⚛️ biophysics

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

This study utilizes cryo-EM to reveal that the human Ndc80 and Ska complexes form a phosphorylation-regulated, polyglutamylation-sensitive assembly across neighboring microtubule protofilaments, where a SKA3 tether links the complexes to jointly sandwich tubulin tails and enable robust, dynamic kinetochore-microtubule coupling during mitosis.

Original authors: Zhou, J., Saha, I., Siahaan, V., Zhao, Y., Janke, C., Yildiz, A., Nogales, E.

Published 2026-10-10

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CC BY 4.0

Original authors: Zhou, J., Saha, I., Siahaan, V., Zhao, Y., Janke, C., Yildiz, A., Nogales, E.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Inside every dividing cell, a microscopic tug-of-war determines whether life continues or fails. The goal is to split the genetic material, carried on thread-like structures called chromosomes, into two perfect halves. To do this, the cell builds a scaffold of protein fibers known as microtubules. These fibers act as ropes, pulling the chromosomes apart. But for the pull to work, the chromosomes must be firmly attached to the ropes without getting stuck or slipping off. If the connection is too weak, the chromosomes drift away; if it is too rigid, they cannot move to their correct positions. This delicate balance is managed by a specialized structure on the chromosome called the kinetochore, which acts as a molecular clamp. For decades, scientists have known that two specific protein machines, the Ndc80 complex and the Ska complex, work together to hold this clamp onto the microtubule rope. However, the exact way these two machines fit together, and how they manage to stay attached while the rope itself is constantly growing and shrinking, has remained a mystery.

A team of researchers has now solved this puzzle by creating a detailed three-dimensional map of the human kinetochore clamped onto a microtubule. Using a powerful imaging technique that freezes molecules in place to reveal their atomic structure, they discovered that these two protein complexes do not just sit side by side; they interlock in a precise, cooperative embrace. The Ndc80 complex binds to one strand of the microtubule rope, while the Ska complex binds to the strand immediately next to it. Crucially, they are not independent; they are physically linked by a flexible tether made of a protein segment called SKA3. This tether reaches across the gap to grab a specific bend in the Ndc80 machine, effectively zipping the two complexes together into a single, unified unit.

This structural union creates a unique grip on the microtubule. The researchers found that the two complexes work together to sandwich a tiny, flexible tail of the microtubule protein between them. This tail, which usually flutters freely and is hard to see, becomes locked in place when the two complexes hold it. This locking mechanism is sensitive to chemical tags on the tail, specifically a type of modification called polyglutamylation. The study shows that when these tags are present, the grip becomes much stronger. This explains why the attachment is more robust on certain types of microtubules found in the brain compared to others, suggesting that the cell can tune the strength of the connection by changing the chemical decoration of the rope itself.

Perhaps the most significant discovery is how this arrangement allows the chromosome to track the very tip of the microtubule as it grows and shrinks. The Ska complex has a natural preference for the "fresh" end of the microtubule, where the protein subunits are in a slightly more extended shape. Because the Ska complex is now physically tethered to the Ndc80 complex, it drags the Ndc80 machine along with it. This means the Ndc80, which is the primary force-bearing element, is no longer just a passive anchor; it is actively guided to the growing tip of the rope by its partner. Without this tether, the Ndc80 complex would struggle to stay with the moving end, potentially losing its grip during the rapid changes of cell division.

The researchers also demonstrated that this connection is regulated by the cell's internal clock. Specific sites on the tether are designed to be modified by the cell's chemical signals, which act like switches to tighten or loosen the bond between the two complexes. When these switches are turned on, the connection strengthens, ensuring the chromosome holds tight. When they are turned off, the connection weakens, allowing the machinery to release or adjust. This regulation is vital for correcting errors; if a chromosome is attached incorrectly, the cell can weaken the grip to let it go and try again.

By visualizing these interactions at such a fine level, the study reveals that the kinetochore is not a static clamp but a dynamic, self-correcting machine. The two protein complexes form a coordinated assembly that reads the chemical state of the microtubule and the physical shape of its end. This allows the chromosome to maintain a firm hold on the rope while still being able to slide along it as the rope changes length. The findings provide a clear structural explanation for how cells avoid the chaos of uneven chromosome distribution, a failure that can lead to developmental disorders and cancer. The work confirms that the secret to faithful cell division lies in the precise, regulated handshake between these two molecular partners, ensuring that life's genetic blueprint is copied and divided with perfect accuracy.

Technical Summary: Structural Basis of Human Kinetochore-Microtubule Coupling by the Ndc80 and Ska Complexes

Problem Statement
Faithful chromosome segregation relies on kinetochore-microtubule (KT-MT) attachments that must be mechanically robust, dynamic, and tightly regulated. In human cells, the Ndc80 complex (Ndc80C) serves as the primary force-coupling element, while the Ska complex (SkaC) reinforces mature attachments and supports end-tracking. However, the molecular architecture governing how these two complexes are organized on the microtubule (MT) surface, how they cooperate to sense MT lattice states, and how they remain coupled to dynamic MT plus ends has remained unresolved. Previous studies proposed various binding modes and interaction interfaces, but a unified structural model explaining the regulation of this assembly by post-translational modifications (PTMs) and phosphorylation was lacking.

Methodology
The authors employed an integrative approach combining high-resolution cryo-electron microscopy (cryo-EM), biochemical assays, and single-molecule fluorescence microscopy (TIRF).

Cryo-EM: Recombinant full-length human Ndc80C and a phosphomimetic SkaC-6D (mimicking CDK1 phosphorylation) were co-incubated with GMPCPP-stabilized microtubules. Data were processed using RELION and FREALIGN, involving symmetry expansion, focused 3D classification, and local refinement to resolve distinct binding registries. AlphaFold3 (AF3) was utilized to model flexible regions and validate interfaces identified by cross-linking mass spectrometry (XL-MS).
Biochemical Assays: MT pelleting assays were performed using tubulin from wild-type and knockout mouse brains (lacking specific polyglutamyltransferases TTLL1 and TTLL7) and HeLa cells. Subtilisin treatment was used to remove tubulin C-terminal tails (CTTs).
Single-Molecule Imaging: TIRF microscopy was used to assess binding affinities, dissociation kinetics, and lattice-state preferences (comparing GMPCPP-capped GDP MTs with GMPCPP lattices mimicking GTP-like ends). Dynamic MT assays measured polymerization/depolymerization rates, catastrophe frequencies, and tip-tracking capabilities.

Key Contributions and Results

Structural Architecture of the Ndc80C-SkaC Assembly:
The cryo-EM structure reveals that Ndc80C and SkaC form a coupled assembly across neighboring protofilaments (PFs). Ndc80C binds along a single PF with tubulin-monomer periodicity, while SkaC binds at the interdimer interface between adjacent PFs. The two complexes do not bind the same tubulin dimer but are positioned on adjacent PFs.
- The "Sandwich" Interface: A critical finding is the "sandwiching" of the C-terminal tail of $\alpha$ -tubulin ( $\alpha$ -CTT, residues 438–445) between the NDC80 calponin-homology domain (CHD) and the SKA1 MT-binding domain (MTBD). This interaction is stabilized by salt bridges involving conserved residues (e.g., NDC80 K115, SKA1 K183/K184) and the $\alpha$ -CTT.
- SKA3 Tether: The study identifies a previously uncharacterized kink in the Ndc80C coiled-coil (NDC80 $^{288-292}$ /NUF2 $^{170-174}$ ). A segment of SKA3 (residues 355–380), containing a short $\alpha$ -helix ("tether") and phosphorylation sites (T358, T360), binds to this kink. This tether physically links SkaC to Ndc80C, enabling cooperative binding.
Regulation by Tubulin PTMs and Phosphorylation:
- Polyglutamylation: The $\alpha$ -CTT is a preferred site for polyglutamylation. Binding assays demonstrated that Ndc80C and SkaC exhibit stronger binding to highly polyglutamylated porcine brain MTs compared to low-PTM HeLa MTs. Crucially, binding was significantly reduced on MTs from TTLL1 knockout mice (lacking $\alpha$ -tubulin polyglutamylation) but not on TTLL7 knockouts (lacking $\beta$ -tubulin polyglutamylation), indicating that $\alpha$ -tubulin polyglutamylation specifically promotes the cooperative recruitment of both complexes.
- Phosphorylation: The SKA3 tether contains phosphorylation sites (T358, T360) that enhance the interaction with the Ndc80C kink. Mutational analysis (SkaC-6D vs. deletion mutants) confirmed that this tether is essential for the cooperative stabilization of the complex but dispensable for basal SkaC-MT binding.
Lattice-State Selectivity and Tip Tracking:
- Lattice Bias: SkaC preferentially recognizes the extended, GTP-like lattice (mimicked by GMPCPP) over the compacted, GDP lattice. Ndc80C alone shows less lattice preference. However, when coupled via the SKA3 tether, SkaC biases Ndc80C toward the extended lattice.
- Tip Tracking: In depolymerization assays, SkaC tracks MT plus ends, while Ndc80C alone does not. The presence of the SKA3 tether allows SkaC to confer tip-tracking activity onto Ndc80C. Deletion of the SKA3 tether abolishes this transfer of tracking ability, confirming the tether as the structural conduit for lattice-state recognition and end-tracking.
Cooperative Clustering:
The study elucidates how Ndc80C forms cooperative clusters along a PF. Inter-complex contacts are mediated by the NDC80 N-terminal tail (NTT) and hydrophobic patches, stabilized by the MT lattice. This intrinsic cooperativity positions SkaC on adjacent PFs, creating a robust, multivalent attachment.

Significance
The paper defines the structural basis for the cooperation between Ndc80C and SkaC in human cells. It proposes a mechanism where the SKA3 tether acts as a phosphorylation-sensitive switch that couples Ndc80C to the structural state of the MT lattice. By sandwiching the $\alpha$ -CTT, the assembly integrates signals from tubulin PTMs (specifically polyglutamylation) and mitotic kinases to regulate attachment strength and dynamics. This architecture allows the kinetochore to maintain robust, load-bearing attachments while remaining adaptable to the remodeling of dynamic MT plus ends during chromosome segregation. The findings provide a mechanistic explanation for how errors in tubulin modification or phosphorylation could lead to chromosome mis-segregation and aneuploidy, linking the "tubulin code" directly to outer kinetochore function.

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