Fig 3. A minimal model of the Tol-Pal system.

(A) Non-dividing cell: Here TolQRA is homogeneously distributed throughout the inner membrane. As a result, TolB is efficiently captured by the TolQRA machine and moved back into the inner periplasm. Pal is predominately free of TolB and hence binds to the peptidoglycan layer slowing its diffusion. (B) Dividing cell: TolQRA complexes become localised to the cell septum [7,8] such that TolB transport by TolQRA is localised to the centre of the cell. This leads to less overall transport of TolB, resulting in more TolB in the outer periplasm, where it binds Pal, increasing Pal mobility everywhere except at the septum. In this way TolB can mobilise Pal in dividing cells. (C) System of partial differential equations describing the main components of the Tol-Pal system. See the main text for explanation.