The roles of a flagellar HSP40 ensuring rhythmic beating.

HSP40s are regarded as co-chaperones perpetually shuttling client polypeptides to HSP70s for refolding. However, many HSP40s central for disparate processes diverge from this paradigm. To elucidate the noncanonical mechanisms, we investigated the HSP40 in the radial spoke (RS) complex in flagella. Disruption of the gene by the MRC1 transposon in Chlamydomonas resulted in jerky flagella. Traditional electron microscopy, cryo-electron tomography and sub-tomogram analysis revealed RSs of various altered morphologies which, unexpectedly, differed in the two RS species. This indicates that HSP40 locks the RS into a functionally rigid conformation, facilitating its interactions with the adjacent central pair apparatus for transducing locally varied mechanical feedback which permits rhythmic beating. Missing HSP40, like missing RSs, could be restored in a tip-to-base direction when HSP40 mutants fused with a HSP40 donor cell. However, without concomitant de novo RS assembly, the repair was exceedingly slow, suggesting HSP40/RS-coupled intraflagellar trafficking and assembly. Biochemical analysis and modeling uncovered spoke HSP40's co-chaperone traits. Taken together, we propose that HSP40 accompanies its client RS precursor when traveling to the flagellar tip. Upon arrival, both refold concertedly to assemble into the mature configuration. HSP40's roles in chaperoning and structural maintenance shed new light on its versatility and flagellar biology.