Proteomics reveals extensive phosphoregulation of outer kinetochore protein KNL1

Proteomic analysis of KNL1 in HEK 293T/17 cells treated with microtubule-disrupting agents reveals 111 phosphorylation sites, demonstrating that this outer kinetochore protein is extensively phosphoregulated in response to defective kinetochore-microtubule attachments.

The Gist

Imagine a cell dividing like a busy construction site where a massive crane (the spindle) is trying to lift two heavy crates (the chromosomes) to opposite sides of the building. To do this safely, the crane needs to hook onto the crates using strong, flexible ropes called microtubules. The specific hook-up point on the crate is called the kinetochore.

If the crane hooks onto the wrong side of the crate, or if the rope is too loose, the whole building could collapse. To prevent this disaster, the cell has a "Safety Inspector" system.

The Main Character: KNL1

In this story, the protein KNL1 is the foreman standing right at the hook-up point. Its job is to keep an eye on the connection.

• When things are good: The rope is tight, the hook is secure, and the foreman stays calm.
• When things are bad: If the rope is loose or the hook is wrong, the foreman gets a signal to "raise the alarm."

The Alarm System: Phosphorylation

How does the foreman raise the alarm? He doesn't use a megaphone; he uses sticky notes. In biology, this process is called phosphorylation. Imagine the foreman (KNL1) has a long coat with hundreds of pockets. When a problem is detected, he quickly sticks "Warning" sticky notes (phosphate groups) onto specific pockets.

These sticky notes act as a signal flare. They tell other workers:

1. Stop the construction! (This is the Spindle Assembly Checkpoint—it halts the cell division until the error is fixed).
2. Send the rescue team! (This recruits the fibrous corona, a giant safety net that helps the crane re-grab the rope).

The Experiment: Breaking the Rules

The scientists in this paper wanted to know: Does the foreman use the same sticky notes for every type of mistake?

To find out, they created three different "disaster scenarios" in a lab using human cells:

1. The "No Rope" Scenario (Nocodazole): They dissolved the ropes entirely. The crane has nothing to grab.
2. The "Frozen Rope" Scenario (Paclitaxel): They glued the ropes so they couldn't move. The crane can't adjust its grip.
3. The "One-Sided Crane" Scenario (STLC): They broke the crane's base so it only has one arm. The ropes get tangled on one side.

They then purified the foreman (KNL1) from these cells and looked closely at his coat to see which sticky notes were attached.

The Big Discovery: A Complex Language of Sticky Notes

The results were surprising and fascinating:

• The Foreman is Covered in Notes: They found 111 different sticky notes on the foreman's coat! This means the foreman is incredibly sensitive and uses a very complex language to communicate.
• The "Universal" Alarm: Some notes (like the one at position S32) were always there, no matter what kind of mistake happened. This suggests these are the "base level" alarms that are always ready.
• The "Specific" Alarms: Other notes appeared only in specific disaster scenarios.
  • One note appeared only when the ropes were frozen.
  • Another appeared only when the crane was one-sided.
  • This suggests the foreman can tell the difference between a "no rope" problem and a "tangled rope" problem, and he uses different combinations of sticky notes to describe the specific problem to the rest of the cell.

Why Does This Matter?

Think of it like a car's dashboard.

• A generic "Check Engine" light tells you something is wrong.
• But a modern car has specific lights for "Low Oil," "Overheating," or "Tire Pressure."

This paper shows that the cell's safety system is like that modern dashboard. It doesn't just say "Error!"; it gives a detailed report on what kind of error occurred. This allows the cell to react with the perfect amount of precision to fix the problem before it causes cancer or birth defects.

In short: The scientists discovered that the cell's safety foreman (KNL1) is covered in a massive, intricate map of signals. By studying how these signals change when the cell's machinery breaks, we are learning a new, detailed language that cells use to ensure our DNA is copied and divided perfectly.

Technical Summary

1. Problem Statement

Chromosome segregation relies on the precise attachment of microtubules to kinetochores. Defective attachments lead to aneuploidy and cancer. Cells employ the Spindle Assembly Checkpoint (SAC) and the fibrous corona to detect and correct these errors. Both mechanisms are triggered by the phosphorylation of the outer kinetochore protein KNL1, specifically at conserved MELT and SHT motifs by kinases TTK (MPS1) and PLK1.

While the role of KNL1 phosphorylation in unattached kinetochores is well-established, it remains unclear whether KNL1 phosphorylation patterns, interaction partners, and downstream signaling differ across distinct erroneous kinetochore-microtubule attachment states (e.g., lack of attachment vs. syntelic attachments vs. restricted dynamics). A comprehensive map of KNL1 phosphorylation sites across these specific states was lacking.

2. Methodology

The study utilized a combination of CRISPR-Cas9 genome editing, chemical biology, and high-resolution mass spectrometry to profile KNL1 in human cells under different mitotic arrest conditions.

• Cell Model: HEK 293T/17 cells were engineered via CRISPR-Cas9 to introduce a 3xFLAG-6xHis epitope tag at the endogenous KNL1 locus.
• Experimental Conditions: Cells were treated with three distinct microtubule-disrupting compounds to induce specific attachment defects, alongside a DMSO control:
  • Nocodazole (300 nM): Inhibits polymerization, causing complete loss of attachments.
  • Paclitaxel (1 µM): Stabilizes microtubules, restricting dynamics and preventing end-on attachments.
  • STLC (3 µM): Inhibits kinesin Eg5, causing monopolar spindles and syntelic attachments (sister kinetochores attached to the same pole).
• Sample Preparation:
  • Cells were harvested, cryogenically milled, and lysed.
  • Immunoprecipitation (IP): Anti-FLAG magnetic beads were used to purify KNL1 and its interactors. Beads were crosslinked to prevent antibody leaching.
  • Proteomics: Purified samples underwent SDS-PAGE, in-gel trypsin digestion, and LC-MS/MS analysis using an Orbitrap Eclipse Tribrid mass spectrometer with FAIMS (High-field Asymmetric Waveform Ion Mobility Spectrometry) to enhance sensitivity.
• Data Analysis:
  • Phosphorylation sites were identified with >75% confidence and $\ge$2 phospho-peptide spectrum matches (PSMs).
  • Coverage and PSM counts were normalized across three biological replicates.
  • Specific attention was paid to MELT motif phosphorylation and attachment-state-specific sites.

3. Key Contributions

• Comprehensive Phosphoproteome: The study identified 111 phosphorylation sites on KNL1, significantly expanding the known phospho-regulatory landscape of this protein.
• State-Specific Profiling: Unlike previous studies focusing solely on unattached kinetochores, this work systematically compared KNL1 regulation across three distinct erroneous attachment states (nocodazole, paclitaxel, STLC).
• Interaction Mapping: Confirmed that KNL1 interacts with a core set of proteins (Mis12, Ndc80, SAC components like Bub1/BubR1, and TTK) regardless of the attachment defect, though interaction intensity varies.
• Novel Site Discovery: Identified several novel, potentially attachment-state-specific phosphorylation sites (e.g., S207, T1410, T552, S1831).

4. Key Results

• Interaction Consistency: SDS-PAGE and MS data revealed that KNL1 binding partners (including Ndc80, TTK, Bub3, Bub1, and BubR1) were consistent across all treatment conditions. However, compound treatments generally enhanced the abundance of these interactions compared to DMSO controls, likely due to mitotic arrest. No unique, compound-specific binding partners were identified, suggesting a "core" response to attachment defects.
• Global Phosphorylation Increase: Compound-treated cells showed 2–3 times more phosphorylation sites than DMSO-treated cells.
  • DMSO: 35 sites.
  • Nocodazole: 67 sites.
  • Paclitaxel: 69 sites.
  • STLC: 79 sites.
• MELT Motif Phosphorylation: Phosphorylation at MELT motifs (critical for SAC activation) increased with treatment:
  • DMSO: 1 site.
  • Nocodazole/Paclitaxel: 5 sites.
  • STLC: 7 sites.
• High-Abundance Sites:
  • S32: The most heavily phosphorylated residue across all conditions (~45–50% of peptides), suggesting a fundamental regulatory role.
  • T539, S765, S956: Enriched in mitotically arrested cells. S765 is a potential Aurora B site; T539 and S956 are followed by proline, suggesting regulation by CDK1 (proline-directed kinases).
• Attachment-State Specific Sites:
  • STLC-specific: Phosphorylation at T1115 (within a MELT motif) and T552 was unique to syntelic attachment (STLC).
  • Nocodazole-specific: S207 was detected only in nocodazole-treated cells.
  • Paclitaxel-specific: T1410 was unique to paclitaxel treatment.
  • S1831: A known non-MELT TTK substrate, enriched in paclitaxel and STLC conditions compared to nocodazole.

5. Significance

• Mechanistic Insight: The data demonstrates that KNL1 is not just a binary switch for SAC activation but is extensively and dynamically phosphoregulated depending on the specific nature of the kinetochore-microtubule error.
• Functional Hypotheses:
  • The enrichment of S765 (Aurora B site) in arrested cells suggests a role in the error correction pathway for low-tension attachments.
  • The presence of CDK1 consensus sites (T539, S956) links KNL1 regulation directly to cell cycle progression.
  • The identification of unique sites (e.g., T1115 in STLC) implies that the cell may utilize distinct phosphorylation codes to distinguish between different types of attachment errors (e.g., syntelic vs. unattached).
• Future Directions: This study provides a foundational map for future mutagenesis studies to determine the functional necessity of specific phospho-sites in SAC signaling and fibrous corona expansion. It also establishes a protocol for investigating the phosphoregulation of other kinetochore components across diverse attachment states.