The native structure of the Trichonympha centriole cartwheel reveals a zigzag stacking pattern

Centrioles are essential cellular organelles with complex architectures. Their earliest assembly intermediate-the cartwheel-has a ninefold symmetry established by Spindle assembly abnormal protein 6 (SAS-6). Using cryoelectron tomography and subtomogram averaging, we resolved the native structure of the SAS-6 rings within the cartwheel of the exceptionally long Trichonympha proximal centriole, identifying a 16-nm axial periodicity was formed by stacked rings of V-shaped SAS-6 tetramers, which comprise the fundamental unit of the cartwheel central hub. Within these stacked rings, SAS-6 N-terminal head domains adopted a zigzag stacking pattern. Additionally, our improved resolution provides insights into the function of the previously observed cartwheel inner density, nine asymmetric densities that bridge adjacent tetramers, potentially imparting polarity and enhancing structural stability. Molecular dynamics simulations indicate increased rigidity in intertetramer interactions, supporting efficient ring formation and revealing how cartwheel architecture and polarity are established and stabilized.