Figure 1.

Hypothetical model for Mce-mediated lipid transport in M. tuberculosis and organization of the mce1 and mce4 operons. (A) Stage 1 depicts a process where Mce proteins bind and transport the lipid substrates across the exterior portion of the mycobacterial cell wall and pseudoperiplasmic space. Stage 2 illustrates the final translocation of lipid substrates across the cytoplasmic membrane by a putative permease complex. (B) The substrate-specific or ‘core proteins’ of the Mce1 and Mce4 complexes are encoded within the mce1 and mce4 operons. The putative subunits shared by the Mce1 and Mce4 complexes (LucA, MceG and OmamAB) are encoded by genes outside of the mce1 and mce4 operons.