Proteins with proximal-distal asymmetries in axoneme localisation control flagellum beat frequency

Abstract

The 9+2 microtubule-based axoneme within motile flagella is well known for its symmetry. However, examples of asymmetric structures and proteins asymmetrically positioned within the 9+2 axoneme architecture have been identified in multiple different organisms, particularly involving the inner or outer dynein arms, with a range of functions. Here, mapped, genome-wide, conserved proximal-distal asymmetries in the uniflagellate trypanosomatid eukaryotic parasites. Building on the genome-wide localisation screen in Trypanosoma brucei we identified conserved proteins with an analogous asymmetric localisation in the related parasite Leishmania mexicana. Using deletion mutants, we map which are necessary for normal cell swimming, flagellum beat parameters and axoneme ultrastructure, and using combinatorial endogenous fluorescent tagging and deletion, map co-dependencies for assembly into their normal asymmetric localisation. This revealed 15 proteins, 8 known and 7 novels, with a conserved proximal or distal axoneme-specific localisation. Most were outer dynein arm associated, and showed that there are at least two distinct classes of proximal-distal asymmetry - one dependent on the docking complex, and one independent. Many were necessary for normal frequency of the tip-to-base symmetric flagellar waveform, and our comprehensive mapping reveals unexpected contribution of proximal-specific axoneme components to frequency of distal waveform initiation.