The centromere localization domain of kinetoplastid kinetochore protein KKT2 recognizes the free N-terminus of histone H3

Technical Summary: The Centromere Localization Domain of KKT2 Recognizes the Free N-terminus of Histone H3

Problem Statement
Kinetochores are essential macromolecular complexes that mediate chromosome segregation. In most eukaryotes, kinetochore assembly is specified by the centromere-specific histone H3 variant, CENP-A. However, kinetoplastids (a group of early-diverging eukaryotes including Trypanosoma brucei) lack CENP-A and canonical kinetochore proteins, possessing instead a unique set of kinetochore components. Despite the identification of these unique proteins, the mechanism by which kinetochores are specifically assembled at centromeres in the absence of CENP-A remains unknown. Specifically, it is unclear how the kinetoplastid kinetochore protein KKT2, which contains a central centromere localization (CL) domain, targets centromeric regions.

Methodology
The study employed a combination of structural biology, biophysical characterization, and biochemical assays to investigate the function of the KKT2 CL domain:

• Structural Analysis: The authors utilized AlphaFold3 to generate models of T. brucei KKT2 and KKT3 CL domains and compared them with existing crystal structures of KKT2 from other kinetoplastids. Structural homology searches (DALI, Foldseek) and topology comparisons were conducted to identify domain similarities.
• NMR Spectroscopy: Nuclear Magnetic Resonance (NMR) was the primary tool for characterizing the KKT2 CL domain (residues 562–630). This included:
  • Resonance assignment using triple-resonance experiments on isotopically labeled ($^{15}$N, $^{13}$C) protein.
  • Assessment of backbone dynamics via heteronuclear NOE (hetNOE) and Residual Dipolar Couplings (RDCs).
  • Titration experiments monitoring chemical shift perturbations (CSPs) upon addition of synthetic histone H3 peptides (and variants) to $^{15}$N-labeled KKT2 CL.
  • Analysis of mutant variants (e.g., D622E) to probe specific residue roles.
• Isothermal Titration Calorimetry (ITC): ITC was used to determine the thermodynamic parameters (binding affinity $K_d$, stoichiometry $N$, enthalpy $\Delta H$, and entropy $\Delta S$) of the interaction between KKT2 CL and histone H3 peptides.
• Peptide Modification: Synthetic peptides representing the N-terminal tails of histones H3, H2A, H2B, and H4 were used. These included unmodified peptides and those with specific post-translational modifications (PTMs) on side chains (phosphorylation, methylation, acetylation) and N-terminal ($\text{N}\alpha$) modifications (acetylation, mono-methylation).
• SEC-MALS: Size-exclusion chromatography coupled with multi-angle light scattering was used to confirm the monomeric state of the purified KKT2 CL protein.

Key Contributions and Results

1. Structural Homology to ZZ Domains:
   The study identified that the KKT2 CL domain shares a structural fold with the ZZ-type zinc finger domain, specifically those known to bind the N-terminus of histone H3 (e.g., p300 ZZ). Although the domains are circularly permuted (explaining previous failures in homology detection), they share a conserved topology and a critical invariant aspartate residue (D622 in T. brucei KKT2).

2. Specific Binding to Histone H3 N-terminus:
   NMR titration experiments demonstrated that the KKT2 CL domain binds specifically to the N-terminal tail of histone H3.

   • Significant chemical shift perturbations were observed for residues surrounding D622 upon H3 peptide addition.
   • The binding was specific to histone H3; peptides derived from H2A, H2B, and H4 induced negligible changes.
   • The interaction was abolished by the D622E mutation, confirming the essential role of the invariant aspartate in recognizing the peptide.

3. Sensitivity to N-terminal ($\text{N}\alpha$) Modifications:
   A critical finding was that the interaction is strictly dependent on the unmodified, free N-terminal amino group of histone H3.

   • $\text{N}\alpha$ Acetylation: Completely abolished binding, consistent with the neutralization of the positive charge required for interaction with the acidic aspartate.
   • $\text{N}\alpha$ Methylation: Strikingly, even mono-methylation of the N-terminus abolished binding. The authors suggest this is likely due to steric hindrance preventing the N-terminus from docking, rather than a charge effect, as methylation retains a positive charge.
   • Side-chain PTMs: Various side-chain modifications (methylation of R2/K4, phosphorylation of T3/T6, acetylation of K4) did not enhance binding. In fact, T3 phosphorylation weakened the interaction.

4. Thermodynamics:
   ITC confirmed a 1:1 stoichiometry for the KKT2 CL–H3 complex with a dissociation constant ($K_d$) of approximately 60 $\mu$M. The interaction was found to be exothermic and predominantly enthalpy-driven. The D622E mutant and $\text{N}\alpha$-monomethylated H3 peptide showed no detectable binding in ITC assays.

Significance and Claims
The paper proposes a mechanism for kinetochore specification in kinetoplastids that relies on the absence of specific histone modifications rather than the presence of a unique histone variant.

• Mechanism: The authors hypothesize that centromere-specific kinetochore assembly in T. brucei is ensured by the absence of $\text{N}\alpha$ methylations and acetylations on histone H3 specifically at centromeric regions. In non-centromeric regions, the abundant $\text{N}\alpha$ methylations of histone H3 (a known feature of T. brucei) prevent KKT2 CL from binding.
• Novelty: This study provides the first biochemical evidence of how a kinetoplastid kinetochore protein recognizes chromatin in the absence of CENP-A. It highlights a unique regulatory mechanism where the "default" state of histone modification (abundant $\text{N}\alpha$ methylation) acts as a barrier to kinetochore assembly, with centromeres defined by a lack of these modifications.
• Future Directions: The authors note that while the mechanism is likely conserved in KKT3 (which shares the invariant aspartate), the specific methyltransferase responsible for the $\text{N}\alpha$ methylation of histone H3 in trypanosomes remains unidentified, as the known eukaryotic enzymes (METTL11A/B and METTL13) do not match the H3 N-terminal consensus motif.

The study concludes that the KKT2 CL domain functions as a sensor for the unmodified N-terminus of histone H3, offering a plausible explanation for how kinetochores are positioned on chromosomes lacking CENP-A.