Extended Data Fig. 8 |. Model of radial spoke assembly.

Proposed model of radial spoke assembly. Monomeric spokehead lobes, comprising RSP1-7 and RSP9-12, assemble in the cell body^28,34–36 before being imported into the cilium by intraflagellar transport (IFT)^34,75. In the cilium, the axonemal doublet microtubules are bound by the CCDC39/40 coiled coil. Specific sequences within the coiled coil are recognized by molecular adaptors FAP253 and FAP91 that establish the binding sites for RS1 and RS2. These molecular adaptors recruit LC8 and FAP207, although the arrangement of these elements is different in the two stalks. RSP3 in the precursor binds the LC8 multimers (Fig. 4b), helping dock the spokehead lobe onto the preassembled stalks. Two lobes can bind a single stalk. Binding of RSP16 is presumably a relatively late step that dimerizes the lobes²⁸. At a similar time, RS-specific proteins bind; RSP14 to RS1 and RSP8 to RS2.