Abstract
BackgroundMany bacterial surface exposed proteins mediate the host-pathogen interaction more effectively in the presence of Ca2+. 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 Ca2+ binds to a Big domains, which would provide a novel functional role of the proteins containing Big fold.Principal FindingsWe 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., 9th (Lig A9) and 10th 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 Ca2+ 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.ConclusionsWe demonstrate that the Lig are Ca2+-binding proteins, with Big domains harbouring the binding motif. We conclude that despite differences in sequence, a Big motif binds Ca2+. This work thus sets up a strong possibility for classifying the proteins containing Big domains as a novel family of Ca2+-binding proteins. Since Big domain is a part of many proteins in bacterial kingdom, we suggest a possible function these proteins via Ca2+ binding.
Highlights
Bacterial immunoglobulin-like (Big) folds, known as Bacterial immunoglobulin-like Domains (BID), are present in many bacterial proteins of 74–90 amino acids in tandem repeats [1]
We demonstrate that the Lig are Ca2+-binding proteins, with Bacterial immunoglobulin like (Big) domains harbouring the binding motif
This work sets up a strong possibility for classifying the proteins containing Big domains as a novel family of Ca2+-binding proteins
Summary
Bacterial immunoglobulin-like (Big) folds, known as Bacterial immunoglobulin-like Domains (BID), are present in many bacterial proteins of 74–90 amino acids in tandem repeats [1]. Leptospiral immunoglobulin-like (Lig) outer membrane proteins of Leptospira interrogans are members of the Big family [3,4,5] that are upregulated during infection of the host and are thought to play a role in the pathogenesis of leptospirosis, a worldwide zoonotic disease [6,7]. Three related Lig proteins, LigA, LigB and LigC have been identified in Leptospira. LigA and LigB have 13 and 12 Big-like domains [8], whereas ligC is considered a pseudogene in serovars Grippotyphosa and Copenhageni [3,9]. 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 bc-crystallin folds, we here explore the important question whether Ca2+ binds to a Big domains, which would provide a novel functional role of the proteins containing Big fold
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