinct HrcN forms: I, 48 kDa (monomer); II, approximately 300 kDa (putative hexamer); III, 575 kDa (dodecamer); and IV, approximately 3.5 MDa. Form III is the predominant form of HrcN at the membrane, and its ATPase activity is dramatically stimulated (>700-fold) over the basal activity of Form I. We propose that TTS ATPases catalyze protein translocation as activated homo-oligomers at the plasma membrane.

Structure/function relationships in the minicollagen of Hydra nematocysts.

The minicollagens found in the inner layer of the Hydra nematocyst walls are the smallest collagens known with 12-16 Gly-X-Y repeats. Minicollagen-1, the best characterized member of this protein family so far, consists of a central collagen triple helix of 12 nm in length flanked at both ends by a polyproline stretch and a conserved cysteine-rich domain. The cysteine-rich tails are proposed to function in the assembly of soluble minicollagen trimers to high molecular structures by a switch of the disulfide linkage from intramolecular to intermolecular bonds. In this study, we investigate the trimeric nature of minicollagen-1 and its capacity to form disulfide-linked polymers in vitro. A fusion protein of minicollagen-1 with maltose-binding protein is secreted as a soluble trimer with only intrachain and no interchain disulfide bridges as confirmed by melting the collagen triple helix under reducing and non-reducing conditions. The conversion of minicollagen-1 trimers to monomers takes place between 40 and 55 degrees C with the melting point being approximately 45 degrees C. Oxidative reshuffling of the minicollagen-1 trimers leads to the formation of high molecular aggregates, which upon reduction show distinct polytrimeric states. Minicollagen trimers in isolated nematocyst capsules proved to be sensitive to SDS and were engaged in polymeric structures with additional cross-links that were resistant to reducing agent.

Removing an interhelical salt bridge abolishes coiled-coil formation in a de novo designed peptide.

Alpha-helical coiled coils represent a common protein oligomerization motif that are mainly stabilized by hydrophobic interactions occurring along their coiled-coil interface, the so-called hydrophobic seam. We have recently de novo designed and optimized a series of two-heptad repeat long coiled-coil peptides which are further stabilized by a complex network of inter- and intrahelical salt bridges. Here we have extended the de novo design of such two heptad-repeat long peptides by removing the central and most important g-e' Arg to Glu (g-e'RE) ionic interhelical interaction and replacing these residues by alanine residues. The effect of the missing interhelical ionic interaction on coiled-coil formation and stability has been analyzed by CD spectroscopy, analytical ultracentrifugation, and X-ray crystallography. We show that the peptide, while being highly alpha-helical, is no longer able to form a parallel coiled-coil structure but rather assumes an octameric globular helical assembly devoid of any coiled-coil interactions.