The other arm of the crosstalk: H2Bub licenses H3K4 methylation through ASH2L and relieves a built-in COMPASS autoinhibition — and MLL3 does not respond at all
also called H2Bub-H3K4me crosstalk · ASH2L trans-regulation · COMPASS activation by H2Bub
The same ubiquitin tag that lets DOT1L work also switches on the enzymes that put the H3K4 mark, and it does so by releasing a brake built into the enzyme itself. Strikingly, it works on MLL1 and not on MLL3 — so the tag does not simply turn up every related enzyme.
The same thing, for a scientist
H2Bub has two downstream arms, and this graph had neither. The H3K4 arm runs through the WRAD module the graph already carries in detail. ASH2L is **essential** for H2Bub-dependent H3K4 methylation; deleting or mutating **K99** of ASH2L's N-terminal winged-helix motif abrogates H2Bub-dependent regulation; the crosstalk can occur **in trans** and does not require the ubiquitin to be on nucleosomes or histones at all; trans-regulation by ubiquitin promotes MLL activity for **all three** methylation states; and — the specificity result — **MLL3 does not respond to H2Bub** [E1]. Structurally, cryo-EM of an extended COMPASS catalytic module on ubiquitinated and free nucleosomes shows the module clamps the nucleosome disc face to capture the H3 N-terminal tail and sandwiches an autoinhibitory Set1 arginine-rich motif; H2Bub activates the assembly **without enhancing COMPASS–nucleosome binding**, by packing against Swd1 and relieving the ARM's inhibition once the ARM is fastened to the acidic patch [E2]. That is the same class of mechanism as the DOT1L arm — allosteric activation rather than recruitment — arrived at independently.
What this adds to the graph's WRAD entries, and the contradiction it must not paper over
The graph already holds gene-ash2l, gene-rbbp5, gene-wdr5, gene-dpy30, ana-wrad-subunit-discrimination and subtraction-kmt2a-win-wrad-interface. What it did not hold is that WRAD is the receiver of the H2Bub signal, and that ASH2L specifically carries the receiving surface 1.
The graph's own gene-rbbp5 entry already flags the counter-argument, and this entry adopts it rather than contradicting it: the H2Bub–RBBP5 contact is real biochemistry, but the MLL1 SET domain is dispensable for MLL-AF9 leukemogenesis in mouse, so a therapeutic rationale built on MLL1 catalysis is load-bearing on an assumption that has been tested and failed. The same caution applies here in full. Nothing in this entry argues that H2Bub-dependent H3K4 methylation by MLL1 is a KMT2A-fusion dependency. What it argues is narrower and defensible:
- The H3K4 arm exists and is mechanistically well characterised, so any intervention on H2Bub hits it as well as the H3K79 arm — which is a selectivity cost, not a benefit.
- The wild-type KMT2A allele retained in every patient is a COMPASS-family enzyme whose activity is H2Bub-regulated; removing H2Bub perturbs normal KMT2A function, and normal KMT2A function is required for haematopoiesis.
- The specificity result — MLL3 unresponsive — means H2Bub is not a general SET1/MLL-family amplifier, which constrains any model that treats it as one.
Two mechanistic details worth keeping
The ubiquitin does not have to be on a histone. 1 reports that the crosstalk can occur in trans and does not require ubiquitin to be on nucleosomes or histones to exert its regulatory effect. Taken with the yeast tethering result in mechanism-h2bub-dot1l-trans-histone-crosstalk [E8 there], the picture is that what these enzymes sense is a ubiquitin presented in the right geometry, not a specific modified residue. Therapeutically that is bad news: a strategy aimed at a single ubiquitination site is aimed at the wrong variable.
H2Bub relieves an autoinhibitor rather than recruiting the enzyme. 2 is explicit that activation occurs without enhancing COMPASS–nucleosome interaction. The Set1 arginine-rich motif is an intramolecular brake; the ubiquitin packs against Swd1 and alleviates it. Enzymes that are switched on by relief of autoinhibition are, in principle, more tractable targets than enzymes switched on by recruitment, because the inhibited state is a real conformation that a small molecule could stabilise. No such compound exists.
| arm | H2BK120ub -> SET1/MLL-family COMPASS -> H3K4me1/2/3 |
|---|---|
| receiver subunit | ASH2L, N-terminal winged-helix motif, residue K99 |
| mechanism class | relief of intramolecular autoinhibition (Set1 arginine-rich motif), not recruitment |
| specificity | MLL1 responds; MLL3 does not |
| in trans | the ubiquitin does not have to be attached to the nucleosome or to a histone |
| why it matters here | it is the selectivity cost of any intervention on H2Bub, not a second opportunity |
graph cross references
[ "gene-ash2l", "gene-rbbp5", "gene-wdr5", "gene-dpy30", "subtraction-kmt2a-win-wrad-interface", "ana-wrad-subunit-discrimination" ]
- Any agent that lowers H2Bub lowers H3K4me2/3 through COMPASS as well as H3K79me through DOT1L. The single property that made DOT1L an attractive target - that inhibiting it barely moved global expression - does not transfer to anything upstream.
- Because the crosstalk works in trans and with ubiquitin presented at more than one position, a site-directed strategy against H2BK120 specifically is aimed at a variable the enzymes do not strictly read.
- MLL3 is unresponsive to H2Bub, so cells that have already lost MLL3/UTX function - a described resistance state in this disease - are by construction less exposed to any intervention on this layer.
- Is ASH2L K99 mutated or altered in any KMT2A-rearranged patient sample? A residue that switches off an entire crosstalk arm is worth checking against the cohorts this graph already cites, and nobody has.
- Does the retained wild-type KMT2A allele's H3K4 activity depend on H2Bub in a leukaemia cell? If yes, lowering H2Bub hits the normal allele, which is the arm of the system that normal blood needs.
- Why does MLL3 not respond to H2Bub? MLL3/KMT2C loss is a documented resistance route in this disease (`resistance-mll34-utx-tumour-suppressor-failure`), and an H2Bub-unresponsive enzyme behaves differently under any intervention on the ubiquitin layer.
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
ASH2L in the MLL complex is essential for H2Bub-dependent H3K4 methylation; deleting or mutating K99 of the N-terminal winged-helix motif of ASH2L abrogates H2Bub-dependent regulation; the crosstalk can occur in trans and does not require ubiquitin to be on nucleosomes or histones to exert regulatory effects; trans-regulation by ubiquitin promotes MLL activity for all three methylation states; and MLL3, an MLL homolog, does not respond to H2Bub.
ASH2L regulates ubiquitylation signaling to MLL: trans-regulation of H3 K4 methylation in higher eukaryotes. · Wu L, Lee SY, Zhou B, Nguyen UT, Muir TW, Tan S, Dou Y · Mol Cell 2013What this evidence cannot show
Abstract read in full; full text not read. Reconstituted biochemistry with semisynthetic ubiquitylated histones; 'MLL' here means the MLL1 core complex in vitro, not a KMT2A fusion protein and not a leukaemia cell. The in-trans result is a striking in vitro observation whose behaviour in chromatin is not established by the abstract. The MLL3 negative result is from the same in vitro system and could reflect construct boundaries rather than biology.
Evidence that argues against this (1)
- This graph's own gene-rbbp5 entry records the decisive caution: whatever the biochemistry of H2Bub-dependent MLL1 catalysis, the MLL1 SET domain is dispensable for MLL-AF9 leukemogenesis in mouse, so the H3K4 arm cannot be assumed to be a KMT2A-fusion dependency. This entry adopts that position rather than disputing it. Oncofusion Knowledge Graph et al. 2026
- E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Cryo-EM structures of an extended COMPASS catalytic module bound to H2Bub and to free nucleosome show the module clamps onto the nucleosome disk-face via an extensive interface to capture the flexible H3 N-terminal tail, sandwiching a critical Set1 arginine-rich motif that autoinhibits COMPASS; H2Bub activates the enzymatic assembly without enhancing COMPASS-nucleosome interaction, by packing against Swd1 and alleviating the inhibitory effect of the Set1 ARM upon fastening it to the acidic patch.
Structural Basis of H2B Ubiquitination-Dependent H3K4 Methylation by COMPASS. · Hsu PL, Shi H, Leonen C, Kang J, Chatterjee C, Zheng N · Mol Cell 2019What this evidence cannot show
Abstract read in full; full text not read. Yeast COMPASS (Set1, Swd1), not the human MLL1 complex, so the mapping of Swd1 to RBBP5 and of Set1 to KMT2A is an inference by homology. Resolution and construct boundaries are not stated in the abstract. A structure of an activated state does not establish that the state is rate-limiting in cells.
Evidence that argues against this (1)
- Cryo-EM of an EXTENDED COMPASS catalytic module on a nucleosome shows how the module clamps and how H2Bub is read. It is a construct rather than the holo-complex, and the KMT2A-family member in leukaemia is a fusion missing the SET domain entirely, so the structural mechanism described applies to the wild-type enzyme this disease has lost. Molecular Cell 2019
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
H2B ubiquitylation directly stimulates hSET1-dependent H3K4 di- and trimethylation in a reconstituted human system, downstream of and separable from the transcription reaction itself.
RAD6-Mediated transcription-coupled H2B ubiquitylation directly stimulates H3K4 methylation in human cells. · Kim J, Guermah M, McGinty RK, Lee JS, Tang Z, Milne TA, Shilatifard A, Muir TW, Roeder RG · Cell 2009What this evidence cannot show
Abstract read in full; full text not read. hSET1, not MLL1 and not a KMT2A fusion.
Evidence that argues against this (1)
- Direct stimulation of hSET1-dependent H3K4 methylation in a reconstituted human system establishes the biochemical arm. SET1 and the KMT2A family are different enzymes with different complex compositions, and the entry generalises from SET1 to the family without marking the step. Cell 2009
the H3K4 arm of the same crosstalk
- depends on →ASH2L — the WRAD subunit that has DPY30's essentiality profile and none of its co-dependency signal
ASH2L is essential for H2Bub-dependent H3K4 methylation; K99 of the winged-helix motif carries the receiving surface
the H2Bub-RBBP5 contact the graph already records; its therapeutic reading is constrained by SET-domain dispensability
- regulates →KMT2A / MLL1 — the gene that breaks
trans-regulation by ubiquitin promotes MLL1 activity for all three H3K4 methylation states; MLL3 does not respond