Figure 10. Comparison of DCsAPFs images to bAPF images of Fig. 2. The bAPF images are those proposed to be formed by hexamers (purple circles) or octamers (red circles). Averaged and individual BCsAPF images are illustrated in the two rows below the bAPF images. The blue circles near the middle of the light-shaded rings of the BCSAPFs have diameters predicted for walls of S3 β-barrels (if N is the number of monomers proposed for the APF and S/N = 1.0 for 18-32mers, 2/3 for APFs developed from hexamers, and 3/4 for APFs developed from octamers).

SsAPFs

Many sAPF images have no discernable cogs. These super smooth APFs (SsAPFs) simply have light-shaded rings with darker shading on their exterior and interiors. However, some do appear to have radial symmetry, as indicated by the radially-averaged images. Our working hypothesis is that cogs appear when triple-stranded S1a-S1b-S2 comprise the outer β-barrel because the S1a and S1b strands are much shorter than the S2 strands (Fig. 6), and that cogs do not appear when S1-S2 hairpins comprise the outer barrel because the continuous S1 strand is about the same length as the S2 strand (Fig. 11). There are two mechanisms by which the outer and inner S1-S2 β-barrels can have larger and smaller diameters than those of the central S3 β-barrels. In most cases the numbers of strands in the outer and inner barrels are likely greater and less than those of the S3 barrels. This situation could develop from bAPF precursors if S1-S2 strands inside the bAPF ring become inner barrel strands and those outside the ring become outer barrel strands. As the SsAPF’s increase in size the ratio of outer to inner barrel strands should decrease, and for very-large SsAPF’s it likely becomes 1.0. In these large SsAPFs the S/N values of the outer and inner barrels likely become greater or less than those of the S3 barrels. Sizes of the SsAPF vary greatly, and many larger ones are less circular than those illustrated in Fig. 11.

The thickness of the light rings of SsAPF’s appears to be between 3 and 4 nm regardless of the size or shape of the SsAPF. This thickness is consistent with three or four β-sheet layers that are each about 1 nm apart.