Fig. 3. Structure of polymerized gp17.1 forming the tail tube of the bacteriophage SPP1.

a Final ten lowest-energy structures of a gp17.1 subunit which consist of a central β-sandwich-type fold (turquoise) that is flanked by an α-helix (pink), a large loop and an extended C-terminal arm (C-arm). b Six gp17.1 monomers form a hexameric ring. The inner β-sheets of the β-sandwiches organize in a β-barrel motif that forms the lumen of the tube. c These hexameric rings stack onto each other in a helical fashion creating a hollow tube. Ring-to-ring contacts are mediated by the two loop regions (highlighted in red)—especially by the C-arm that folds onto the subjacent ring. d The molecular lipophilicity potential of gp17.1 reveals a hydrophobic patch on the surface of one subunit i. The color gradient represents the lipophilicity potential. e, f This unpolar area is obscured by the C-arm (pink) of the superjacent subunit j within the complex of the tail-tube—by anchoring the sidechain of Gln162 into a pocket. g The loop of subunit i (turquoise) features mostly electrostatic interactions with five neighboring subunits (gray, purple, green, beige, and orange) within the complex. Charged amino acids are colored in red (negative) and blue (positive). The direction of the tail structure is baseplate upwards.