The eukaryotic motile cilia and flagella are made up of a complex arrangement of microtubules and associated proteins. The core of the flagellum has a characteristic ‘9 microtubule doublets + 2 central pair’ arrangement. However, the exact structure of these multiprotein complexes has been the subject of intense debate in the field, and better structural information is needed. To this end, Takashi Ishikawa and colleagues (Poghosyan et al., 2020) have now obtained radial spoke (RS) complexes of Chlamydomonas flagella by using an improved purification protocol and analysed them with single-particle cryo-EM. This approach demonstrated a two-fold symmetry of the spoke head region, extending towards the neck, and the absence of any symmetry in the stalk region. The authors also identified protein–protein interactions within the RS complex by performing on-bead cross-linking mass spectrometry. This revealed that radial spoke protein 2 (RSP2), located in the neck region, interacts with RSP4 from the head region. To further assign the other parts of the complex, analytical ultracentrifugation and multi-angle light-scattering measurements were employed, which show that RSP10 is present as a dimer, thus further refining the overall RS structure and its assembly. These new details of the RS structure might help to provide a better understanding of flagellar motion and allow its comparison between species.
