B A

Prestroke

Prestroke

Poststroke

▲ EXPERIMENTAL FIGURE 20-28 In vitro dynein-mediated sliding of doublet microtubules requires ATP.

(a) Electron micrograph of two doublet microtubules in a protease-treated axoneme incubated with ATP In the absence of cross-linking proteins, which are removed by the protease, doublet microtubules slide excessively. The dynein arms can be seen projecting from A tubules and interacting with B tubule of the top microtubule. (b) Single-headed dynein molecules in their prestroke and poststroke states. Thousands of images of purified inner-arm dynein were recorded in an electron microscope and then averaged. A comparison of dynein containing ADP and vanadate, a state mimicking the ADP-Pi state, with dynein absent of any bound nucleotide, suggests that the difference in structure may be related to the conformational changes taking place in the ATP cycle. A model of the force-generation mechanism suggests that the head changes orientation relative to the stem, causing a movement of the microtubule-binding stalk. [Part (a) courtesy of P Satir. Part (b) from S. A. Burgess et al., 2003, Nature 421:715; courtesy of S. A. Burgess.]

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