RTX calcium binding motifs are intrinsically disordered in the absence of calcium: implication for protein secretion

Mutations affecting the extreme C terminus of Escherichia coli haemolysin A reduce haemolytic activity by altering the folding of the toxin

Escherichia coli haemolysin A (HlyA), an RTX toxin, is secreted probably as an unfolded intermediate, by the type I (ABC transporter-dependent) pathway, utilizing a C-terminal secretion signal. However, the mechanism of translocation and post-translocation folding is not understood. We identified a mutation (hlyA99) at the extreme C terminus, which is dominant in competition experiments, blocking secretion of the wild-type toxin co-expressed in the same cell. This suggests that unlike recessive mutations which affect recognition of the translocation machinery, the hlyA99 mutation interferes with some later step in secretion. Indeed, the mutation reduced haemolytic activity of the toxin and the activity of β-lactamase when the latter was fused to a C-terminal 23 kDa fragment of HlyA carrying the hlyA99 mutation. A second mutant (hlyAdel6), lacking the six C-terminal residues of HlyA, also showed reduced haemolytic activity and neither mutant protein regained normal haemolytic activity in in vitro unfolding/refolding experiments. Tryptophan fluorescence spectroscopy indicated differences in structure between the secreted forms of wild-type HlyA and the HlyA Del6 mutant. These results suggested that the mutations affected the correct folding of both HlyA and the β-lactamase fusion. Thus, we propose a dual function for the HlyA C terminus involving an important role in post-translocation folding as well as targeting HlyA for secretion.

Pertussis toxin and adenylate cyclase toxin: key virulence factors of Bordetella pertussis and cell biology tools

Pertussis toxin and adenylate cyclase toxin are two important virulence factors of Bordetella pertussis, the bacterial cause of the respiratory disease pertussis or whooping cough. In addition to studies on the structure, function and role in pathogenesis of these two toxins, they are both used as cell biology tools for a variety of applications owing to their ability to enter mammalian cells, perform enzymatic activities and modify cell signaling events. In this article, recent data from the research literature that enhance our understanding of the nature of these two toxins, their role in the pathogenesis of B. pertussis infection and disease, particularly in modulating host immune responses, and their use as tools for other areas of research will be outlined.

Order and disorder in the domain organization of the plasmid partition protein KorB

The plasmid partition protein KorB has a dual role: it is essential for the correct segregation of the low copy number broad host range RK2 plasmid while also being an important regulator of transcription. KorB belongs to the ParB family of proteins, and partitioning in RK2 has been studied as a simplified model of bacterial chromosome segregation. Structural information on full-length ParB proteins is limited, mainly due to the inability to grow crystals suitable for diffraction studies. We show, using CD and NMR, that KorB has regions of significant intrinsic disorder and hence it adopts a multiplicity of conformations in solution. The biophysical data are consistent with bioinformatic predictions based on the amino acid sequence that the N-terminal region and also the region between the central DNA-binding domain and the C-terminal dimerization domain are intrinsically disordered. We have used small angle x-ray scattering data to determine the ensemble of solution conformations for KorB and selected deletion mutants, based on models of the known domain structures. This conformational range of KorB is likely to be biologically required for DNA partitioning and for binding to a diverse set of partner proteins.

Calcium-induced folding of a beta roll motif requires C-terminal entropic stabilization

Beta roll motifs are associated with several proteins secreted by the type 1 secretion system (T1SS). Located just upstream of the C-terminal T1SS secretion signal, they are believed to act as calcium-induced switches that prevent folding before secretion. Bordetella pertussis adenylate cyclase (CyaA) toxin has five blocks of beta roll motifs (or repeats-in-toxin motifs) separated by linkers. The block V motif on its own has been reported to be non-responsive to calcium. Only when the N- and C-terminal linkers, or flanking groups, were fused did the motif bind calcium and fold. In an effort to understand the requirements for beta roll folding, we have truncated the N- and C-terminal flanks at several locations to determine the minimal essential sequences. Calcium-responsive beta roll folding occurred even in the absence of the natural N-terminal flank. The natural C-terminal flank could not be truncated without decreased calcium affinity and only partially truncated before losing calcium-responsiveness. Globular protein fusion at the C-terminus likewise enabled calcium-induced folding but fusions solely at the N-terminus failed. This demonstrates that calcium-induced folding is an inherent property of the beta roll motif rather than the flanking groups. Given the disparate nature of the observed functional flanking groups, C-terminal fusions appear to confer calcium-responsiveness to the beta roll motif via a non-specific mechanism, suggesting that entropic stabilization of the unstructured C-terminus can enable beta roll folding. Increased calcium affinity was observed when the natural C-terminal flank was used to enable calcium-induced folding, pointing to its cooperative participation in beta roll formation. This work indicates that a general principle of C-terminal entropic stabilization can enable stimulus-responsive repeat protein folding, while the C-terminal flank has a specific role in tuning calcium-responsive beta roll formation. These observations are in stark contrast to what has been reported for other repeat proteins.

Interdomain Ca(2+) effects in Escherichia coli alpha-haemolysin: Ca(2+) binding to the C-terminal domain stabilizes both C- and N-terminal domains

alpha-Haemolysin (HlyA) is a toxin secreted by pathogenic Escherichia coli, whose lytic activity requires submillimolar Ca(2+) concentrations. Previous studies have shown that Ca(2+) binds within the Asp and Gly rich C-terminal nonapeptide repeat domain (NRD) in HlyA. The presence of the NRD puts HlyA in the RTX (Repeats in Toxin) family of proteins. We tested the stability of the whole protein, the amphipathic helix domain and the NRD, in both the presence and absence of Ca(2+) using native HlyA, a truncated form of HlyADeltaN601 representing the C-terminal domain, and a novel mutant HlyA W914A whose intrinsic fluorescence indicates changes in the N-terminal domain. Fluorescence and infrared spectroscopy, tryptic digestion, and urea denaturation techniques concur in showing that calcium binding to the repeat domain of alpha-haemolysin stabilizes and compacts both the NRD and the N-terminal domains of HlyA. The stabilization of the N-terminus through Ca(2+) binding to the C-terminus reveals long-range inter-domain structural effects. Considering that RTX proteins consist, in general, of a Ca(2+)-binding NRD and separate function-specific domains, the long-range stabilizing effects of Ca(2+) in HlyA may well be common to other members of this family.

Calcium-dependent conformational flexibility of a CUB domain controls activation of the complement serine protease C1r

C1, the first component of the complement system, is a Ca(2+)-dependent heteropentamer complex of C1q and two modular serine proteases, C1r and C1s. Current functional models assume significant flexibility of the subcomponents. Noncatalytic modules in C1r have been proposed to provide the flexibility required for function. Using a recombinant CUB2-CCP1 domain pair and the individual CCP1 module, we showed that binding of Ca(2+) induces the folding of the CUB2 domain and stabilizes its structure. In the presence of Ca(2+), CUB2 shows a compact, folded structure, whereas in the absence of Ca(2+), it has a flexible, disordered conformation. CCP1 module is Ca(2+)-insensitive. Isothermal titration calorimetry revealed that CUB2 binds a single Ca(2+) with a relatively high K(D) (430 mum). In blood, the CUB2 domain of C1r is only partially (74%) saturated by Ca(2+), therefore the disordered, Ca(2+)-free form could provide the flexibility required for C1 activation. In accordance with this assumption, the effect of Ca(2+) on the autoactivation of native, isolated C1r zymogen was proved. In the case of infection-inflammation when the local Ca(2+) concentration decreases, this property of CUB2 domain could serve as subtle means to trigger the activation of the classical pathway of complement. The CUB2 domain of C1r is a novel example for globular protein domains with marginal stability, high conformational flexibility, and proteolytic sensitivity. The physical nature of the behavior of this domain is similar to that of intrinsically unstructured proteins, providing a further example of functionally relevant ligand-induced reorganization of a polypeptide chain.

Characterization of the regions involved in the calcium-induced folding of the intrinsically disordered RTX motifs from the bordetella pertussis adenylate cyclase toxin

Repeat in toxin (RTX) motifs are nonapeptide sequences found among numerous virulence factors of Gram-negative bacteria. In the presence of calcium, these RTX motifs are able to fold into an idiosyncratic structure called the parallel beta-roll. The adenylate cyclase toxin (CyaA) produced by Bordetella pertussis, the causative agent of whooping cough, is one of the best-characterized RTX cytolysins. CyaA contains a C-terminal receptor domain (RD) that mediates toxin binding to the eukaryotic cell receptor. The receptor-binding domain is composed of about forty RTX motifs organized in five successive blocks (I to V). The RTX blocks are separated by non-RTX flanking regions of variable lengths. It has been shown that block V with its N- and C-terminal flanking regions constitutes an autonomous subdomain required for the toxicity of CyaA. Here, we investigated the calcium-induced biophysical changes of this subdomain to identify the respective contributions of the flanking regions to the folding process of the RTX motifs. We showed that the RTX polypeptides, in the absence of calcium, exhibited the hallmarks of intrinsically disordered proteins and that the C-terminal flanking region was critical for the calcium-dependent folding of the RTX polypeptides, while the N-terminal flanking region was not involved. Furthermore, the secondary and tertiary structures were acquired concomitantly upon cooperative binding of several calcium ions. This suggests that the RTX polypeptide folding is a two-state reaction, from a calcium-free unfolded state to a folded and compact conformation, in which the calcium-bound RTX motifs adopt a beta-roll structure. The relevance of these results to the toxin physiology, in particular to its secretion, is discussed.

Toxins from bacteria

Bacterial toxins damage the host at the site of bacterial infection or distant from the site. Bacterial toxins can be single proteins or oligomeric protein complexes that are organized with distinct AB structure-function properties. The A domain encodes a catalytic activity. ADP ribosylation of host proteins is the earliest post-translational modification determined to be performed by bacterial toxins; other modifications include glucosylation and proteolysis. Bacterial toxins also catalyze the non-covalent modification of host protein function or can modify host cell properties through direct protein-protein interactions. The B domain includes two functional domains: a receptor-binding domain, which defines the tropism of a toxin for a cell and a translocation domain that delivers the A domain across a lipid bilayer, either on the plasma membrane or the endosome. Bacterial toxins are often characterized based upon the secretion mechanism that delivers the toxin out of the bacterium, termed types I-VII. This review summarizes the major families of bacterial toxins and also describes the specific structure-function properties of the botulinum neurotoxins.

A FRET-based method for probing the conformational behavior of an intrinsically disordered repeat domain from Bordetella pertussis adenylate cyclase

A better understanding of the conformational changes exhibited by intrinsically disordered proteins is necessary as we continue to unravel their myriad biological functions. In repeats in toxin (RTX) domains, calcium binding triggers the natively unstructured domain to adopt a beta roll structure. Here we present an in vitro Forster resonance energy transfer (FRET)-based method for the investigation of the conformational behavior of an RTX domain from the Bordetella pertussis adenylate cyclase consisting of nine repeat units. Equilibrium and stopped-flow FRET between fluorescent proteins, attached to the termini of the domain, were measured in an analysis of the end-to-end distance changes in the RTX domain. The method was complemented with circular dichroism spectroscopy, tryptophan fluorescence, and bis-ANS dye binding. High ionic strength was observed to decrease the calcium affinity of the RTX domain. A truncation and single amino acid mutations yielded insights into the structural determinants of beta roll formation. Mutating the conserved Asp residue in one of the nine repeats significantly reduced the affinity of the domains for calcium ions. Removal of the sequences flanking the repeat domain prevented folding, but replacing them with fluorescent proteins restored the conformational behavior, suggesting an entropic stabilization. The FRET-based method is a useful technique that complements other low-resolution techniques for investigating the dynamic conformational behavior of the RTX domain and other intrinsically disordered protein domains.