Figure 4.

Protein systems implicated in PL transport in Gram-negative bacteria. Mechanism(s) for bulk anterograde PL transport are elusive, but the IM protein PbgA/YejM has been suggested to be involved in the transport of CLs to the OM. Both the Tol–Pal complex and the OmpC–Mla system are important for retrograde transport of PLs from the inner and outer leaflet of the OM, respectively. The directionality of the latter pathway has been a subject of controversy. Pqi and Yeb protein systems are putative PL transporters, but their biological relevant substrate(s) and functions are unknown. For each of these systems, additional experimental evidence is necessary to ascertain direct involvement in and/or directionality of PL transport.