AUC analysis and visualization of ceSAS-6 oligomers. (A) AUC velocity analysis of ceN-CC[S123E/I154E] (blue) at 50 μM concentration shows a narrow sedimentation distribution corresponding to the molecular mass of a dimer. ceN-CC[S123E], which can oligomerize through its coiled coil and the low-affinity N-N interface, shows a distribution of small oligomers at 50 μM concentration (red). In contrast, ceN-CC[S123E/I154W], which exhibits high N-N dimerization affinity, forms larger oligomers at 25 μM concentration (green). (B and C) Electron micrographs of negatively stained ceN-CC[S123E/I154W] (B) after or (C) before sample centrifugation. The stabilized ceN-CC forms spirals at higher concentrations, which in turn, give rise to micrometer-long filaments that show evidence of spiral intertwining. The white arrowhead in C indicates the point where the two strands separate. (D) Close-up view of a representative ceN-CC[S123E/I154W] micrograph from B compared with our crystallographic model of ceN-CC spirals. (Scale bar: 10 nm.) (E and F) Histograms of spiral diameter and pitch, respectively. The values are consistent with the dimensions of our crystallographic model of ceN-CC spirals (black solid lines; 17-nm diameter, 33-nm pitch) but fit better to the dimensions of a 4.5-fold symmetric spiral (red lines; 15-nm diameter, 31-nm pitch). Experimental mean values and SDs are shown by dotted lines. (G) Histogram of filament diameter from C and similar migrographs (Fig. S7B).