Structure of the RZZ complex and molecular basis of Spindly‐driven corona assembly at human kinetochores

In metazoans, a ≈1 megadalton (MDa) multiprotein complex comprising the dynein–dynactin adaptor Spindly and the ROD–Zwilch–ZW10 (RZZ) complex is the building block of a fibrous biopolymer, the kinetochore fibrous corona. The corona assembles on mitotic kinetochores to promote microtubule capture and spindle assembly checkpoint (SAC) signaling. We report here a high‐resolution cryo‐EM structure that captures the essential features of the RZZ complex, including a farnesyl‐binding site required for Spindly binding. Using a highly predictive in vitro assay, we demonstrate that the SAC kinase MPS1 is necessary and sufficient for corona assembly at supercritical concentrations of the RZZ–Spindly (RZZS) complex, and describe the molecular mechanism of phosphorylation‐dependent filament nucleation. We identify several structural requirements for RZZS polymerization in rings and sheets. Finally, we identify determinants of kinetochore localization and corona assembly of Spindly. Our results describe a framework for the long‐sought‐for molecular basis of corona assembly on metazoan kinetochores.