Structural basis of α-catenin recognition by EspB from enterohaemorrhagic E. coli based on hybrid strategy using low-resolution structural and protein dissection.

Enterohaemorrhagic E. coli (EHEC) induces actin reorganization of host cells by injecting various effectors into host cytosol through type III secretion systems. EspB is the natively partially folded EHEC effector which binds to host α-catenin to promote the actin bundling. However, its structural basis is poorly understood. Here, we characterize the overall structural properties of EspB based on low-resolution structural data in conjunction with protein dissection strategy. EspB showed a unique thermal response involving cold denaturation in the presence of denaturant according to far-UV circular dichroism (CD). Small angle X-ray scattering revealed the formation of a highly extended structure of EspB comparable to the ideal random coil. Various disorder predictions as well as CD spectra of EspB fragments identified the presence of α-helical structures around G41 to Q70. The fragment corresponding to this region indicated the thermal response similar to EspB. Moreover, this fragment showed a high affinity to C-terminal vinculin homology domain of α-catenin. The results clarified the importance of preformed α-helix of EspB for recognition of α-catenin.

Cytoskeleton-modulating effectors of enteropathogenic and enterohemorrhagic Escherichia coli: a case for EspB as an intrinsically less-ordered effector.

Enterohemorrhagic and enteropathogenic Escherichia coli produce various effector proteins that are directly injected into the host-cell cytosol through the type III secretion system. E. coli secreted protein (Esp)B is one such effector protein, and affects host-cell morphology by reorganizing actin networks. Unlike most globular proteins that have well-ordered, rigid structures, the structures of type III secretion system effectors from pathogenic Gram-negative bacteria, including EspB, are often less well-ordered. This minireview focuses on the functional relationship between the structural properties of these proteins and their roles in type III secretion system-associated pathogenesis.

Molecular basis of actin reorganization promoted by binding of enterohaemorrhagic Escherichia coli EspB to alpha-catenin.

EspB is a multifunctional protein associated with the type III secretion system of enterohaemorrhagic Escherichia coli, and interacts with various biomolecules including alpha-catenin in the host cell. The binding of EspB to alpha-catenin is thought be involved in actin reorganization during bacterial infection, although the precise mechanism of this phenomenon is still unclear. Recent research shows that dimerization of alpha-catenin dissociates it from E-cadherin/beta-catenin/alpha-catenin complexes, and that the dimer suppresses Arp2/3-mediated actin branching or polymerization. These results inspired us to evaluate the effect of EspB on the functions of alpha-catenin. Based on a series of in vitro biochemical approaches, including pull-down, co-sedimentation and pyrene-actin polymerization assays combined with transmission electron microscopy, we conclude that EspB promotes all the functions of dimeric alpha-catenin described above. These results clarified the molecular basis of reorganization of actin filaments during infection with enterohaemorrhagic Escherichia coli.

EspB from enterohaemorrhagic Escherichia coli is a natively partially folded protein.

The structural properties of EspB, a virulence factor of the Escherichia coli O157 type III secretion system, were characterized. Far-UV and near-UV CD spectra, recorded between pH 1.0 and pH 7.0, show that the protein assumes alpha-helical structures and that some tyrosine tertiary contacts may exist. All tyrosine side-chains are exposed to water, as determined by acrylamide fluorescence quenching spectroscopy. An increase in the fluorescence intensity of 8-anilinonaphthalene-1-sulfonate was observed at pH 2.0 in the presence of EspB, whereas no such increase in fluorescence was observed at pH 7.0. These data suggest the formation of a molten globule state at pH 2.0. Destabilization of EspB at low pH was shown by urea-unfolding transitions, monitored by far-UV CD spectroscopy. The result from a sedimentation equilibrium study indicated that EspB assumes a monomeric form at pH 7.0, although its Stokes radius (estimated by multiangle laser light scattering) was twice as large as expected for a monomeric globular structure of EspB. These data suggest that EspB, at pH 7.0, assumes a relatively expanded conformation. The chemical shift patterns of EspB 15N-1H heteronuclear single quantum correlation spectra at pH 2.0 and 7.0 are qualitatively similar to that of urea-unfolded EspB. Taken together, the properties of EspB reported here provide evidence that EspB is a natively partially folded protein, but with less exposed hydrophobic surface than traditional molten globules. This structural feature of EspB may be advantageous when EspB interacts with various biomolecules during the bacterial infection of host cells.