Ring formation requires both the α0 helix and the CTE of Shs1. (A) Purified complexes containing either WT Shs1, Shs1(Δα0), or Shs1(ΔCTE) in high salt were examined in the EM. The histogram shows the size distribution (number of subunits) determined as in Fig. 1 C. Shs1(Δα0)-containing complex (2,241 particles total): 49% octamers, 21% heptamers, and 30% hexamers. Shs1(ΔCTE)-containing complex (4,027 particles total): 72% octamers, 7% heptamers, and 21% hexamers. Samples of each of these three preparations were diluted into low salt conditions for 30 min and then applied to grids and examined in the EM. (B–D) Representative fields are shown for Shs1(Δα0; B), Shs1(ΔCTE; C), and WT Shs1 (D). Representative octameric class averages are shown for Shs1(Δα0; B) and Shs1(ΔCTE; C) in the insets.