Fig. 4.

The quest for anterograde transport: Potential mechanisms. Possible mechanisms to mediate transport of GPLs from the inner membrane to the outer membrane. (A) Transport using a periplasmic chaperone. Similar to Lol, transport could be mediated via a periplasmic chaperone that would bind to GPLs either nonspecifically or in a head group-dependent manner. (B) Transport via a periplasmic-spanning bridge. Similar to the Lpt system, this model would rely on a continuous protein bridge consisting of an inner membrane complex, periplasmic components, and an outer membrane partner. (C) Passive diffusion via Bayer’s junctions. Bayer’s junctions have been proposed to form between the outer leaflet of the inner membrane and the inner leaflet of the outer membrane. These junctions could allow for the passive diffusion of GPLs between both inner and outer membranes continuously.