analytical ultracentrifugation; modeling; signal transduction; small-angle X-ray scattering; structural domains

We have used an antibody-Fab tag to mark the position of the cytoplasmic dynein amino-terminal tail domain, as it emerges from the main mass of the motor. Electron microscopy and single-particle image analysis reveal that the tag does not assume a rigidly fixed position, but instead can be found at various locations around the planar ring that comprises the motorʹs backbone. The work suggests that the tail is attached to the motor at a point near the ring center, and that the sequence immediately adjacent to this connection is flexible. Such flexibility argues against a simple-lever arm model for dynein force production.