Figure 1.
A schematic illustration of the protein ensemble near the TFP base inside the cell wall which are responsible for the retraction process.
The retraction/elongation process involves a large number of minor and primary proteins such as PilT, PilB molecular motors, PilC platform protein as well as the pore PilQ, all spread across the periplasm of the cell. Pilins are stowed in the inner membrane after de-polymerization(retraction)and are subsequently recruited during polymerization (elongation). Dashed arrows indicate direction of motion. O.M.: Outer Membrane and I.M.: Inner Membrane [9], [18], [28].
Figure 2.
Model simplification of TFP biological apparatus and their consequences.
(a) A simplified reduction of the TFP processing bio-system into an axi-symmetric structure with a sliced view of TFP-protein/periplasm interfaces. The cylindrical retraction apparatus(RA) sits below the TFP base on the cytoplasmic part of the cell and the shallow cylindrical polar complex at the end of the TFP(shown in dotted red lines) is an electrostatic complex which is essential for recruiting pilins for elongation [18], [31] (b) the top part of the RA (only PilT shown) forming the RA-plane is responsible for the binding regime of the retraction process and is assumed to be very closely packed with PilT units sitting close to the base of the TFP. Note that the empty space surrounding the TFP and above the RA plane in this figure is actually filled by PilQ, enclosing periplasm and embedded minor proteins. (c) binding energy at zero deformation as a function of size of the RA-plane indicating three distinct zones and a strongly saturating characteristic assuming a van-derWalls type binding. The x axis is RA radius normalized by the pilus radius and y-axis is current binding energy normalized by that of an infinite plane. (Inserts: White circle indicates the size of RA plane and black the TFP cross section). (d)Normalized force-radius characteristic of TFP. The numbers on the loading curve (green) represent (Insert: Free body diagram of loaded TFP,
is binding force due to RA).
Figure 3.
Free body diagram of the section of the homogenized cylindrical type 4 pilus (TFP) with individual strands shown as colored filaments.
A section of an individual filament is depicted in the lower left corner. (Insert: Cylindrical coordinates).
Figure 4.
(a)Force-Bead velocity (in )comparison of the model with experiments [13](Top Insert: Experimental setup)(b)traction-separation diagram showing progressive debonding
and its effect on force-radius and force-velocity relationship which is now capable of reproducing the higher velocity mode of retraction
.
is the lower cohesive strength(Inserts: Effect of progressive debonding on force-radius and force-velocity characteristics).
Figure 5.
Stall-map indicating variation of normalized stall force with mechanical cohesive energy and normalized chemical binding activation free energy.
Dashed lines show phase boundaries. The black circle shows the location corresponding to the experiments [12], [13]. BF: Binding Failure, MC: Mechano-Chemical, BD: Binding Dominated and ID: Interface Dominated.