Pigino et al. describe the three-dimensional structure of radial spokes, key regulatory complexes that determine how cilia and flagella move.
Radial spokes connect the central pair of microtubules in cilia and flagella axonemes to the nine outer microtubule doublets and are thought to regulate how dynein motors slide these microtubules to generate movement. Each spoke contains at least 23 proteins, with two or three spokes (depending on the species) clustering together at regular intervals along the axoneme. How the spokes function is unclear, however, so Pigino et al. used cryoelectron tomography to observe their structure in various cilia and flagella.
Pigino et al.’s reconstructions of flagella from the alga Chlamydomonas revealed that spokes project from outer microtubule doublets and split at their necks to form two closely apposed heads near the center of the axoneme. The symmetry of these structures supported the idea—previously suggested by biochemical experiments—that radial spokes assemble through the dimerization of smaller, precursor structures. In addition, by comparing the structures of radial spokes in Chlamydomonas mutants lacking particular spoke proteins, the researchers were able to assign many of these components to specific locations within the complex.
One surprise was that Chlamydomonas, whose radial spokes are thought to exist in pairs, actually has a short protrusion next to each pair, exactly where the third spoke exists in the cilia of species like the protozoan Tetrahymena. Senior author Takashi Ishikawa says it's unclear whether Chlamydomonas lost, or Tetrahymena gained, a full third spoke during evolution.