Figure 5.

Structural analysis of the T-antigen interface within the polymerized pilus. (a) Ribbon diagram of the T18.1 dimer show that pilins stack end-on-end. The electron density of the sortase motif (S) connecting the two pilins is only partially interpretable. (b) Top view of the T18.1 dimer showing a 120° rotation between each pilin along the long axis, which is identical to that measured in T1 antigen crystal packing [21]. (c) The interface between the two T18.1 pilin monomers (green) is virtually identical to the T1 crystal packing interface (magenta), as is the distance between the pilin lysine (k) and the C-terminus of the next pilin (dashed line). (d) The small interface supports a model in which T-antigen inter-pilin contacts are relatively weak, potentially functioning like a knee joint whereby the pilin flexes in only one direction as the covalent isopeptide bond prevents bending in the opposite direction, or complete disassociation. The 120° rotation between each successive pilin allows 360° flexibility along the length of the pilus. (T18.1 = green, T1 = magenta, K = pilin lysine, S = sortase motif).