Yang et al., coordinated movements require the radial spoke, which keeps the center of the flagellum in touch with its outer parts.
Most flagella have a pair of central microtubules surrounded by nine outer microtubule doublets. The central pair is transiently connected to each outer pair by a protein complex called the radial spoke. But not all flagella have spokes and a central pair, leaving scientists to wonder just what these structures do.
The creation of mutant flagella to address this question has been less than successful. Most of the flagellum assembles in units, so loss of one protein demolishes a chunk of the structure and leaves it paralyzed. Yang et al. averted this problem by getting rid of a protein that is added on its own, late in spoke assembly. The latecomer, a chaperone assistant called HSP40, hooks onto the spoke. Using RNAi, the group fully depleted HSP40 from Chlamydomonas and created mutants with only subtle structural defects in the spoke, near the central pair.
The loss of HSP40 resulted in herky-jerky flagella that paused sporadically midstroke and occasionally switched directions prematurely. The authors conclude that the spoke helps time the beating movements.
For Chlamydomonas, this timing coordination probably entails the sequential activation—from flagellum base to tip—of dynein motors along outer doublets. Dyneins slide one doublet past another to drive bending. The team imagines that mechanical or molecular signals travel along the spoke from the outer doublet to the central pair, then back out to the outer pair, and so forth. Near the head of the spoke, HSP40 is well-positioned to stabilize other spoke proteins into a rigid structure that might transduce these signals.
Flagella that naturally lack the central pair have a different beating pattern and evidently use an alternative coordination system. The nodal flagella that establish left–right asymmetry, for example, move in a swirling pattern rather than the more powerful breaststroke-like movements of Chlamydomonas flagella. Spoke-driven coordination might thus be the basis for this extra power.