Model of SAS-6 ring assembly in zebrafish. (A) Ribbon presentation of a modeled SAS-6 tetramer based on the observed coiled-coil and head-to-head dimers. The distance between the base regions of the two coiled-coil domains is indicated. (B) Cryo-electron microscopic image of a face-on view of a thin crystal of N-SAS-61-217. The pixel size is 3.74 Å/pixel. Scale bar = 60 Å. The image was Fourier-filtered and symmetry-averaged. Overlay shows the SAS-6 tetramer presented in (A). The overlaid structure is based on N-SAS-61-179, which has a shorter coiled-coil domain than N-SAS-61-217. (C) Models of SAS-6 rings with different symmetries. The approximate diameters of these rings (the double-distance from head domain center to ring center) are indicated above the modeled rings. The diameter of cartwheel hubs observed in procentrioles by cryo-electron tomography is shown as a dotted circle. To model these rings, a change in the orientation of the head domains relative to the coiled-coil domain was allowed for. To compare the required changes, the root-mean-square deviation between the N-SAS-61-179 structure and two equivalent head domains in the modeled ring was calculated. These values are indicated under the modeled rings. Reprinted with permission .