Figure 4.

Dodecamer models. Parameters of the barrel are listed below the octadecamer schematics. The color code is the same as in the previous figures. The structures of the core S2 and S3 strands of the hexamer are maintained; the core S1a-S1b hairpins are displaced outwardly to the third barrel. S2 and S3 strands of six additional subunits are added to the middle barrel between the core S2 strands. (a) Wedge representations of the central cross-section; the six additional peripheral subunits are stippled. (b-d) Flattened schematics of four subunits of (b) the inner 6-stranded S3 β-barrel (same as hexamer), (c) the middle 18-stranded S2 and S3 β-barrel, and (d) the outer 24-stranded S1a-S1b β-barrel. Strands of the six additional subunits have shaded backgrounds. (e-h) Atomic-scale models of the dodecamer. (e & f) Rainbow colored ribbon illustration of the backbone as viewed from the top and side. (g & h) Cross-sections with side chains colored by element viewed along the barrels’ axis and from the side. Note that most buried side chains are tightly packed and hydrophobic (gray carbons, white hydrogens, and yellow sulfurs), and that most surface side chains have polar red oxygen or blue nitrogen atoms with white hydrogens.