Figure 4

A model of fibril-mediated pilin-derived PNT assembly. The ΔK122 pilin (PDB ID 1QVE) [71] exists in a monomer-dimer equilibrium in solution. Upon addition of a hydrophobic initiator such as MPD to the protein solution, the ΔK122 pilin forms fibrils that can then assemble into PNTs through the coalescence of three fibrils in a three-start helical assembly as observed in native T4P [67]. Once formed, PNTs are able to form bundles (Figure 3C) and retain binding characteristics similar to native T4P [47, 50, 58–60]. The common structural features of the type IV pilins are highlighted on the ΔK122 monomer the 4-stranded antiparallel β-sheet (green) that is packed onto a (truncated) N-terminal α-helix (cyan) and connected by variable loop regions (gold); receptor and surface interactions by the pilins are mediated by the C-terminal D-region (magenta). The three ΔK122 fibrils that comprise the full pilin-derived PNT are shown in purple, green and blue, respectively.