Kinesin-14 lever positions. (a) A crystal structure of a wild-type Ncd dimer shows a symmetrical structure with the heads in equivalent positions (PDB 1CZ7). The amino terminus of the head is highlighted in red and the carboxyl terminus in green The alpha4 relay helix is magenta. The blow-up on the right of the region linking the motor heads to their tails (the lever) shows the carboxyl termini of the two heads in essentially the same conformation. (b) In a dimer formed of mutant Ncd subunits (N600K, PDB 1N6M), one head is rotated relative to the other by about 70°. The new structure of Heuston et al.  of the mutant T436S shows this arrangement. (c) Same data as in (b) above, with one head rotated to overlay the other, showing the coiled coil tail can adopt two stable positions relative to the head.