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Background  Many bacterial surface exposed proteins mediate the host-pathogen interaction more effectively in the presence of Ca 2+ . Leptospiral immunoglobulin-like (Lig) proteins, LigA and LigB, are surface exposed proteins containing Bacterial immunoglobulin like (Big) domains. The function of proteins which contain Big fold is not known. Based on the possible similarities of immunoglobulin and βγ-crystallin folds, we here explore the important question whether Ca 2+  binds to a Big domains, which would provide a novel functional role of the proteins containing Big fold.    Principal Findings  We selected six individual Big domains for this study (three from the conserved part of LigA and LigB, denoted as Lig A3, Lig A4, and LigBCon5; two from the variable region of LigA, i.e., 9 th  (Lig A9) and 10 th  repeats (Lig A10); and one from the variable region of LigB, i.e., LigBCen2. We have also studied the conserved region covering the three and six repeats (LigBCon1-3 and LigCon). All these proteins bind the calcium-mimic dye Stains-all. All the selected four domains bind Ca 2+  with dissociation constants of 2–4 µM. Lig A9 and Lig A10 domains fold well with moderate thermal stability, have β-sheet conformation and form homodimers. Fluorescence spectra of Big domains show a specific doublet (at 317 and 330 nm), probably due to Trp interaction with a Phe residue. Equilibrium unfolding of selected Big domains is similar and follows a two-state model, suggesting the similarity in their fold.    Conclusions  We demonstrate that the Lig are Ca 2+ -binding proteins, with Big domains harbouring the binding motif. We conclude that despite differences in sequence, a Big motif binds Ca 2+ . This work thus sets up a strong possibility for classifying the proteins containing Big domains as a novel family of Ca 2+ -binding proteins. Since Big domain is a part of many proteins in bacterial kingdom, we suggest a possible function these proteins  via  Ca 2+  binding.

Big Domains Are Novel Ca2+-Binding Modules: Evidences from Big Domains of Leptospira Immunoglobulin-Like (Lig) Proteins