Mechanisms of flagellar propulsion
ABSTRACT Flagellar bending is caused by the action of molecular motors - dyneins - distributed internally along the length of the organelle. The basic internal structure of eukaryotic flagella is the axoneme, which consists of a set of nine doublet microtubules arranged cylindrically around a pair of single microtubules. A computer model of axonemal structure has been generated, based on electron micrographs of flagella prepared in different ways, and is being used to evaluate the relationships between features of the axoneme. Flagella can adopt a wide range of bend patterns, both twoand three-dimensional, with different degrees of asymmetry. Application of objective methods shows that planar bends on both smooth and hispid flagella in vivo consist of circular arcs separated by straight regions, suggesting that the bend shape is an intrinsic characteristic of the axoneme. The interaction of a flagellum with its liquid environment produces propulsive forces which are dominated by viscosity. Calculations of the propulsive thrust yield velocities in reasonable agreement with observation. The energy dissipated against external viscous forces can be calculated and used in assessments of motor action. During bend formation and propagation, the motor molecules, which are arranged along each doublet in two rows of composite structures known as inner and outer dynein arms, undergo cyclic activity to cause microtubule sliding. The outer dynein arms appear to control beat frequency while the inner arms influence bend symmetry. Complete arms and individual motor molecules extracted from flagella can be distributed on a glass surface and activated to transport isolated microtubules. Dynamic computer modelling is used to study the activity and mutual interaction of arms in assemblies for comparison with these in vitro preparations. Theoretical predictions based on separate models incorporating either random or co-ordinated arm action are in accord with experimental results, and more experimental data are needed to differentiate between them. Development of the computer models will allow the incorporation of mechanical parameters, such as the forces generated by the motors and the elastic properties of the microtubules, with the ultimate goal of constructing a functional model of the flagellar axoneme.