The second Kunitz domain of human tissue factor pathway inhibitor: cloning, structure determination and interaction with factor Xa.

Tissue Factor Pathway Inhibitor (TFPI) is a 36 kDa glycoprotein that helps maintain haemostasis by inhibiting Factor Xa and the Factor VIIa/Tissue Factor (TF) complex. TFPI contains three tandemly linked Kunitz inhibitor domains, of which the second inhibits factor Xa. We have undertaken a multidisciplinary approach to study the structure and function of the second Kunitz domain of TFPI, with a view towards the rational design of factor Xa inhibitors. Amino acid residues 93 to 154 of the mature TFPI protein, corresponding to the second Kunitz domain (TFPI-kII), were expressed in Escherichia coli. The protein was purified to near homogeneity by ion exchange, hydrophobic interaction, and size exclusion chromatography, respectively. TFPI-kII is a potent factor Xa inhibitor with a Ki of 1.5 x 10(-10) M, a value that does not differ significantly from that of intact TFPI. The three-dimensional structure of TFPI-kII in aqueous solution was determined by 1H nuclear magnetic resonance spectroscopy (NMR). A set of 30 conformers was calculated with the program DIANA using 906 distance constraints derived from nuclear Overhauser effects and 23 dihedral angle constraints. This set, representing the solution structure of TFPI-kII, has an average root-mean-square deviation of 0.78 A for the backbone atoms and 1.38 A for all heavy atoms of residues 1 to 58. The structure of TFPI-kII has also been determined in complex with porcine trypsin using X-ray crystallographic techniques. The complex has been solved to a resolution of 2.6 A, with a final R-factor of 16.2%. Comparison of the NMR derived structure with that of TFPI-kII in complex with trypsin reveals little divergence of the two structures, with the exception of residue Tyr17. Superposition of the trypsin:TFPI-kII complex on factor Xa provides insights into macromolecular determinants for the inhibition of factor Xa. Complexation would require a degree of reorganisation of factor Xa residues, in particular of TyrF99, but also perhaps of the F148-loop. The interaction was further investigated using restrained molecular dynamics. Electrostatic interactions would appear to play a major role. The reorganisation of factor Xa is in contrast to the proposed factor Xa:TAP interaction, where TAP would bind to the "ground state" structure of factor Xa.

Hydrogen exchange studies of the Arc repressor: evidence for a monomeric folding intermediate.

The hydrogen exchange rates of the backbone amide and labile side-chain protons of the dimeric Arc repressor have been measured. For the slowly exchanging amides in the alpha-helical regions, these rates show a concentration dependence. To account for this dependence, the role of the monomer-dimer equilibrium was considered. Extrapolating the observed exchange rates to zero dimer concentration provides estimates of these rates in the monomer and shows that they are significantly retarded compared to those of an unfolded polypeptide. This suggests that the monomer is in a structured "molten globule" like state. In particular, the two helices of Arc retain a high degree of their secondary structure and it is proposed that the two amphiphilic helices are packed together with their hydrophobic faces. Evidence for a partially folded structure in the Arc monomer was reported earlier in two other studies [J. L. Silva, C. F. Silveira, A. Correia, Jr., and L. Pontes (1992) Journal of Molecular Biology, Vol. 223, pp. 545-555; X. Peng, J. Jonas, and J. L. Silva (1993) Proceedings of the National Academy of Science USA, Vol. 90, pp. 1776-1780]. By combining the results of these studies and ours, a folding pathway of the dimeric Arc repressor involving four different stages is proposed. Due to the low concentration of Arc repressor in the cell, the protein is present either as a free monomer or it is bound to DNA presumably as a tetramer. Therefore the folding pathway can be regarded as an integral part of the overall DNA binding process.

Solution structure of dimeric Mnt repressor (1-76).

Wild-type Mnt repressor of Salmonella bacteriophage P22 is a tetrameric protein of 82 residues per monomer. A C-terminal deletion mutant of the repressor denoted Mnt (1-76) is a dimer in solution. The structure of this dimer has been determined using NMR. The NMR assignments of the majority of the 1H, 15N, and 13C resonances were obtained using 2D and triple-resonance 3D techniques. Elements of secondary structure were identified on the basis of characteristic sequential and medium range NOEs. For the structure determination more than 1000 NOEs per monomer were obtained, and structures were generated using distance geometry and restrained simulated annealing calculations. The discrimination of intra- vs intermonomer NOEs was based upon the observation of intersubunit NOEs in [15N,13C] double half-filtered NOESY experiments. The N-terminal part of Mnt (residues 1-44), which shows a 40% sequence homology with the Arc repressor, has a similar secondary and tertiary structure. Mnt (1-76) continues with a loop region of irregular structure, a third alpha-helix, and a random coil C-terminal peptide. Analysis of the secondary structure NOEs, the exchange rates, and the backbone chemical shifts suggests that the carboxy-terminal third helix is less stable than the remainder of the protein, but the observation of intersubunit NOEs for this part of the protein enables the positioning of this helix. The rsmd's between the backbone atoms of the N-terminal part of the Mnt repressor (residues 5-43, 5'-43') and the Arc repressor is 1.58 A, and between this region and the corresponding part of the MetJ repressor 1.43 A.

Nuclear magnetic resonance solution structure of the Arc repressor using relaxation matrix calculations.

The Arc repressor of Salmonella bacteriophage P22 is a dimeric sequence-specific DNA-binding protein. The solution structure of Arc has been determined from 2D NMR data using an "ensemble" iterative relaxation matrix approach (IRMA) followed by direct NOE refinement with DINOSAUR. A set of 51 structures was generated with distance geometry and further refined with a combination of restrained energy minimization and restrained molecular dynamics in a parallel refinement protocol. Distance constraints were obtained from an extensive set of NOE build-ups in H2O and 2H2O via relaxation matrix calculations from the ensemble of structures. Methyl group rotation, aromatic ring flaps and internal mobility effects (via order parameters obtained from a free molecular dynamics run in water) were included in these calculations. The best structures were finally refined with direct NOE constraints following a slow-cooling simulated annealing protocol. In this final refinement stage, theoretical NOE intensities were directly compared with the experimental data and forces were derived using a simple two-spin approximation for the gradient of the NOE function. Dynamic assignment was applied to the peaks involving unassigned diastereotopic groups. The structure is determined to a precision (r.m.s.d. from the average excluding the ill defined C and N-terminal region) of 0.55 and 1.10 A for backbone and all atoms, respectively. The final structures, with R factor values around 0.35, have good stereochemical qualities, contain an extensive network of hydrogen bonds consistent with the secondary structure elements and structural features in concordance with genetic data. The overall folding of the solution and crystal structures is the same.

Observation of inter-subunit nuclear Overhauser effects in a dimeric protein. Application to the Arc repressor.

For the structure determination of symmetric protein dimers it is necessary to distinguish between intra- and inter-subunit NOEs. A method is presented to measure selectively the inter-subunit NOEs using uniform 15N and 13C isotope labelling. This is accomplished by double filtered 2D NOE experiments on mixtures of native protein with isotope-labeled protein. The method has been applied to the Arc repressor and allows the characterization of virtually all proton-proton NOEs in terms of their intra- or inter-subunit nature.

Structure of Arc repressor in solution: evidence for a family of beta-sheet DNA-binding proteins.

The Arc repressor, which is involved in the switch between lysis and lysogeny of Salmonella bacteriophage P22, does not belong to any of the known classes of DNA-binding proteins. Mutagenesis studies show that the DNA-binding region is located in the 15 N-terminal amino-acid residues. We have now determined the three-dimensional structure of the Arc dimer from an extensive set of interproton-distance data obtained from 1H NMR spectroscopy. A priori, intra- and inter-monomer nuclear Overhauser effects (NOEs) cannot be distinguished for a symmetric dimer. But by using the homology with the Escherichia coli Met repressor we could interpret the NOEs unambiguously in an iterative structure refinement procedure. The final structure satisfies a large set of NOE constraints (1,352 for the dimer). It shows a strongly intertwined dimer, in which residues 8-14 of different monomers form an antiparallel beta-sheet. A model for the Arc repressor-operator complex can account for all available biochemical and genetic data. In this model two Arc dimers bind with their beta-sheet regions in successive major grooves on one side of the DNA helix, similar to the Met repressor interaction. Thus, Arc and Met repressors are members of the same family of proteins, which contain an antiparallel beta-sheet as the DNA-binding motif.