Fig. 3: Structure of the pointy tip.

a Structure of the pointy tip in ribbon representation with the C domain of pIII shown in orange, pVI in purple and pVIII in light blue. The tip is shown in two views 90° apart to show a side view and a view looking down the phage from the pointy tip. Proteins pIII and pVI are shown individually as well as in the phage assembled pentameric structure. Density for the pIII N1-N2 domains, plus the final 2 residues at the C-terminus, is not visible in our map and is not shown. The disulphide bond in pIII is shown in green and labelled S-S. The * highlights the β-hairpin loop. Regions 1 and 2 are boxed. Box 1 shows hydrogen bonding within the pIII β-hairpin (blue dashed lines) and the cysteine residues within the correct distance to form a disulphide bond. Box 2 shows a view looking down the phage towards the pointy tip, and a hydrogen bond formed between the sidechain of Arg 402 and the mainchain carbonyl oxygen of Asn 399 from a neighbouring pIII chain. pIII is shown as sticks, pVI as ribbons, hydrogen bonds as blue dashed lines. b Interactions between pIII and pVI. Regions 3 and 4 are boxed and show the hydrogen bonds (blue dashed lines) observed between one pIII molecule and two neighbouring pVI molecules, and the salt bridge between Glu 277 and Arg 12. c Pointy tip shown with pVI bundle only, and pIII/pVI bundle, in side view shown as ribbons, and as surface views of hydrophobicity and electrostatic potential. The most hydrophilic residues are shown in cyan and the most hydrophobic in mustard yellow; negative residues are shown in red and positive residues in blue. * denotes the rings of positive charge in pVI.