Fig. 4. Subunit contacts in the hexamer. (A) The trimer. Regions involved in the contacts are highlighted in red, green and blue, respectively, for each subunit. C-terminal helices α3 and α4 from the blue subunit pack against helix α1 and the capping motif of the red subunit. Additional contacts involve the K3 and P-loops in yellow. (B) Dimer contacts. One subunit of the bottom trimer makes contact with all three subunits of the top trimer, mostly through loops at the edge of the central β-sheet, the C-terminus of the capping motif and the C-terminus of helix α3. On the 2-fold axis, density in a (2F_o – F_c) map countoured at 1σ is interpreted as a set of water molecules. It bridges His173 from the bottom and top subunits.