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Background  The  mce4  operon is one of the four homologues of mammalian cell entry ( mce ) operons of  Mycobacterium tuberculosis . The  mce4A  (Rv3499c) gene within this operon is homologous to  mce1A  (Rv0169), that has a role in host cell invasion by  M. tuberculosis . Our earlier reports show that  mce4  operon is expressed during the stationary phase of growth of the bacillus in culture and during the course of infection in mammalian hosts.  M. tuberculosis  carrying mutation in  mce4  operon shows growth defect and reduced survival in infected mice. However, the intracellular localization of Mce4A protein and its direct role in cell entry or survival of the bacillus has not been demonstrated so far.    Results  By transmission electron microscopy we have demonstrated that recombinant Mce4A protein facilitates the invasion of non-pathogenic strain of  E. coli  into non-phagocytic HeLa cells. We observe that  mce4A  gene has a role comparable to  mce1A  in the survival of recombinant  E. coli  in human macrophages. Using antibodies raised against Mce4A protein, we show that the protein is localized in the cell wall fraction of  M. tuberculosis  H37Rv stationary phase culture only.    Conclusion  Mce4A protein is expressed during the stationary phase of broth culture and localizes in the cell wall fraction of  M. tuberculosis . Mce4A protein expressed in non-pathogenic  E. coli  enables it to enter and survive within HeLa cells and the macrophages. As Mce4A protein is expressed during later phase of mycobacterial growth, our results raise the possibility of it playing a role in maintenance of persistent tubercular infection.

Characterization of Mce4A protein of Mycobacterium tuberculosis: role in invasion and survival