Figure 4.

Superposition of cross sections from the different fibril morphologies suggests the presence of a common protofilament structure. (a) The protofilaments fit well when compared individually between the two- and four-protofilament fibrils. (b) The pair of protofilaments fits well in size with pairs of protofilaments in the six-protofilament fibril. (c) The ribbon could hold six-protofilaments although their boundaries are not clear. (d and e) Two possible, simple models for the cross section of the compact, four-protofilament fibril. (f) Average of the repeats extracted from cryo-EM images of the compact fibrils and aligned as single particles. (g) Reprojection of the 3D map of the compact fibril. (h and i) Reprojections of the 3D models whose cross sections are shown in d and e, respectively.