Structural Basis of Human Kinetochore-Microtubule Coupling by the Ndc80 and Ska Complexes.

Faithful chromosome segregation depends on kinetochore-microtubule attachments that must be load-bearing, dynamic, and tightly regulated. In human cells, the Ska and Ndc80 complexes (SkaC and Ndc80C) cooperate to sustain these attachments, but how they are organized on microtubules and coupled to end dynamics remains unclear. Here, our cryo-EM structure shows that Ndc80C and SkaC form a coupled assembly across neighboring protofilaments. A phosphorylation-sensitive SKA3 tether binds a defined kink in the Ndc80 coiled-coil, positioning SKA1 beside Ndc80C. These two complexes jointly sandwich the α-tubulin tail and sense its polyglutamylation state. SkaC recognizes the extended microtubule lattice and, through the SKA3 tether, confers this selectivity onto Ndc80C, enabling Ndc80C tip tracking while strengthening microtubule stabilization. Together, our findings define the structural basis of Ndc80C-SkaC cooperation and uncover a phosphorylation-regulated, polyglutamylation-sensitive, and lattice-state-dependent mechanism for robust and regulated KT-MT coupling during chromosome segregation in mitosis.