Among the seventeen species of the Gram-negative genus Yersinia, three have been shown to be virulent and pathogenic to humans and animals—Y. enterocolitica, Y. pseudotuberculosis, and Y. pestis. In order to be so, they are armoured with various factors that help them adhere to tissues and organelles, cross the cellular barrier and escape the immune system during host invasion. The group of proteins that mediate pathogen–host interactions constitute adhesins. Invasin, Ail, YadA, YadB, YadC, Pla, and pH 6 antigen belong to the most prominent and best-known Yersinia adhesins. They act at different times and stages of infection complementing each other by their ability to bind a variety of host molecules such as collagen, fibronectin, laminin, β1 integrins, and complement regulators. All the proteins are anchored in the bacterial outer membrane (OM), often forming rod-like or fimbrial-like structures that protrude to the extracellular milieu. Structural studies have shown that the anchor region forms a β-barrel composed of 8, 10, or 12 antiparallel β-strands. Depending on the protein, the extracellular part can be composed of several domains belonging to the immunoglobulin fold superfamily, or form a coiled-coil structure with globular head domain at the end, or just constitute several loops connecting individual β-strands in the β-barrel. Those extracellular regions define the activity of each adhesin. This review focuses on the structure and function of these important molecules, and their role in pathogenesis.

Human cystatin C (HCC) is a family 2 cystatin inhibitor of papain‐like (C1) and legumain‐related (C13) cysteine proteases. In pathophysiological processes, the nature of which is not understood, HCC is codeposited in the amyloid plaques of Alzheimer’s disease or Down’s syndrome. The amyloidogenic properties of HCC are greatly increased in a naturally occurring L68Q variant, resulting in fatal cerebral amyloid angiopathy in early adult life. In all crystal structures of cystatin C studied to date, the protein has been found to form 3D domain‐swapped dimers, created through a conformational change of a β‐hairpin loop, L1, from the papain‐binding epitope. We have created monomer‐stabilized human cystatin C, with an engineered disulfide bond (L47C)–(G69C) between the structural elements that become separated upon domain swapping. The mutant has drastically reduced dimerization and fibril formation properties, but its inhibition of papain is unaltered. The structure confirms the success of the protein engineering experiment to abolish 3D domain swapping and, in consequence, amyloid fibril formation. It illustrates for the first time the fold of monomeric cystatin C and allows verification of earlier predictions based on the domain‐swapped forms and on the structure of chicken cystatin. Importantly, the structure defines the so‐far unknown conformation of loop L1, which is essential for the inhibition of papain‐like cysteine proteases.

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Background: It is unknown why patients with autoantibodies against complement factor H (CFH) lack homologous CFHR1 protein.Results: The autoantibody epitope on CFH was identified, and the structure of the corresponding part of CFHR1 was solved.Conclusion: The autoantigenic epitope of CFH and its homologous site in CFHR1 are structurally different.Significance: A plausible explanation for formation of autoantibodies due to CFHR1 deficiency in autoimmune atypical hemolytic uremic syndrome was obtained.

Background: Borrelia burgdorferi OspE protein recruits complement regulator FH onto the bacteria for immune evasion. Results: We solved the structure of OspE and the OspE⅐FH complex by NMR and x-ray crystallography. Conclusion: The OspE⅐FH structure shows how Borrelia evade complement attack by mimicking how host cells protect themselves. Significance: This explains how the bacteria survive in the host and facilitates vaccine design against borreliosis.