Multiple phosphorylations control recruitment of the KMN network onto kinetochores

The microtubule-binding interface of the kinetochore is of central importance in chromosome segregation. Although kinetochore components that stabilize, translocate on, and affect the polymerization state of microtubules have been identified, none have proven essential for kinetochore-microtubule interactions. Here, we examined the conserved KNL-1/Mis12 complex/Ndc80 complex (KMN) network, which is essential for kinetochore-microtubule interactions in vivo. We identified two distinct microtubule-binding activities within the KMN network: one associated with the Ndc80/Nuf2 subunits of the Ndc80 complex, and a second in KNL-1. Formation of the complete KMN network, which additionally requires the Mis12 complex and the Spc24/Spc25 subunits of the Ndc80 complex, synergistically enhances microtubule-binding activity. Phosphorylation by Aurora B, which corrects improper kinetochore-microtubule connections in vivo, reduces the affinity of the Ndc80 complex for microtubules in vitro. Based on these findings, we propose that the conserved KMN network constitutes the core microtubule-binding site of the kinetochore.

Homology modeling aims to build three-dimensional protein structure models using experimentally determined structures of related family members as templates. SWISS-MODEL workspace is an integrated Web-based modeling expert system. For a given target protein, a library of experimental protein structures is searched to identify suitable templates. On the basis of a sequence alignment between the target protein and the template structure, a three-dimensional model for the target protein is generated. Model quality assessment tools are used to estimate the reliability of the resulting models. Homology modeling is currently the most accurate computational method to generate reliable structural models and is routinely used in many biological applications. Typically, the computational effort for a modeling project is less than 2 h. However, this does not include the time required for visualization and interpretation of the model, which may vary depending on personal experience working with protein structures.

Mitotic cells face the challenging tasks of linking kinetochores to growing and shortening microtubules and actively regulating these dynamic attachments to produce accurate chromosome segregation. We report here that Ndc80/Hec1 functions in regulating kinetochore microtubule plus-end dynamics and attachment stability. Microinjection of an antibody to the N terminus of Hec1 suppresses both microtubule detachment and microtubule plus-end polymerization and depolymerization at kinetochores of PtK1 cells. Centromeres become hyperstretched, kinetochore fibers shorten from spindle poles, kinetochore microtubule attachment errors increase, and chromosomes severely mis-segregate. The N terminus of Hec1 is phosphorylated by Aurora B kinase in vitro, and cells expressing N-terminal nonphosphorylatable mutants of Hec1 exhibit an increase in merotelic attachments, hyperstretching of centromeres, and errors in chromosome segregation. These findings reveal a key role for the Hec1 N terminus in controlling dynamic behavior of kinetochore microtubules.