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Best direct Swi6 Kd + paired ChIP evidence
The clearest match to “most direct Swi6 Kd measurements” and “paired ChIP experiments” in your provided data is the CKII/HP1(Swi6) phosphorylation paper that reports explicit nucleosome-binding Kd values (in vitro, purified proteins + reconstituted nucleosomes) and pairs them with genome-wide Swi6 and H3K9me2/me3 ChIP-seq showing phospho-mutant-specific spreading defects.
Long Explanation
Evidence battle-test: direct Swi6 Kd + paired ChIP
Focus: identify the most direct Swi6 binding Kd measurements and the most tightly paired ChIP experiments from the provided dataset.
1) Candidate set from provided research data
Direct Swi6 Kd + paired genome-wide ChIP-seq:
No explicit Swi6 Kd values were provided for the other included studies in your dataset snippets (e.g., RNAi linkage to H3K9 methylation, Swi6 recruitment by siRNA-DNA hybrids, or Swi6 S160 phosphorylation controlling localization). These are mechanistically informative, but do not satisfy “most direct Swi6 Kd measurements” as written.
2) Most direct Swi6 Kd: quantitative anchors (from the paired Kd+ChIP study)
The study reports in vitro affinity measurements for unphosphorylated Swi6 (unpSwi6) and phosphorylated Swi6 (pSwi6) toward unmethylated and H3K9me3 nucleosomes/peptides. The most “Kd-direct” values are the reported Kd estimates for:
(i) nucleosome binding on unmethylated vs H3K9me3 nucleosomes; and
(ii) peptide binding to H3K9me3.
Figure A — Reported nucleosome binding Kd (pSwi6 vs Swi6)
How to read: Kd is an inverse proxy for binding strength. In the reported numbers, phosphorylation shifts binding such that pSwi6 is dramatically weaker on unmethylated nucleosomes (higher Kd) while remaining substantially tighter on H3K9me3 nucleosomes (lower Kd).
Figure B — H3K9me3 peptide Kd (pSwi6 vs Swi6)
Peptide binding Kd values provide an additional check on recognition specificity for the H3K9me3 motif (reported as pSwi6 ~2.45 μM vs unpSwi6 ~8.17 μM).
3) Paired ChIP experiments: what they show relative to the Kd shifts
The same study couples the biochemical Kd changes to chromatin outcomes by measuring genome-wide H3K9me2/me3 distributions and Swi6 behavior in vivo for phospho-mutant alleles (notably Swi6 S18/24A). The provided extracted results specify:
Swi6 S18/24A shows reduced H3K9me3 across most spreading regions (with more limited effects at some nucleators), while H3K9me2 is elevated in many nucleation regions.
Across genome-wide bins, conversion of H3K9me2 → H3K9me3 is impaired outside nucleation centers.
Critically: these ChIP observations are paired with the Kd measurements within the same mechanistic narrative, but the excerpts provided do not include a direct residue-by-residue mechanistic dissection linking each Kd change to each genomic bin’s conversion dynamics (i.e., “tightest pairing” is at the level of shared mutants and shared substrate specificity shift).
4) Minimal, direct “best evidence” recommendation (what to cite first)
Evidence layer
Most direct matching item (from provided data)
Strength / match
Swi6 Kd (in vitro)
Reported Kd estimates for pSwi6 vs unpSwi6 to unmethylated nucleosomes and to H3K9me3 nucleosomes/peptides.
Paired ChIP (in vivo)
Genome-wide H3K9me3/H3K9me2 and Swi6 distribution differences in phospho-mutants (notably S18/24A), with impaired H3K9me2→H3K9me3 conversion outside nucleation centers.
5) Skeptical audit: where this “best evidence” could still mislead
In vitro–in vivo mapping risk: Kd values are measured with purified proteins and reconstituted nucleosomes; the excerpted limitations note that in vitro phosphorylation states may not fully recapitulate in vivo phospho-regulation dynamics.
Mutant pleiotropy: phosphosite mutant alleles (e.g., S18/24A) may affect protein behavior beyond the intended binding specificity shift, so ChIP differences might not be solely due to nucleosome competition.
ChIP signal interpretation: antibody specificity and cross-reactivity can influence ChIP outcomes; also mapping complexity is noted (duplicate reporter sequences complicate read assignment).
6) What would most strongly falsify the Kd↔ChIP pairing (within this mechanism)
If S18/24A phosphorylation did not alter nucleosome binding affinities/oligomerization (i.e., Kd-shifts absent), yet ChIP still showed normal H3K9me3 spreading, that would weaken the causal substrate-competition explanation.
If pSwi6 did not relieve Clr4-mediated conversion bottlenecks in single-turnover assays, while still producing ChIP conversion changes, the linkage between nucleosome affinity shifts and Clr4 chemistry would be suspect.
If ChIP-seq showed no differential H3K9me2→H3K9me3 conversion outside nucleation centers between WT and swi6 S18/24A, despite the reported Kd shifts, then the claimed genomic spreading consequence would be unpaired.
Derive and visualize a small table/plot of reported pSwi6 vs unpSwi6 Kd values and substrate classes from URL_0, then compute fold-changes to quantify phosphorylation’s substrate bias alongside the reported ChIP qualitative trends.
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Hypothesis Graveyard
A “S18/24A only affects localization, not substrate competition” model becomes less likely if (as reported) pSwi6/unpSwi6 show large opposite Kd shifts on nucleosome substrates and if ChIP-seq conversion outside nucleation centers tracks those affinity changes (not just foci brightness).
A “ChIP differences are antibody artifact only” model is unlikely at the strength level of concordant nucleosome Kd shifts plus multiple ChIP readouts (H3K9me2/me3 and Swi6 distribution) reported in the same study, though antibody specificity remains a stated limitation.
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