Dynein is a microtubule motor that powers motility of cilia and flagella. There is evidence that the relative sliding of the doublet microtubules is due to a conformational change in the motor domain that moves a microtubule bound to the end of an extension known as the stalk. A predominant model for the movement involves a rotation of the head domain, with its stalk, toward the microtubule plus end. However, stalks bound to microtubules have been difficult to observe. Here, we present the clearest views so far of stalks in action, by observing sea urchin, outer arm dynein molecules bound to microtubules, with a new method, “cryo-positive stain” electron microscopy. The dynein molecules in the complex were shown to be active in in vitro motility assays. Analysis of the electron micrographs shows that the stalk angles relative to microtubules do not change significantly between the ADP·vanadate and no-nucleotide states, but the heads, together with their stalks, shift with respect to their A-tubule attachments. Our results disagree with models in which the stalk acts as a lever arm to amplify structural changes. The observed movement of the head and stalk relative to the tail indicates a new plausible mechanism, in which dynein uses its stalk as a grappling hook, catching a tubulin subunit 8 nm ahead and pulling on it by retracting a part of the tail (linker).
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