Leptospira immunoglobulin-like protein B (LigB) binding to the C-terminal fibrinogen αC domain inhibits fibrin clot formation, platelet adhesion and aggregation

Leptospira immunoglobulin-like (Lig) proteins including LigA and LigB are adhesins that bind to fibronectin, collagen, laminin and elastin. In addition, Lig proteins are fibrinogen (Fg)-binding proteins, although the physiological role of the Lig-Fg interaction is unclear. In this study, a previously identified Fg-binding region, LigBCen2 (amino acids 1014-1165 of LigB), has been further localized to LigBCen2R, which consists of the partial 11th and entire 12th Ig-like domain (amino acids 1014-1119). LigBCen2R was found to bind to the C-terminal alpha C domain of Fg (Fg alpha CC; amino acids 392-644 in Fg alpha chain; isothermal titration calorimetry, K-D = 0.375 mu M; fluorescence spectrometry, K-D = 0.364 mu M). The quenching and blue shift observed for the maximum wavelength intensities of the tryptophan fluorescence spectra for Fg alpha CCY570W upon LigBCen2RW1073C binding suggested an RGD motif close to the sole tryptophan on Fg alpha CCY570W was buried in LigBCen2R upon saturation with FgaCC. A conformational change in LigBCen2R when bound to the Fg alpha CC RGD motif blocked further binding to integrin alpha(IIb)beta 3 on platelets, thus preventing their aggregation. LigBCen2R binding to Fg alpha CC reduced clot formation but did not affect plasminogen and tissue-type plasminogen activator interactions with Fg alpha CC. This study is the first to report that a spirochaetal protein binds to the C-terminal alpha C domain of Fg, which regulates thrombosis and fibrinolysis, and may help explain the pulmonary haemorrhage and thrombocytopenia seen in clinical cases of leptospirosis.