Figure 1: The structure of nucleotide-free kinesin bound to tubulin.

(a) Overview of the complex crystallized. (b) Two Fobs-Fcalc map (contoured at the 1σ level) of the nucleotide-free complex centred at the nucleotide-binding site of (apo-) kinesin. ADP and Mg²⁺ are from kinesin–ADP (1BG2) after superposition of the kinesin P-loop subdomains (see Fig. 2a) in these two structures. In the apo-kinesin nucleotide site, no water molecule has been identified but an SO₄²⁻ ion that largely overlaps with the ADP β-phosphate and accounts for the observed electron density has been modelled. A stereo image of the electron density is presented in Supplementary Fig. 1. (c) The α4 N-terminal extension is stabilized both by direct and water (red spheres)-mediated interactions with α-tubulin and the motor domain core. The kinesin is in green with the extension of the α4 helix, which forms in all nucleotide states on binding to tubulin^11,14, in darker green (as in a); α-tubulin is in cyan. All figures of structural models were generated with Pymol⁵¹.