Docked and undocked positions of the carboxy-terminal neck-linker in kinesin-5. A kinesin-5 in the docked conformation (PDB 1X88) with a structure in the undocked conformation (PDB 1II6) superimposed. With the alpha4 relay helix in the UP position (shown in pink), the neck-linker (green) can dock into a site on the main part of the motor head. With the alpha4 DOWN (shown in magenta) the neck-linker docking site is sterically blocked, and the neck-linker adopts an alternative position. In the magnified portion of the structure shown on the right, the arrow indicates intrusion of the carboxyl terminus of alpha4 (magenta) into the docking site of the carboxy-terminal neck linker (green).