Schematic picture depicting spoke HSP40 and the radial spoke in the 9 + 2 axoneme. (A) Spoke HSP40 contains the signature domains of the type II HSP40. (B) A model highlights the predicted position of HSP40 relative to RS, CP, and the dynein motors. Outer and inner dynein arms (ODA and IDA) power the interdoublet sliding that is subsequently converted to bend and oscillatory beating. It is postulated that the conversion is locally controlled by transient interactions of RS with CP resulting in tilt (θ), lengthening (δ), and strain of the engaged RS (Warner and Satir, 1974). The shape and size of the spoke HSP40 homodimer are largely based on the U-shaped dimeric type II HSP40 SIS1 (3 × 7 × 9 nm; Sha et al., 2000) relative to the ∼40-nm-long RS (Witman et al., 1978). Dimeric HSP40 possibly interacts with RSP2 and/or RSP23 (2 and 23, gray ovals) underneath the bulbous spokehead. The latter two are depicted as homodimers as well, based on the predicted resemblance of their DPY-30 domain to the homodimerized RIIa domain of cAMP-dependent protein kinase. The unfilled circles in the spokehead represent the five head proteins.