(A–D) View towards the MT with the plus end towards the top of MT-bound Kin3 reconstructions (shown as blue transparent density) in (A) Mg-ADP, model shown in light blue, (B) no nucleotide (NN), model shown in the mid-blue, (C) Mg-AMPPNP, model shown in navy blue, and (D) Mg-ADPAlFx, model shown in dark blue; (E–G) Same view of MT-bound Kin1 reconstructions (shown is green transparent density in E) no nucleotide (NN), model shown in light green, (F) Mg-AMPPNP, model shown in dark green, (G) Mg-ADPAlFx, model shown in olive green. In Mg-ADP/NN states of Kin3 (A and B) and the NN state of Kin1 (E) helix-α6 terminates before helix-α4 leaving a gap (chevrons). Additional regions of density (arrows) at the helix-α6 C-terminus likely represent conformers of the initial portion of the neck linker (fuchsia), most of which is invisible and presumably flexible. However, in AMPPNP/ADPAlFx states of both Kin3 (C and D) and Kin1 (F and G), tilting of the motor domain allows helix-α6 to extend, closing the gap between helix-α4 and allowing neck linker docking, for which extra density is seen alongside the core β-sheet (arrowheads). Neck linker docking allows CNB formation with the N-terminus (orange). In all reconstructions, density for the motor domain was contoured to an equivalent volume.