Fig. 3. Structural changes at the active site and the filament-binding region in kinesin and myosin. (A,B) Comparison of human kinesin (PDB 1BG2) (Kull et al., 1996) and rat kinesin (PDB 2KIN, monomer structure) (Sack et al., 1997) showing the nucleotide-binding site with bound ADP and movements of helices 
4 and 5 (human, orange; rat, red). Helix 4 can be seen in (A) and is the upper red/orange helix pair in (B). The lower helix pair is 5. The superposed structures show little difference at the active site in the P-loop (green), switch I (purple), or switch II (cyan), but helices 4 and 5 are displaced relative to one another by a simple translation of 4-5Å. The kinesin neck linker, observed in the rat structure, is shown in magenta. (C,D) Superposed KIF1A-ADP (PDB 1I5S) and KIF1A-AMP·PCP (PDB 1I6I) (Kikkawa et al., 2001). The movement of helices 4 and 5 (KIF1A-ADP, orange; KIF1A-AMP·PCP, red) is more complex than in (B), and consists of a translation coupled with a rotation. There is also a substantial structural rearrangement of the switch I region (C) (KIF1A-ADP, pink-purple; KIF1A-AMP·PCP, yellow), which changes the short loop-helix-loop-helix (KIF1A-ADP) to a short pseudo-ß-hairpin (KIF1A-AMP·PCP). (E,F) Myosin-ADP·BeF3 CLOSED (F.J.K. & K.C. Holmes, unpublished) compared with myosin-ADP OPEN (PDB 1G8X) (Kliche et al., 2001). The relay helix (analogous to kinesin helix 4) in the CLOSED form (red) is translated along its axis 
4.5Å toward the nucleotide-binding site with little rotational movement at its N-terminus. However, the pronounced bend in the middle of the relay helix in the CLOSED structure causes a 100° rotation of its C-terminal end, causing the position of the helix end in the OPEN (orange) and CLOSED (red) forms to differ by 15 Å. This movement is further amplified by the myosin converter domain, and is thought to drive the myosin power stroke. All of the comparisons and distances described in this paper are based on a least-squares alignment of 19 -carbon atoms in the structurally conserved P-loop, the preceding ß-strand, and the N-terminal end of the following -helix (residues 77-95 of human kinesin, 78-96 of rat kinesin, 89-107 of KIF1A, 466-484 of Kar3 and 171-189 of Dictyostelium myosin II).
