Microtubule Engagement by the WD40-Containing Tail of Kinesin-4 KIF21B

Kinesin tails are structurally diverse and mediate a range of functions, including autoinhibition, cargo binding, and microtubule regulation. Within the kinesin-4 family, KIF21A and KIF21B have emerged as key regulators of microtubule network organization and dynamics in neurons and immune cells, yet the molecular basis of this activity has remained unclear. Here, we combined single-particle cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) to examine how the KIF21B tail engages microtubules. We find that conserved residues in the WD40 {beta}-propeller and an adjacent N-terminal linker contact successive tubulin dimers along a single protofilament, forming an extended longitudinal binding mode that spans both the intradimer and interdimer interfaces. Cryo-EM 3D classification further revealed two distinct engagement states, a tilted state, in which the {beta}-propeller makes partial contacts with the microtubule while the linker remains anchored, and a flat state, in which the {beta}-propeller lies flush with the lattice surface. Cryo-ET of full-length KIF21B reveals multiple binding configurations on the microtubule lattice, including orientations consistent with crosslinking adjacent microtubules. Together, these findings provide a structural framework for tail-mediated microtubule attachment by a neuronal kinesin-4 and suggest how its distinctive WD40 domain may contribute to microtubule regulation and organization of microtubule arrays.