Recombinant Mycobacterium tuberculosis protein associated with mammalian cell entry

Abstract

The ability to gain entry and resist the antimicrobial intracellular environment of mammalian cells is an essential virulence property of Mycobacterium tuberculosis. A purified recombinant protein expressed by a 1362 bp locus (mce1) in the M. tuberculosis genome promoted uptake into HeLa cells of polystyrene latex microspheres coated with the protein. N‐terminus deletion constructs of Mce1 identified a domain located between amino acid positions 106 and 163 that was needed for this cell uptake activity. Mce1 contained hydrophobic stretches at the N‐terminus predictive of a signal sequence, and colloidal gold immunoelectron microscopy indicated that the corresponding native protein is expressed on the surface of the M. tuberculosis organism. The complete M. tuberculosis genome sequence revealed that it contained four homologues of mce (mce1, mce2, mce3, mce4) and that they were all located within operons composed of genes arranged similarly at different locations in the chromosome. Recombinant Mce2, which had the highest level of identity (67%) to Mce1, was unable to promote the association of microspheres with HeLa cells. Although the exact function of Mce1 is still unknown, it appears to serve as an effector molecule expressed on the surface of M. tuberculosis that is capable of eliciting plasma membrane perturbations in non‐phagocytic mammalian cells.

Mycobacterium tuberculosis Mce3E Suppresses Host Innate Immune Responses by Targeting ERK1/2 Signaling

“…Mammalian cell entry (Mce) proteins were initially characterized as invasion-like proteins with putative export signal sequences at the N-terminal end (16,17). Previous studies suggest that latex beads coated with Mce1A, Mce3A, and Mce3E are internalized by nonphagocytic HeLa cells (18,19). Additional studies have demonstrated that the M. tuberculosis strains with mce operon disruption were attenuated in mice (20,21).…”

Mycobacterium tuberculosis Mce1 protein complex initiates rapid induction of transcription of genes involved in substrate trafficking

“…Originally thought to be simply involved in gaining entry to host cells, 3 more recent work 9 suggests that the Mce1 protein complex is also involved in the recycling of extracellular mycolic acids for use as a carbon source. Of particular interest, we show that the genes that were significantly upregulated by the macrophages in the period immediately after infection by M. tuberculosis H37Rv, but not D-mce1 H37Rv, were primarily involved in substrate trafficking.…”

“…13 Besides being used as an energy source, cholesterol is essential for receptormediated internalization through clathrin-coated pits. 14 Previous studies have suggested that the active domain of the Mce1 protein complex for gaining entry into the host cell is a 58 amino acid long domain located between position 105 and 163 of the Mce1; the most hydrophilic part of the protein, 3,15 and that this peptide induces a membrane invagintion with structures resembling clathrin-coated pits. 3 For this study, the upregulation of the gene Abca1 by 15 min was only observed post H37Rv infection (18.5-fold) and not post D-mce1 H37Rv (1.8-fold), suggesting that the Mce1 protein complex is involved in the manipulation of cholesterol efflux during the initial stages of infection.…”

“…2 All six Mce1 proteins localize to the cell wall fraction of M. tuberculosis, and Mce1A is exposed on the surface of M. tuberculosis. 3 Further analyses of the M. tuberculosis genome showed that M. tuberculosis contains four copies of the mce operon (mce1-4) organized in a similar manner to mce1. The genes within the mce operons 1-4 have been shown to be conserved in all members of the M. tuberculosis complex, the Mycobacterium avium complex and Mycobacterium scrofolaceum, except mce3, which is absent from Mycobacterium bovis, Mycobacterium microti, Mycobacterium pinnipedii and a subgroup of Mycobacterium africanum.…”

“…6 It has also recently been shown that the mce1 operon, in the M. bovis BCG strain, is an important component in the switch from slow to fast growth, suggesting an involvement in the transition between acute and persistent infection. 7 Previous studies have shown that the M. tuberculosis Mce1 protein complex facilitates the invasion of the bacterium into host cells, possibly through cholesterol-dependent clathrin-coated pits, 3 and that infection with an Mce1-deficient M. tuberculosis strain fail to induce a protective pro-inflammatory response, causing a defective granuloma formation with subsequent uncontrolled replication of the bacterium. 6 However, more recent studies show that the mce operons encode for lipid uptake transporters 8 and that the mce1 operon is involved in fatty acid transport/metabolism; and more specifically, mycolic acid recycling.…”