CcdB at pH 4 Forms a Partially Unfolded State with a Dry Core.

pH is an important factor that affects the protein structure, stability, and activity. Here, we probe the nature of the low-pH structural form of the homodimeric CcdB (controller of cell death B) protein. Characterization of CcdB protein at pH 4 and 300 K using circular dichroism spectroscopy, 8-anilino-1-naphthalene-sulphonate binding, and Trp solvation studies suggests that it forms a partially unfolded state with a dry core at equilibrium under these conditions. CcdB remains dimeric at pH 4 as shown by multiple techniques, such as size-exclusion chromatography coupled to multiangle light scattering, analytical ultracentrifugation, and electron paramagnetic resonance. Comparative analysis using two-dimensional 15N-1H heteronuclear single-quantum coherence NMR spectra of CcdB at pH 4 and 7 suggests that the pH 4 and native state have similar but nonidentical structures. Hydrogen-exchange-mass-spectrometry studies demonstrate that the pH 4 state has substantial but anisotropic changes in local stability with core regions close to the dimer interface showing lower protection but some other regions showing higher protection relative to pH 7.

Stepwise Assembly of ß-Sheet Structure during the Folding of an SH3 Domain Revealed by a Pulsed Hydrogen Exchange Mass Spectrometry Study.

Dissecting temporally the sequence of secondary structural changes, and determining how these specific changes modulate conformational heterogeneity, remain major goals of protein folding studies. In this study, the folding of the SH3 domain of PI3 kinase has been characterized using pulsed hydrogen exchange mass spectrometry (HX-MS). The folding could be described as a four-state process, U ↔ IVE ↔ IE ↔ N, where IVE and IE are structurally heterogeneous intermediate ensembles. Compared to U, early intermediate IVE has a marginally increased level of protection against HX of amides along the entire length of the polypeptide. Sequential assembly into ß-sheet structure has been resolved temporally. Three of the five ß-strands acquire nativelike structure before the rate-limiting step. ß-Strands 2 and 5 acquire nativelike structure in IVE, while ß-strand 4 does so in IE. ß-Strand 1 acquires nativelike structure only during the last step of the folding process. Hence, the HX-MS study has resolved the order of assembly of the ß-strands for the formation of the two ß-sheets, which previous studies utilizing Φ-value analysis of several different SH3 domains had been unable to accomplish. Moreover, it is shown that structural heterogeneity decreases in a stepwise manner during the three stages of folding.

Molecular Determinants of Mutant Phenotypes, Inferred from Saturation Mutagenesis Data.

Understanding how mutations affect protein activity and organismal fitness is a major challenge. We used saturation mutagenesis combined with deep sequencing to determine mutational sensitivity scores for 1,664 single-site mutants of the 101 residue Escherichia coli cytotoxin, CcdB at seven different expression levels. Active-site residues could be distinguished from buried ones, based on their differential tolerance to aliphatic and charged amino acid substitutions. At nonactive-site positions, the average mutational tolerance correlated better with depth from the protein surface than with accessibility. Remarkably, similar results were observed for two other small proteins, PDZ domain (PSD95pdz3) and IgG-binding domain of protein G (GB1). Mutational sensitivity data obtained with CcdB were used to derive a procedure for predicting functional effects of mutations. Results compared favorably with those of two widely used computational predictors. In vitro characterization of 80 single, nonactive-site mutants of CcdB showed that activity in vivo correlates moderately with thermal stability and solubility. The inability to refold reversibly, as well as a decreased folding rate in vitro, is associated with decreased activity in vivo. Upon probing the effect of modulating expression of various proteases and chaperones on mutant phenotypes, most deleterious mutants showed an increased in vivo activity and solubility only upon over-expression of either Trigger factor or SecB ATP-independent chaperones. Collectively, these data suggest that folding kinetics rather than protein stability is the primary determinant of activity in vivo This study enhances our understanding of how mutations affect phenotype, as well as the ability to predict fitness effects of point mutations.

Homodimeric Escherichia coli Toxin CcdB (Controller of Cell Division or Death B Protein) Folds via Parallel Pathways.

The existence of parallel pathways in the folding of proteins seems intuitive, yet remains controversial. We explore the folding kinetics of the homodimeric Escherichia coli toxin CcdB (Controller of Cell Division or Death B protein) using multiple optical probes and approaches. Kinetic studies performed as a function of protein and denaturant concentrations demonstrate that the folding of CcdB is a four-state process. The two intermediates populated during folding are present on parallel pathways. Both form by rapid association of the monomers in a diffusion limited manner and appear to be largely unstructured, as they are silent to the optical probes employed in the current study. The existence of parallel pathways is supported by the insensitivity of the amplitudes of the refolding kinetic phases to the different probes used in the study. More importantly, interrupted refolding studies and ligand binding studies clearly demonstrate that the native state forms in a biexponential manner, implying the presence of at least two pathways. Our studies indicate that the CcdA antitoxin binds only to the folded CcdB dimer and not to any earlier folding intermediates. Thus, despite being part of the same operon, the antitoxin does not appear to modulate the folding pathway of the toxin encoded by the downstream cistron. This study highlights the utility of ligand binding in distinguishing between sequential and parallel pathways in protein folding studies, while also providing insights into molecular interactions during folding in Type II toxin-antitoxin systems.

The utilization of competing unfolding pathways of monellin is dictated by enthalpic barriers.

Direct evidence of the presence of competing pathways for the folding or unfolding reactions of proteins is difficult to obtain. A direct signature for multiple pathways, seen so far only rarely in folding or unfolding studies, is an upward curvature in the dependence of the logarithm of the observed rate constant (λU) of folding on denaturant concentration. In this study, the unfolding mechanism of the wild-type (wt) and E24A variants of monellin has been investigated, and both variants are shown to display upward curvatures in plots of log λU versus denaturant concentration. Curvature is distinctly more pronounced for E24A than for the wt protein. Kinetic unfolding studies of E24A were conducted over a range of denaturant concentrations and across a range of temperatures, and the kinetic data were globally analyzed assuming two parallel pathways L and H, which proceed through transition states TS(L) and TS(H), respectively. The observation of the upward curvature in the unfolding kinetics permitted a thermodynamic analysis of how unfolding switches from one pathway to the other upon a change in unfolding conditions. The m −N and ΔCp values indicate that TSL is more compact than TS(H). The major contribution to the free energy of activation on either pathway is seen to be enthalpic and not entropic in origin.

A buried ionizable residue destabilizes the native state and the transition state in the folding of monellin.

A buried ionizable residue can have a drastic effect on the stability of a native protein, but there has been only limited investigation of how burial of an ionizable residue affects the kinetics of protein folding. In this study, the effect of burial of ionizable residues on the thermodynamics and kinetics of folding and unfolding of monellin has been investigated. The stability of wild-type (wt) monellin is known to decrease with an increase in pH from 4 to 10. The Glu24 → Ala mutation makes the stability of the resultant E24A mutant protein independent of pH in the range from 4 to 8. An additional mutation, Cys42 → Ala, results in the stability becoming independent of pH in the range from 4 to 10. Like the wt protein, E24A folds via very fast, fast, and slow folding pathways. Compared to that of the wt protein, the rate of slow folding pathway of E24A is ~7-fold faster, the rate of fast folding pathway is ~1.5-fold faster, while the rate of very fast folding pathway is similar. E24A unfolds ~7-fold slower than the wt. The extent of stabilization of the transition state (TS) observed for the slow pathway of refolding and for unfolding is the same, indicating that unfolding occurs via the TS populated on the slow pathway of refolding. The stabilization of the TS of folding (1.1 kcal mol(-1)) is less than that of the native state (2.3 kcal mol(-1)) of E24A, indicating that structure has only partially formed in the vicinity of Glu24 in the TS of folding.

Equilibrium unfolding studies of monellin: the double-chain variant appears to be more stable than the single-chain variant.

To improve our understanding of the contributions of different stabilizing interactions to protein stability, including that of residual structure in the unfolded state, the small sweet protein monellin has been studied in both its two variant forms, the naturally occurring double-chain variant (dcMN) and the artificially created single-chain variant (scMN). Equilibrium guanidine hydrochloride-induced unfolding studies at pH 7 show that the standard free energy of unfolding, ΔG°(U), of dcMN to unfolded chains A and B and its dependence on guanidine hydrochloride (GdnHCl) concentration are both independent of protein concentration, while the midpoint of unfolding has an exponential dependence on protein concentration. Hence, the unfolding of dcMN like that of scMN can be described as two-state unfolding. The free energy of dissociation, ΔG°(d), of the two free chains, A and B, from dcMN, as measured by equilibrium binding studies, is significantly lower than ΔG°(U), apparently because of the presence of residual structure in free chain B. The value of ΔG°(U), at the standard concentration of 1 M, is found to be ∼5.5 kcal mol(-1) higher for dcMN than for scMN in the range from pH 4 to 9, over which unfolding appears to be two-state. Hence, dcMN appears to be more stable than scMN. It seems that unfolded scMN is stabilized by residual structure that is absent in unfolded dcMN and/or that native scMN is destabilized by strain that is relieved in native dcMN. The value of ΔG°(U) for both protein variants decreases with an increase in pH from 4 to 9, apparently because of the thermodynamic coupling of unfolding to the protonation of a buried carboxylate side chain whose pK(a) shifts from 4.5 in the unfolded state to 9 in the native state. Finally, it is shown that although the thermodynamic stabilities of dcMN and scMN are very different, their kinetic stabilities with respect to unfolding in GdnHCl are very similar.

Heterologous expression, purification and characterization of heterodimeric monellin.

Monellin is an intensely sweet-tasting protein present in the berry of Dioscoreophyllum cumminsii. Naturally occurring monellin (double chain monellin) is a heterodimer of two subunits commonly referred to as chain A and chain B. Monellin is a good model system for structural and dynamic studies of proteins. Single chain monellin has been generated by covalently linking the two subunits of naturally occurring double chain monellin, and has been used extensively for folding and unfolding studies, as well as for protein aggregation studies. There are, however, relatively few reports on such studies with double chain monellin. The primary difficulty associated with studies using double chain monellin appears to be the lack of a standard purification method. Here, a simple method for the purification of double chain monellin is presented. The genes encoding the two chains of monellin were cloned into a modified pETDUET vector under separate T7 promoters. The expression vector containing the genes of the two chains was expressed in E. coli BL21 Star (DE3). The expressed protein was purified using two steps of chromatography, ion exchange chromatography and gel filtration chromatography. This expression system consistently produced 40 mg of pure double chain monellin per litre of E. coli culture, in the correctly folded native state. The purity of the protein was confirmed by mass spectrometry and SDS-PAGE analysis. The purified protein was characterized using different spectroscopic methods, and the spectra obtained were in good agreement with the published spectra of naturally occurring double chain monellin.

Mechanism of CcdA-Mediated Rejuvenation of DNA Gyrase.

Most biological processes involve formation of transient complexes where binding of a ligand allosterically modulates function. The ccd toxin-antitoxin system is involved in plasmid maintenance and bacterial persistence. The CcdA antitoxin accelerates dissociation of CcdB from its complex with DNA gyrase, binds and neutralizes CcdB, but the mechanistic details are unclear. Using a series of experimental and computational approaches, we demonstrate the formation of transient ternary and quaternary CcdA:CcdB:gyrase complexes and delineate the molecular steps involved in the rejuvenation process. Binding of region 61-72 of CcdA to CcdB induces the vital structural and dynamic changes required to facilitate dissociation from gyrase, region 50-60 enhances the dissociation process through additional allosteric effects, and segment 37-49 prevents gyrase rebinding. This study provides insights into molecular mechanisms responsible for recovery of CcdB-poisoned cells from a persister-like state. Similar methodology can be used to characterize other important transient, macromolecular complexes.

Kinetic studies of the folding of heterodimeric monellin: evidence for switching between alternative parallel pathways.

Determining whether or not a protein uses multiple pathways to fold is an important goal in protein folding studies. When multiple pathways are present, defined by transition states that differ in their compactness and structure but not significantly in energy, they may manifest themselves by causing the dependence on denaturant concentration of the logarithm of the observed rate constant of folding to have an upward curvature. In this study, the folding mechanism of heterodimeric monellin [double-chain monellin (dcMN)] has been studied over a range of protein and guanidine hydrochloride (GdnHCl) concentrations, using the intrinsic tryptophan fluorescence of the protein as the probe for the folding reaction. Refolding is shown to occur in multiple kinetic phases. In the first stage of refolding, which is silent to any change in intrinsic fluorescence, the two chains of monellin bind to one another to form an encounter complex. Interrupted folding experiments show that the initial encounter complex folds to native dcMN via two folding routes. A productive folding intermediate population is identified on one route but not on both of these routes. Two intermediate subpopulations appear to form in a fast kinetic phase, and native dcMN forms in a slow kinetic phase. The chevron arms for both the fast and slow phases of refolding are shown to have upward curvatures, suggesting that at least two pathways each defined by a different intermediate are operational during these kinetic phases of structure formation. Refolding switches from one pathway to the other as the GdnHCl concentration is increased.