RTX adhesins are long, multi-domain proteins present on the outer membrane of many Gram-negative bacteria. From this vantage point, adhesins use their distal ligand-binding domains for surface attachment leading to biofilm formation. To expand the reach of the ligand-binding domains, RTX adhesins maintain a central extender region of multiple tandem repeats, which makes up most of the proteins' large molecular weight. Alignments of the 10-15-kDa extender domains show low sequence identity between adhesins. Here we have produced and structurally characterized protein constructs of four tandem repeats (tetra-tandemers) from two different RTX adhesins. In comparing the tetra-tandemers to each other and already solved structures from Marinomonas primoryensis and Salmonella enterica, the extender domains fold as diverse beta-sandwich structures with widely differing calcium contents. However, all the tetra-tandemers have at least one calcium ion coordinated in the linker region between beta-sandwich domains whose role appears to be the rigidification of the extender region to help the adhesin extend its reach.
Keywords: Adhesion protein; AhLap, Aeromonas hydrophila long adhesion protein; Bacterial adhesin; Beta-sandwich domains; Calcium coordination; MhLap, Marinobacter hydrocarbonoclasticus long adhesion protein; MpIBP, Marinomonas primoryensis ice-binding protein; RTX, Repeats in toxin; vWFA, von Willebrand Factor A.
© 2020 Published by Elsevier Inc.