Solution structure of a bacterial immunoglobulin-like domain of the outer membrane protein (LigB) from Leptospira

Leptospiral immunoglobulin-like (Lig) proteins are surface proteins expressed in pathogenic strains of Leptospira. LigB, an outer membrane protein containing tandem repeats of bacterial Ig-like (Big) domains and a no-repeat tail, has been identified as a virulence factor involved in adhesion of pathogenic Leptospira interrogans to host cells. A Big domain of LigB, LigBCen2R, was reported previously to bind the GBD domain of fibronectin, suggesting its important role in leptospiral infections. In this study, we determined the solution structure of LigBCen2R by nuclear magnetic resonance (NMR) spectroscopy. LigBCen2R adopts a canonical immunoglobulin-like fold which is comprised of a beta-sandwich of ten strands in three sheets. We indicated that LigBCen2R is able to bind to Ca(2+) with a high affinity by isothermal titration calorimetry assay. NMR perturbation experiment identified a number of residues responsible for Ca(2+) binding. Structural comparison of it with other Big domains demonstrates that they share a similar fold pattern, but vary in some structural characters. Since Lig proteins play a vital role in the infection to host cells, our study will contribute a structural basis to understand the interactions between Leptospira and host cells.