Fig. 3. High-resolution EM maps of plus-end-directed motors bound to microtubules. (A) Surface representation showing kinesin-1 bound to a MT (Sindelar and Downing, 2007). The secondary structures (superimposed in cartoon form) from kinesin-ADP crystals (PDB code 1BG2) (Kull et al., 1996) dock rather precisely into the EM map. (B,C) Sections through EM maps (density represented as a grey net) of the Kif1a motor domain attached to tubulin in two nucleotide states (ADP-bound and AMPPNP-bound) with corresponding crystal structures superimposed (from Kikkawa and Hirokawa, 2006). The view from the MT plus end (B) compares two positions of helix 4 (yellow for the ADP-bound state, red for the AMPPNP-bound state) relative to the motor core. In the side view (C), changes in helix 6 (green) are indicated, as well as changes indicating the reduced mobility of the neck linker when AMPPNP is bound.