Figure 9.

Schematic of common modes of protofilament organization. Fibrils can be (A) a single protofilament, (B–D) a twisted ribbon or tubular structure formed by association of several protofilaments, often with rotational symmetry about the fibril axis, or (E) a tape-like side-by-side association of protofilaments. In the top-down schematics, blue squares with red arrows represent top-down views of subunits, with the red arrows showing their relative orientation in the subunit plane. In the side-on schematics for (B), blue pentagons represent side views of protofilament subunits. As shown in this panel, twofold-symmetric fibrils or protofilament groups can have an in-register (C₂) association of laterally apposed subunit stacks, or a staggered (pseudo-2₁) organization in which the two stacks are half a β-sheet spacing out of register. While the former optimizes interactions that rely on alignment of the subunits in the same plane, the latter allows interdigitation of sidechains that protrude into the interface. The density map obtained by Mizuno et al. (2011) was used as a template for the schematic in (C).