Model simplification of TFP biological apparatus and their consequences.

<p>(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 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114613#pone.0114613-Craig1" target="_blank">[18]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114613#pone.0114613-Nivaskumar1" target="_blank">[31]</a> (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).