Fig. 6. Models of kinesin-1 dimers bound to tubulin. (A) Crystal structure of the rat kinesin-1 dimer (PDB code 3KIN) (Kozielski et al., 1997) with head 1 in the same orientation as the Kar3 head in Fig. 5. Note that the coiled-coil would clash with tubulin unless it moves from its position in the crystal structure. When MT were decorated with ADP-bound dimers (see C), a rotation of the whole motor domain apparently relieved the clash (Hirose et al., 1999). (B) The kinesin dimer crystal structure with the heads reoriented relative to each other to allow them both to dock into the EM map in D, with head 1 as the directly-bound head. The movement of head 2 is likely to have resulted from conformational changes in head 1 but the details are unknown. The coiled-coil (dark red) is shown as having shifted with head 2, though it was not detectable in D and was probably free to move anywhere. (C,D) Parts of the outer surfaces of low-resolution EM maps of kinesin-1 dimer bound in the ADP-bound and empty states to MTs (Hirose et al., 1999). Directly bound heads (1) and tethered heads (2) (both coloured here in cyan) can be identified. The MT is coloured green.