Fig. 1 |. Structures of radial spokes on and off doublet microtubules.

a, Schematic representation showing biochemical fragmentation of the Chlamydomonas axoneme (viewed in cross-section; center) into mechanoregulator-bound doublet microtubules (left) and isolated radial spokes (right). The axoneme consists of a central pair of microtubules (CP, purple) surrounded by doublet microtubules (DMT; gray) bound by radial spokes (RS; blue), nexin-dynein regulatory complexes (N-DRC; green), inner dynein arm (IDA; yellow), and outer dynein arm (ODA; red). b, Composite density map for on-doublet RS1 and RS2. The maps of the stalks are colored by subunit. The neck and spokehead, which are less well resolved, are colored gray. c, Orthogonal views of a composite density map for isolated RS1 with the map colored by subunit. Dashed lines indicate the positions of the stalks and doublet microtubule which are not present in the reconstruction. d, Orthogonal views of an atomic model for the 96-nm repeat of the doublet microtubule. The model combines atomic models of the doublet-bound stalks of RS1 and RS2, and the stalk, neck, and spokehead of isolated RS1 with model of the doublet microtubule (PDB 6U42)²¹. Individual subunits are colored according to panels b and c. In panels b and d, the minus (−) and plus (+) ends of the doublet microtubule are indicated at the ends of the scale bar.