Flagella are supported by a scaffold termed the axoneme, which comprises nine pairs of microtubules surrounding a central apparatus of two singlet microtubules (C1 and C2) joined by projections. The complex waveforms beating flagella produce arise through spatial regulation of the motor protein dynein along the axoneme. In Chlamydomonas, the central apparatus modulates flagellar waveforms, but its composition is largely undefined. Now, on , Elizabeth Smith and co-workers identify calmodulin, PF6 and four novel proteins as components of a complex that localizes to the C1 microtubule. The pf6 mutant of Chlamydomonas, which lacks the C1a projection, has flagella that only twitch. The authors use anti-PF6 antibodies to immunoprecipitate five additional components of the C1a projection, which they then identify by mass spectrometry. They show that all five proteins plus PF6 form a single complex and identify specific binding partners within the complex. Although four of the polypeptides are novel proteins of unknown function, the fifth polypeptide is calmodulin. This result strongly implicates the axonemal central apparatus in modulation of flagellar waveforms by calcium, an important regulatory mechanism.
