The geometric clutch as a working hypothesis for future research on cilia and flagella

The Geometric Clutch hypothesis contends that the forces transverse to the flagellar axis (t-forces) act on the axonemal scaffold to regulate flagellar beating. T-forces develop as the product of the curvature and the accumulated tension or compression on the doublet microtubules. In this respect, t-force is a mediator of self-organizing behavior. It arises from the collective action of the assemblage of dynein motors on the structural components of the axoneme and, in turn, imparts order to the sequence of activation and deactivation of the dynein. At the switch point of the flagellar beat, the magnitude of the t-force per micron of flagellum is approximately equal to the sum total of dynein force that can be generated per micron of flagellum. This suggests that the t-force could directly overcome the force-producing dynein bridges and terminate their action. However, many questions remain to be answered concerning the behavior of the axonemal scaffold under stress. Little is known of the force-bearing capacity of the radial spokes and the central pair (cp) projections. The properties of these structures will determine how t-force is distributed within the axoneme. The mechanical and elastic properties of the dynein arms and nexin links need to be better understood to determine how they respond to the application of t-force. In the framework of the Geometric Clutch hypothesis these are the issues that are most important to explore if we are to understand how the flagellum works.