3.A.1.27.6
Mce1A-F hetero-hexameric uptake porter for mycolic acid and fatty acids. The structure of Mce1A is known (Asthana et al. 2021); MceE = LprK. Each of the four Mce complexes in Mtb (Mce1-4) comprises six substrate-binding proteins (SBPs; MceA-F), each of which contains four conserved domains (N-terminal transmembrane, MCE, helical and C-terminal unstructured tail domains). In the crystal structure of the MCE domain of Mce4A (MtMce4A39-140) a domain-swapped conformation is observed. The fact that there are six SBPs in each Mtb mce operon suggests that the MceA-F SBPs from Mce1-4 may form heterohexamers. The helical domains interact with the detergent micelle, suggesting that when assembled, the helical domains of MceA-F may form a hydrophobic pore for lipid transport, as observed in PqiB of E. coli (Asthana et al. 2021).