Motile flagellar axonemes with a 9 + 1 microtubule configuration

Electron microscope (EM) studies of the eukaryotic flagellum reveal that the organelle contains a 9 + 2 arrangement of microtubules, the axoneme, with nine doublets surrounding two singlets enveloped by a membrane which is continuous with that of the cell; various linkages and projections are associated with the microtubules. Strong experimental evidence supports the idea that the forces required for bend formation on eukaryotic flagella are derived from active relative sliding of the peripheral doublets. Dynein arms, which project from each peripheral microtubule and possess ATPase activity, interact with a neighbouring doublet and undergo conformational changes which induce sliding. To form and propagate coordinated bends along a flagellum the sliding must be resisted in a controlled manner by structures within the axoneme. The regulatory mechanism responsible for the control of inter-doublet sliding is not known in detail, but ultrastructural studies suggest that interactions between the radial spokes attached to each doublet and the central complex of the axoneme may be involved. We report here the treatment of flagella with a 9 + 2 microtubular structure from the trypanosomid flagellate Crithidia oncopelti to produce motile axonemes with only one central microtubule. We conclude that the complete central complex is not involved in the conversion of microtubule sliding into axonemal bending, but may be both associated with the control of wave propagation and essential for bend initiation.