Interhead tension determines processivity across diverse N-terminal kinesins.

Consistent with their diverse intracellular roles, the processivity of N-terminal kinesin motors varies considerably between different families. Kinetics experiments on isolated motor domains suggest that differences in processivity result from differences in the underlying biochemistry of the catalytic heads. However, the length of the flexible neck linker domain also varies from 14 to 18 residues between families. Because the neck linker acts as a mechanical element that transmits interhead tension, altering its mechanical properties is expected to affect both front and rear head gating, mechanisms that underlie processive walking. To test the hypothesis that processivity differences result from family-specific differences in neck linker mechanics, we systematically altered the neck linker length in kinesin-1, -2, -3, -5, and -7 motors and measured run length and velocity in a single-molecule fluorescence assay. Shortening the neck linkers of kinesin-3 (Unc104/KIF1A) and kinesin-5 (Eg5/KSP) to 14 residues enhanced processivity to match kinesin-1, which has a 14-residue neck linker. After substituting a single residue in the last alpha helix of the catalytic core, kinesin-7 (CENP-E) exhibited this same behavior. This convergence of processivity was observed even though motor speeds varied over a 25-fold range. These results suggest that differences in unloaded processivity between diverse kinesins is primarily due to differences in the lengths of their neck linker domains rather than specific tuning of rate constants in their ATP hydrolysis cycles.