Amino acid sequence of the [4Fe-4S] ferredoxin isolated from Desulfovibrio desulfuricans Norway

Heteronuclear NMR and soft docking: an experimental approach for a structural model of the cytochrome c553-ferredoxin complex

The combination of docking algorithms with NMR data has been developed extensively for the studies of protein-ligand interactions. However, to extend this development for the studies of protein-protein interactions, the intermolecular NOE constraints, which are needed, are more difficult to access. In the present work, we describe a new approach that combines an ab initio docking calculation and the mapping of an interaction site using chemical shift variation analysis. The cytochrome c553-ferredoxin complex is used as a model of numerous electron-transfer complexes. The 15N-labeling of both molecules has been obtained, and the mapping of the interacting site on each partner, respectively, has been done using HSQC experiments. 1H and 15N chemical shift analysis defines the area of both molecules involved in the recognition interface. Models of the complex were generated by an ab initio docking software, the BiGGER program (bimolecular complex generation with global evaluation and ranking). This program generates a population of protein-protein docked geometries ranked by a scoring function, combining relevant stabilization parameters such as geometric complementarity surfaces, electrostatic interactions, desolvation energy, and pairwise affinities of amino acid side chains. We have implemented a new module that includes experimental input (here, NMR mapping of the interacting site) as a filter to select the accurate models. Final structures were energy minimized using the X-PLOR software and then analyzed. The best solution has an interface area (1037.4 A2) falling close to the range of generally observed recognition interfaces, with a distance of 10.0 A between the redox centers.

ZmHox: a novel class of maize homeobox genes

Clones of two highly related genes, ZmHox2a and ZmHox2b (Zea mays homeobox), were isolated from maize embryo cDNA libraries by screening with the ZmHox1a homeobox sequence. The genes map to chromosomes 3 and 8, respectively, and encode mRNA transcripts of 6kb. The encoded proteins, ZmHox2a and b, share 84% sequence identity and exhibit a modular structure with several novel plant-specific protein domains. Interestingly, each ZmHox2a, gene product contains two complete homeodomains which, for Zmhox2a, were both shown to be functional DNA-binding motifs in vitro. Not only probes encoding the homeobox but also DNA fragments corresponding to other ZmHox2 domains hybridize to multiple bands in genomic Southern blots, indicating that related protein domains may be conserved in other maize genes. The ZmHox2a/b genes, therefore, are members of a novel and large class of maize genes, some of which can be expected to encode new transcription factors.

Intramolecular electron transfer in ferredoxin II from Desulfovibrio desulfuricans Norway

In order to elucidate the role of the two (4Fe-4S) clusters in ferredoxins and to determine whether an electron-transfer mechanism may occur between the clusters, the in vitro reduction of cytochrome c3 and cytochrome c553 by Desulfovibrio desulfuricans Norway ferredoxin II was studied using spectrophotometric techniques. Ferredoxin II, covalently cross-linked with either cytochrome c3 or c553, is an obligate intermediate in cytochrome reduction by pyruvate dehydrogenase. Both titration of the complex formation under 1H-NMR spectroscopy and cross-linking experiments between ferredoxin II and either cytochrome c3 or cytochrome c553 gave a stoichiometric ratio of 1:1. Modelling the protein yielded differences between the charge distributions around the two (Fe-S) clusters. The fact that Cluster 2 is blocked in the electron-transfer domain facing the cytochrome interacting heme, indicates Cluster 1 receives electron from pyruvate dehydrogenase. Consecutively, cytochrome reduction occurs owing to an intramolecular electron exchange between the two clusters of the ferredoxin. The properties of two (Fe-S) cluster ferredoxins are compared to those of monocluster ferredoxins and discussed in evolutionary terms.

Identification of the site of interaction between cytochrome c3 and ferredoxin using peptide mapping of the cross-linked complex

Structural studies carried out on a cross-linked complex between cytochrome c3 and ferredoxin I, both isolated from Desulfovibrio desulfuricans Norway, allowed the identification of the site of interaction between the two redox proteins. Staphylococcus aureus proteinase and chymotrypsin digestions led to characterization of peptides containing both cytochrome c3 and ferredoxin sequences. The cytochrome c3 sequences involved in the three isolated cross-linked peptides contained several lysine residues localized around the heme 4 crevice. This analysis stressed the peculiar role of lysines 100, 101, 103, 104 and 113, which could be considered as major cross-link sites, as opposed to the lysines 75, 79 and 82, which could be considered as minor cross-link sites. One cross-linked peptide, containing two ferredoxin sequences joined to one cytochrome c3 sequence, had been isolated, suggesting the possibility of more than one cross-link per covalent complex. All these results led to the identification of heme 4 of cytochrome c3 as the site of interaction for the ferredoxin I. This study confirms the proposal that could be deduced from the hypothetical structure of the complex built by computer graphics modelling (Cambillau, C., Frey, M., Mosse, J., Guerlesquin, F. and Bruschi, M. (1988) Proteins: struct., funct. genet. 4, 63-70).

Ferredoxin electron transfer site on cytochrome c3. Structural hypothesis of an intramolecular electron transfer pathway within a tetra-heme cytochrome

To specify electron exchanges involving Desulfovibrio desulfuricans Norway tetra-heme cytochrome c3, the chemical modification of arginine 73 residue, was performed. Biochemical and biophysical studies have shown that the modified cytochrome retains its ability to both interact and act as an electron carrier with its redox partners, ferredoxin and hydrogenase. Moreover, the chemical modification effects on the cytochrome c3 1H NMR spectrum were similar to that induced by the presence of ferredoxin. This suggests that arginine 73 is localized on the cytochrome c3 ferredoxin interacting site. The identification of heme 4, the closest heme to arginine 73, as the ferredoxin interacting heme helps us to hypothesize about the role of the three other hemes in the molecule. A structural hypothesis for an intramolecular electron transfer pathway, involving hemes 4, 3 and 1, is proposed on the basis of the crystal structures of D. vulgaris Miyazaki and D. desulfuricans Norway cytochromes c3. The unique role of some structural features (alpha helix, aromatic residues) intervening between the heme groups, is proposed.

Novel cysteine-rich motif and homeodomain in the product of the Caenorhabditis elegans cell lineage gene lin-11

The gene lin-11 is required for the asymmetric division of a vulval precursor cell type in the nematode Caenorhabditis elegans. Putative lin-11 complementary DNAs were sequenced and found to encode a protein that contains both a homeodomain and two tandem copies of a novel cysteine-rich motif: C-X2-C-X17-19-H-X2-C-X2-C-X2-C-X7-11-(C)-X8-C. Two tandem copies of this motif are also present amino-terminal to the homeodomains in the proteins encoded by the genes mec-3, which is required for C. elegans touch neuron differentiation, and isl-1, which encodes a rat insulin I gene enhancer-binding protein. The arrangement of cysteine residues in this motif, referred to as LIM (for lin-11 isl-1 mec-3), suggests that this region is a metal-binding domain. The presence in these three proteins of both a potential metal-binding domain and a homeodomain distinguishes them from previously characterized proteins.

Structure of [4Fe-4S] ferredoxin from Bacillus thermoproteolyticus refined at 2.3 A resolution. Structural comparisons of bacterial ferredoxins

The structure of a low-potential ferredoxin isolated from Bacillus thermoproteolyticus has been refined by a restrained least-squares method. The final crystallographic R factor is 0.204 for 2906 reflections with F greater than 3 sigma F in the 6.0 to 2.3 A resolution range. The model contains 81 amino acid residues, one [4Fe-4S] cluster, and 59 water molecules. The root-mean-square deviation from ideal values for bond lengths is 0.018 A, and the mean coordinate error is estimated to be 0.25 A. The present ferredoxin is similar in the topology of the polypeptide backbone to the dicluster-type ferredoxins from Peptococcus aerogenes and Azotobacter vinelandii, but has considerable insertions and deletions of the peptide segments as well as different secondary structures. Although all but the C-terminal C zeta atoms of P. aerogenes ferredoxin superpose on the C alpha atoms of A. vinelandii ferredoxin, only 60% superpose on the C alpha atoms of B. thermoproteolyticus ferredoxin, with a root-mean-square distance of 0.82 A for each pair. The conformations of the peptide segments surrounding the [4Fe-4S] clusters in these three ferredoxins are all conserved. Moreover, the schemes for the NH...S hydrogen bonds in these ferredoxins are nearly identical. The site of the aromatic ring of Tyr27 in B. thermoproteolyticus ferredoxin is close spatially to that of Tyr28 in P. aerogenes ferredoxin with reference to the cluster, but these residues do not correspond in the spatial alignment of their polypeptide backbones. We infer that in monocluster-type ferredoxins, the side-chain at the 27th residue has a crucial effect on the stability of the cluster. Of the four cysteine residues that bind to the second Fe-S cluster in the dicluster-type ferredoxins, two are conserved in the monocluster-type ferredoxins from Desulfovibrio gigas. D. desulfuricans Norway, and Clostridium thermoaceticum. The tertiary structure of B. thermoproteolyticus ferredoxin suggests that in such monocluster-type ferredoxins these two cysteine residues, which in it correspond to Ala21 and Asp53, form a disulfide bridge.

Experimental evidence of an alpha helix in Desulfovibrio desulfuricans Norway ferredoxin I: a two-dimensional NMR study

Desulfovibrio ferredoxins are small proteins involved in biological oxido-reduction reactions and contain either one or two (4Fe-4S) clusters. The conformation of D. desulfuricans Norway ferredoxin I in solution was studied by two-dimensional NMR and various conformational parameters (n.O.e. and J-coupling) indicate the presence of an alpha-helix involving residues 41 to 50. These data confirm an earlier proposal (Fukuyama et al, J. Mol. Biol. 199, 183 (1988] in which the space of the missing cluster in monocluster ferredoxins is occupied by an alpha-helix. The evolutionary relevance of this result is discussed in view of published sequences and structures of related ferredoxins.

Characterization and complete amino acid sequence of ferredoxin II from Desulfovibrio vulgaris Miyazaki

One of 2 ferredoxins, Fd II, purified from Desulfovibrio vulgaris Miyazaki (DvM) has been characterized and its complete amino acid sequence established. Fd II is composed of 63 amino acid residues and contains 7 cysteinyl residues but has only 4 iron atoms in an iron-sulfur cluster of a standard redox potential of -405 mV. The arrangement of cysteinyl residues in the protein suggests that some cysteinyl residues are not directly involved in ligation to the iron-sulfur cluster. Homology is recognized among Fd II (DvM), Fd I (D. desulfuricans Norway), Fd I (D. africanus Benghazi), and Fd (D. gigas). Although Fd I and Fd II in DvM are poorly homologous, the C-terminal half of Fd I is fairly homologous to the N-terminal half of Fd II. Fd II is 40% as effective as Fd I as an electron carrier for pyruvate dehydrogenase coupled with hydrogenase and cytochrome c3.

Tertiary structure of Bacillus thermoproteolyticus [4Fe-4S] ferredoxin. Evolutionary implications for bacterial ferredoxins

The structure of a low-potential [4Fe-4S] ferredoxin from Bacillus thermoproteolyticus has been solved using anomalous scattering data from iron atoms in the diffraction data of native crystals and refined partially to a crystallographic R-factor of 0.33, with 2.3 A (1 A = 0.1 nm) resolution data. The least-squares refinement based on the Bijvoet differences has determined that the four iron atoms in the cluster are an equal distance, approximately 2.8 A, apart. The NH ... S hydrogen bonds between polypeptide nitrogen atoms, and both cysteine and inorganic sulfur atoms, are present, as in ferrodoxin from Peptococcus aerogenes. The polypeptide chain of the B. thermoproteolyticus ferredoxin has a fold closely similar to that of 2[4Fe-4S] ferredoxin from P. aerogenes. The structural correspondence indicates strongly that both types of ferredoxin evolved from a common ancestor. The second cluster-binding region in P. aerogenes ferredoxin corresponds to the alpha-helix in B. thermoproteolyticus ferredoxin. The secondary-structure predictions strongly suggest that the alpha-helix is generally present in the monocluster-type ferredoxins. The conformational change to alpha-helix, insertions of a loop and a protrusion, as well as the absence of the second cluster in B. thermoproteolyticus ferredoxin, result in the lack of 2-fold symmetry present in P. aerogenes ferredoxin. So, the track of gene duplication is no longer detectable in the tertiary structure alone. The evolutionary events that may have occurred in the ferredoxins with the [4Fe-4S] cluster are discussed.

Model of a complex between the tetrahemic cytochrome c3 and the ferredoxin I from Desulfovibrio desulfuricans (Norway strain)

A three-dimensional model of an electron-transfer complex between the tetrahemic cytochrome c3 and the ferredoxin I from the sulfate-reducing bacterium Desulfovibrio desulfuricans (Norway strain) has been generated through computer graphics methods. The model is based on the known X-ray structure of the cytochrome and on a model of the ferredoxin that has been derived through computer graphics modeling and energy minimization methods, from the X-ray structure of the homologous ferredoxin from Peptococcus aerogenes. Four possible models of interaction between the two molecules were examined by bringing in close proximity each of the four hemes and the redox center (4Fe-4S) of the ferredoxin and by optimizing the ion pairs interactions. One of these models shows by far the "best" structure in terms of charges, interactions, and complementarity of the topology of the contact surfaces. In this complex, the distance between the iron atoms of the ferredoxin redox center and the hemic iron atom is 11.8 A, which compares well with those found between redox centers in other complexes. The contact surface area between the two molecules is 170 A2.

Amino acid sequence of [2Fe-2S] ferredoxin from Clostridium pasteurianum

The complete amino acid sequence of the [2Fe-2S] ferredoxin from the saccharolytic anaerobe Clostridium pasteurianum has been determined by automated Edman degradation of the whole protein and of peptides obtained by tryptic and by staphylococcal protease digestion. The polypeptide chain consists of 102 amino acids, including 5 cysteine residues in positions 11, 14, 24, 56, and 60. The sequence has been analyzed for hydrophilicity and for secondary structure predictions. In its native state the protein is a dimer, each subunit containing one [2Fe-2S] cluster, and it has a molecular weight of 23,174, including the four iron and inorganic sulfur atoms. The extinction coefficient of the native protein is 19,400 M-1 cm-1 at 463 nm. The positions of the cysteine residues, four of which are most probably the ligands of the [2Fe-2S] cluster, on the polypeptide chain of this protein are very different from those found in other [2Fe-2S] proteins, and in other ferredoxins in general. In addition, whole sequence comparisons of the [2Fe-2S] ferredoxin from C. pasteurianum with a number of other ferredoxins did not reveal any significant homologies. The likely occurrence of several phylogenetically unrelated ferredoxin families is discussed in the light of these observations.