The structure of Fis mutant Pro61Ala illustrates that the kink within the long alpha-helix is not due to the presence of the proline residue

Unique behaviour of α-helix in bending deformation

Maximum degree of bending that can be tolerated by the rigid rod-like α-helix remains unknown; however, it should be very difficult or even impossible to make α-helices with varying degrees...

DNA recognition by a σ(54) transcriptional activator from Aquifex aeolicus

Transcription initiation by bacterial σ(54)-polymerase requires the action of a transcriptional activator protein. Activators bind sequence-specifically upstream of the transcription initiation site via a DNA-binding domain (DBD). The structurally characterized DBDs from activators all belong to the Fis (factor for inversion stimulation) family of helix-turn-helix DNA-binding proteins. We report here structures of the free and DNA-bound forms of the DBD of NtrC4 (4DBD) from Aquifex aeolicus, a member of the NtrC family of σ(54) activators. Two NtrC4-binding sites were identified upstream (-145 and -85bp) from the start of the lpxC gene, which is responsible for the first committed step in lipid A biosynthesis. This is the first experimental evidence for σ(54) regulation in lpxC expression. 4DBD was crystallized both without DNA and in complex with the -145-binding site. The structures, together with biochemical data, indicate that NtrC4 binds to DNA in a manner that is similar to that of its close homolog, Fis. The greater sequence specificity for the binding of 4DBD relative to Fis seems to arise from a larger number of base-specific contacts contributing to affinity than for Fis.

The role of the local environment of engineered Tyr to Trp substitutions for probing the denaturation mechanism of FIS

Factor for inversion stimulation (FIS), a 98-residue homodimeric protein, does not contain tryptophan (Trp) residues but has four tyrosine (Tyr) residues located at positions 38, 51, 69, and 95. The equilibrium denaturation of a P61A mutant of FIS appears to occur via a three-state (N(2) ⇆ I(2) ⇆ 2U) process involving a dimeric intermediate (I(2)). Although it was suggested that this intermediate had a denatured C-terminus, direct evidence was lacking. Therefore, three FIS double mutants, P61A/Y38W, P61A/Y69W, and P61A/Y95W were made, and their denaturation was monitored by circular dichroism and Trp fluorescence. Surprisingly, the P61A/Y38W mutant best monitored the N(2) ⇆ I(2) transition, even though Trp38 is buried within the dimer removed from the C-terminus. In addition, although Trp69 is located on the protein surface, the P61A/Y69W FIS mutant exhibited clearly biphasic denaturation curves. In contrast, P61A/Y95W FIS was the least effective in decoupling the two transitions, exhibiting a monophasic fluorescence transition with modest concentration-dependence. When considering the local environment of the Trp residues and the effect of each mutation on protein stability, these results not only confirm that P61A FIS denatures via a dimeric intermediate involving a disrupted C-terminus but also suggest the occurrence of conformational changes near Tyr38. Thus, the P61A mutation appears to compromise the denaturation cooperativity of FIS by failing to propagate stability to those regions involved mostly in intramolecular interactions. Furthermore, our results highlight the challenge of anticipating the optimal location to engineer a Trp residue for investigating the denaturation mechanism of even small proteins.

A truncated peptide model of the mutant P61A FIS forms a stable dimer

Factor for inversion stimulation (FIS) is a 98-residue homodimeric DNA-binding protein involved in several different cellular processes including DNA inversion and the regulation of multiple genes. FIS contains a flexible and functionally important N-terminus followed by four helices (A-D), the last two of which consist of the DNA-binding region. Helix B, which comprises the main dimerization interface has a 20 degrees kink at its center that was originally thought to be caused by the presence of a proline at position 61. However, it was later shown that the kink remained largely intact and that FIS retained its native-like function when the proline was mutated to an alanine. We previously showed that the P61A mutation increased the stability of FIS, but decreased its equilibrium denaturation cooperativity apparently due to preferential stabilization of the B-helix. Here we studied a peptide of P61A FIS, corresponding to residues 26-71 (26-71(A3) FIS), which encompasses the dimer interface (helices A and B). Circular dichroism (CD) and size-exclusion chromatography/multi-angle light scattering showed that the peptide was alpha-helical and dimeric, respectively, as expected based on the 3D structure of FIS. Urea-induced equilibrium denaturation experiments monitored by far-UV CD revealed a concentration-dependent transition, and data analysis based on a N2<-->2U model yielded a DeltaG of approximately -10 kcal/mol. Our results suggest that 26-71(A3) FIS can form a stable dimeric structure despite lacking the N- and C-terminus of native FIS.

Common and variable contributions of Fis residues to high-affinity binding at different DNA sequences

Fis is a nucleoid-associated protein that interacts with poorly related DNA sequences with a high degree of specificity. A difference of more than 3 orders of magnitude in apparent Kd values was observed between specific (Kd, approximately 1 to 4 nM) and nonspecific (Kd, approximately 4 microM) DNA binding. To examine the contributions of Fis residues to the high-affinity binding at different DNA sequences, 13 alanine substitutions were generated in or near the Fis helix-turn-helix DNA binding motif, and the resulting proteins were purified. In vitro binding assays at three different Fis sites (fis P II, hin distal, and lambda attR) revealed that R85, T87, R89, K90, and K91 played major roles in high-affinity DNA binding and that R85, T87, and K90 were consistently vital for binding to all three sites. Other residues made variable contributions to binding, depending on the binding site. N84 was required only for binding to the lambda attR Fis site, and the role of R89 was dramatically altered by the lambda attR DNA flanking sequence. The effects of Fis mutations on fis P II or hin distal site binding in vitro generally correlated with their abilities to mediate fis P repression or DNA inversion in vivo, demonstrating that the in vitro DNA-binding effects are relevant in vivo. The results suggest that while Fis is able to recognize a minimal common set of DNA sequence determinants at different binding sites, it is also equipped with a number of residues that contribute to the binding strength, some of which play variable roles.

Location and nature of the residues important for ligand recognition in G-protein coupled receptors

The overall structure of the biogenic amine subclass of the G-protein-coupled receptors, and of their ligand binding sites, is discussed with the aim of highlighting the major structural features of these receptors that are responsible for ligand recognition. A comparison is made between biogenic amine receptors, peptide receptors of the rhodopsin class, and the secretin receptors which all have peptide ligands. The question of where the peptide ligands bind, whether at extracellular sites or within the transmembrane helix bundle, is discussed. The suitability of the rhodopsin crystal structure as a template for construction of homology models is discussed and it is concluded that there are many reasons why a caution should be issued against using it uncritically.

Structural insights into the stability and flexibility of unusual erythroid spectrin repeats

Erythroid spectrin, a major component of the cytoskeletal network of the red cell which contributes to both the stability and the elasticity of the red cell membrane, is composed of two subunits, alpha and beta, each formed by 16-20 tandem repeats. The properties of the repeats and their relative arrangement are thought to be key determinants of spectrin flexibility. Here we report a 2.4 A resolution crystal structure of human erythroid beta-spectrin repeats 8 and 9. This two-repeat fragment is unusual as it exhibits low stability of folding and one of its repeats lacks two tryptophans highly conserved among spectrin repeats. Two key factors responsible for the lower stability and, possibly, its flexibility, are revealed by the structure. A third novel feature of the structure is the relative orientation of the two repeats, which increases the range of possible conformations and provides new insights into atomic models of spectrin flexibility.

Folding mechanism of FIS, the intertwined, dimeric factor for inversion stimulation

FIS, the factor for inversion stimulation, from Escherichia coli and other enteric bacteria, is an interwined alpha-helical homodimer. Size exclusion chromatography and static light scattering measurements demonstrated that FIS is predominately a stable dimer at the concentrations (1-10 microM monomer) and buffer conditions employed in this study. The folding and unfolding of FIS were studied with both equilibrium and kinetic methods by circular dichroism using urea and guanidinium chloride (GdmCl) as the perturbants. The equilibrium folding is reversible and well-described by a two-state folding model, with stabilities at 10 degrees C of 15.2 kcal mol(-1) in urea and 13.5 kcal mol(-1) in GdmCl. The kinetic data are consistent with a two-step folding reaction where the two unfolded monomers associate to a dimeric intermediate within the mixing time for the stopped-flow instrument (<5 ms), and a slower, subsequent folding of the dimeric intermediate to the native dimer. Fits of the burst phase amplitudes as a function of denaturant showed that the free energy for the formation of the dimeric intermediate constitutes the majority of the stability of the folding (9.6 kcal mol(-1) in urea and 10.5 kcal mol(-1) in GdmCl). Folding-to-unfolding double jump kinetic experiments were also performed to monitor the formation of native dimer as a function of folding delay times. The data here demonstrate that the dimeric intermediate is obligatory and on-pathway. The folding mechanism of FIS, when compared to other intertwined, alpha-helical, homodimers, suggests that a transient kinetic dimeric intermediate may be a common feature of the folding of intertwined, segment-swapped, alpha-helical dimers.

Thermodynamic effects of replacements of Pro residues in helix interiors of maltose-binding protein

Introduction of Pro residues into helix interiors results in protein destabilization. It is currently unclear if the converse substitution (i.e., replacement of Pro residues that naturally occur in helix interiors would be stabilizing). Maltose-binding protein is a large 370-amino acid protein that contains 21 Pro residues. Of these, three nonconserved residues (P48, P133, and P159) occur at helix interiors. Each of the residues was replaced with Ala and Ser. Stabilities were characterized by differential scanning calorimetry (DSC) as a function of pH and by isothermal urea denaturation studies as a function of temperature. The P48S and P48A mutants were found to be marginally more stable than the wild-type protein. In the pH range of 5-9, there is an average increase in T(m) values of P48A and P48S of 0.4 degrees C and 0.2 degrees C, respectively, relative to the wild-type protein. The other mutants are less stable than the wild type. Analysis of the effects of such Pro substitutions in MBP and in three other proteins studied to date suggests that substitutions are more likely to be stabilizing if the carbonyl group i-3 or i-4 to the mutation site is not hydrogen bonded in the wild-type protein.

Structural mimicry of proline kinks: tertiary packing interactions support local structural distortions

Proline residues in the helical segments of soluble and transmembrane proteins have received special attention from both a structural and functional perspective. A feature of these helices is the structural distortion termed "proline-kink", which has been associated with the presence of the proline residue. However, a recent report on the yeast heat-shock transcription factor of Kluyveromyces lactis (HSF_KL) suggests that these proline-associated deformations can be achieved in the absence of proline residues, thus raising the question of the mechanisms responsible for the structural mimicry of proline-related features. In this study, the specific interactions responsible for the distortion were characterized by comparative analysis of the atomic details of the packing interactions that surround the evolutionarily conserved proline-kink in the alpha2 helix of HSF_KL and a set of 39 structurally related proteins that lacked the distortion. The mechanistic details inferred from this analysis were confirmed with molecular dynamics simulations. The study shows that the packing interactions between the alpha2 and alpha1 helices in HSF_KL are responsible for the stabilization of the conserved kink, whether a proline residue that divides the helix into segments is present or not. The proline-kink can facilitate the formation of tertiary packing interactions that would otherwise not be possible. However, it is the ability to establish differential packing interactions for the helix segments, rather than the structural properties of the proline-kink itself, that emerges as the key factor for the characteristic distortion.

Proline in alpha-helical kink is required for folding kinetics but not for kinked structure, function, or stability of heat shock transcription factor

The DNA-binding domain of the yeast heat shock transcription factor (HSF) contains a strictly conserved proline that is at the center of a kink. To define the role of this conserved proline-centered kink, we replaced the proline with a number of other residues. These substitutions did not diminish the ability of the full-length protein to support growth of yeast or to activate transcription, suggesting that the proline at the center of the kink is not conserved for function. The stability of the isolated mutant DNA-binding domains was unaltered from the wild-type, so the proline is not conserved to maintain the stability of the protein. The crystal structures of two of the mutant DNA-binding domains revealed that the helices in the mutant proteins were still kinked after substitution of the proline, suggesting that the proline does not cause the alpha-helical kink. So why are prolines conserved in this and the majority of other kinked alpha-helices if not for structure, function, or stability? The mutant DNA-binding domains are less soluble than wild-type when overexpressed. In addition, the folding kinetics, as measured by stopped-flow fluorescence, is faster for the mutant proteins. These two results support the premise that the presence of the proline is critical for the folding pathway of HSF's DNA-binding domain. The finding may also be more general and explain why kinked helices maintain their prolines.

Thermodynamics of replacing an alpha-helical Pro residue in the P40S mutant of Escherichia coli thioredoxin

Escherichia coli thioredoxin is a 108 amino acid oxidoreductase and contains a single Met residue at position 37. The protein contains a long alpha-helical stretch between residues 32 and 49. The central residue of this helix, Pro40, has been replaced by Ser. The stabilities of the oxidized states of two proteins, the single mutant M37L and the double mutant M37L,P40S, have been characterized by differential scanning calorimetry (DSC) and also by a series of isothermal guanidine hydrochloride (GuHCl) melts in the temperature range of 277 to 333 K. The P40S mutation was found to stabilize the protein at all temperatures upto 340 K though both proteins had similar Tm values of about 356 K. At 298 K, the M37L,P40S mutant was found to be more stable than M37L by 1.5 kcal/mol. A combined analysis of GuHCl and calorimetric data was carried out to determine the enthalpy, entropy, and heat capacity change upon unfolding. At 298 K there was a large, stabilizing enthalpic effect in P40S though significant enthalpy-entropy compensation was observed and the two proteins had similar values of deltaCp. Thus, replacement of a Pro in the interior of an alpha helix can have substantial effects on protein stability.

Solution structure of the DNA-binding domain of NtrC with three alanine substitutions

The structure of the 20 kDa C-terminal DNA-binding domain of NtrC from Salmonella typhimurium (residues Asp380-Glu469) with alanine replacing Arg456, Asn457, and Arg461, was determined by NMR spectroscopy. NtrC is a homodimeric enhancer-binding protein that activates the transcription of genes whose products are required for nitrogen metabolism. The 91-residue C-terminal domain contains the determinants necessary for dimerization and DNA-binding of the full length protein. The mutant protein does not bind to DNA but retains many characteristics of the wild-type protein, and the mutant domain expresses at high yield (20 mg/l) in minimal medium. Three-dimensional (1)H/(13)C/(15)N triple-resonance, (1)H-(13)C-(13)C-(1)H correlation and (15)N-separated nuclear Overhauser effect (NOE) spectroscopy experiments were used to make backbone and side-chain (1)H,(15)N, and (13)C assignments. The structures were calculated using a total of 1580 intra and inter-monomer distance and hydrogen bond restraints (88 hydrogen bonds; 44 hydrogen bond restraints), and 88 phi dihedral restraints for residues Asp400 through Glu469 in both monomers. A total of 54 ambiguous restraints (intra or inter-monomer) involving residues close to the 2-fold symmetry axis were also included. Each monomer consists of four helical segments. Helices A (Trp402-Leu414) and B (Leu421-His440) join with those of another monomer to form an antiparallel four-helix bundle. Helices C (Gln446-Leu451) and D (Ala456-Met468) of each monomer adopt a classic helix-turn-helix DNA-binding fold at either end of the protein. The backbone rms deviation for the 28 best of 40 starting structures is 0.6 (+/-0.2) A. Structural differences between the C-terminal domain of NtrC and the homologous Factor for Inversion Stimulation are discussed.

Quantitative evaluation of the chicken lysozyme epitope in the HyHEL-10 Fab complex: free energies and kinetics

The hen (chicken) egg-white lysozyme (HEWL) epitope for the monoclonal antibody HyHEL-10 Fab (Fab-10) was investigated by alanine scan mutagenesis. The association rate constants (k(on)) for the HEWL Fab-10 complexes were obtained from the homogenous solution method described in the preceding paper (Taylor et al., 1998). A new method for determining the dissociation rate constant (k(off)) for the complex, by trapping nascent free antibody with an inactive HEWL mutant is described. The values of k(on) fall within a factor of 2 of the wild-type (WT) HEWL value (1.43+/-0.13 X 10(6)M(-1)s(-1)), while the increases in k(off)more nearly reflect the total change in free energies of the complex (deltadeltaG(D)). The dissociation constants (K(D)) were measured directly in those cases where satisfactory kinetic data could not be obtained. The Y20A, K96A, and K97A HEWL.Fab-10 complexes are destabilized by more than 4 kcal/mol compared to the WT complex. The R21A, L75A, and D101A antibody complexes are moderately destabilized (0.7 < deltadeltaG(D)< or = 1.0 kcal/mol). Additional mutations of the "hotspot" residues (Tyr20, Lys96, Lys97) were constructed to probe, more precisely, the nature of their contributions to complex formation. The results show that the entire hydrocarbon side chains of Tyr20 and Lys97, and only the epsilon-amino group of Lys96, contribute to the stability of the complex. The value of deltadeltaG(D) for the R21A mutant complex is a distinct outlier in the Arg21 replacement series demonstrating the importance of supplementing alanine scan mutagenesis with additional mutations.