Abstract
The non-hydrolysable ATP analogue, β,γ-imido-adenosine-5′-triphosphate (AMPPNP, adenylylimidodiphosphate) does not reduce the stiffness of demembranated sea urchin sperm flagella measured in the rigor state in the absence of MgATP2− and does not cause relaxation of rigor bends. The competitive inhibition of flagellar beat frequency by AMPPNP is not enhanced by vanadate. These observations provide further evidence that the dynein-tubulin crossbridge cycle in flagella is similar to the actin-myosin crossbridge cycle.