Model for T4P assembly. Model for T4P assembly. (A) Side view of the inner membrane assembly platform. In the pilus assembly cycle shown, a single pilin subunit docks into a gap in the growing pilus filament, attracted by complementarity between its negatively charged Glu5 (−) residue and the positively charged main chain amine on the N-terminal residue (+) of the terminal pilin subunit in the growing filament. ATP is hydrolyzed by the assembly ATPase, inducing a conformational change in the inner membrane core protein (IMCP) that extrudes the filament outward a short distance, opening up a new gap ∼120° around the base of the filament for a new pilin subunit to dock. Thus, subunits are added iteratively at 3 sites around the base of the growing filament, corresponding to each of the three helical strands in the T4P, shown in red, blue, and yellow. Each subunit is staggered by an axial distance of 8 to 10 Å along the length of the filament. Only one of three predicted inner membrane core proteins is shown. IM, inner membrane. (B) Top view of the assembly apparatus, looking down on the growing filament. The red subunit is added first, followed by the blue, then the yellow, etc. (Modified from reference 11 with permission from Elsevier.)‏ Stephen Melville, and Lisa Craig Microbiol. Mol. Biol. Rev. 2013; doi:10.1128/MMBR.00063-12