Fig 5.

Models of pilus and pseudopilus fibers. Using structural subunit information, low-resolution EM data, and biochemical data, fiber models of the T4aP (left), T4bP (center), and pseudopilus (right) were generated. The T4aP model can be viewed as a one-start or four-start right-handed helix or a three-start left-handed helix (one strand is pictured, with two subunits shown as a green cartoon) with a diameter of approximately 60 Å. Tight packing of the N-terminal helices holds the structure together, with additional polar interactions between the C-terminal head groups. The three-start left-handed helix T4bP model has a larger diameter, at approximately 90 Å, due to the larger subunit size along with the more loose packing of the subunits, exposing portions of the N-terminal helix and producing deep grooves and bulges along the fiber (model kindly provided by Lisa Craig). The right-handed one-start helix T2S pseudopilus model is slightly larger than the T4aP model, at 65 Å, with hydrophobic and electrostatic interactions stabilizing subunit interactions (model kindly provided by Olivera Francetic). Figures were prepared using MacPymol (DeLano Scientific).