Shs1 substitutes for Cdc11 at the terminal positions in septin octamers. (A) Representative class averages of EM images of septin complexes in high salt (300 mM NaCl). (left) Cdc11-containing complex; (middle) Cdc3–Cdc10–Cdc12 hexameric complex; (right) Shs1-containing complex. Dimensions are indicated. (B) Additional class averages of Shs1-containing complexes; note the range of curvatures. (C) Histogram of the size distribution (number of subunits) for Shs1- and Cdc11-containing complexes and the Cdc3–Cdc10–Cdc12 complex. Each of the three preparations (10 µg/ml) was applied to EM grids and examined as in Fig. S1 B. Particles were assigned to classes, each with a homogeneous population of particle size, and the number of particles within each class for each preparation was counted. Cdc11-containing complex (4,211 particles total): 86% octamers, 10% heptamers, and 4% hexamers. Shs1-containing complex (9,901 particles total): 47% octamers, 15% heptamers, and 38% hexamers. Cdc3–Cdc10–Cdc12 complex (2,093 particles total): 100% hexamers.