Internalization by HeLa cells of latex beads coated with mammalian cell entry (Mce) proteins encoded by the mce3 operon of Mycobacterium tuberculosis.

The mammalian cell entry (Mce) operon 3 (mce3) is one of four homologous mce operons of Mycobacterium tuberculosis, encoding six (Mce3A-F) invasin-like membrane-associated proteins. Previous studies have shown that recombinant expression of Mce1A encoded by the mce1 operon in Escherichia coli allows this non-pathogenic bacterium to invade and survive inside macrophages, and latex beads coated with Mce1A are internalized by non-phagocytic HeLa cells. However, the role of other mce1 operon proteins (Mce1B-F) and proteins encoded by the operons mce2-4 in facilitating the internalization of M. tuberculosis in mammalian cells has not been studied. This study was carried out to determine whether Mce proteins encoded by the mce3 operon also facilitated the internalization of latex beads by HeLa cells. Recombinant pure Mce3A and lipoprotein LprM (Mce3E) were expressed and purified from E. coli cells. Mce1A expressed as a fusion protein with glutathione S-transferase (GST-Mce1A) and GST alone, purified similarly from E. coli cells, were used as control proteins. Fluorescent latex beads coated with purified proteins were used to study their uptake by HeLa cells using fluorescence microscopy, flow cytometry and electron microscopy. Fluorescence microscopy and flow cytometry showed an association of HeLa cells with beads coated with both Mce3A and LprM, whilst GST-Mce1A and GST yielded the expected results. Transmission electron microscopy confirmed the uptake of beads coated with Mce3A or LprM by HeLa cells. The data showed that Mce3A encoded by the mce3 operon facilitated the uptake and internalization of latex beads by HeLa cells. The data also showed, for the first time, the role of another Mce protein (LprM/Mce3E) in facilitating the interaction and internalization of M. tuberculosis by mammalian cells.