The Flagella Beat of Chlamydomonas Has Distinctly Regulated Static and Dynamic Components, which Accord with Curvature Controlled Dynein Activity

Abstract

The axoneme, the mechanical core of eukaryotic cilia and flagella, is a huge macromolecular assembly of microtubules, cross-links and dynein motors. The coordinated activity of the dyneins produces a diversity of flagellar waveforms that drive fluid flows across tissue surfaces or propel micro-swimmers. While it is believed that the flagellar beat is a result of feedback - based on the ability of motors to produce and sense forces - the mechanism by which motors are controlled remains unknown.To elucidate how molecular motors are involved in the generation of flagellar waveforms, we studied the beat of axonemes isolated from Chlamydomonas reinhardtii, a single cell green alga. Chlamydomonas flagella can exhibit both of the most common waveforms found in nature: the asymmetrical breaststroke-like beat as well as the symmetrical sperm-like beat.Using high speed microscopy and image analysis we precisely determine axonemal shapes in space and time. A characterization of the waveform properties at different ATP concentrations shows that the static asymmetry of the breaststroke - which has an approximately circular shape - can be separated from the dynamic beat component. This analysis reveals that the breaststroke can essentially be viewed as a sperm-like beat traveling around a circular shape and allows us to examine both waveform components independently.By comparing the dynamic component of experimentally measured waveforms to a mechanical model of the axoneme, we found that the shapes were consistent with a model in which dynein motors respond to changes in axonemal curvature. We furthermore show that the static asymmetric shape underlying the beat could also result from curvature controlled dynein activity.Together our findings present novel insights into how molecular motors shape the asymmetric waveform of Chlamydomonas flagella.