The N320-C180 clamp is the only KMT2A module whose disruption has ever been shown to change subnuclear localisation - and the fusion is the disruption
also called FYRN-FYRC clamp localisation · MLL C180 diffuse redistribution · N320-C180 subnuclear destination
Normal MLL is cut in two and the halves clip back together; a 2003 paper showed that this clipped-together pair is what gives the protein its position inside the nucleus, and that breaking the clip makes one half spread out evenly instead of sitting in spots. Every leukemia fusion loses the second half entirely, so it can never form that clip - but a 2019 paper looking at the same system saw no change in position at all.
The same thing, for a scientist
Wild-type KMT2A is cleaved by taspase1 into N320 and C180, which heterodimerise through a FYRN-FYRC interaction. The 2003 papers state that this complex 'confers protein stability and subnuclear localization' [E1] and that the fragments 'heterodimerize to stabilize the complex and confer its subnuclear destination' [E2]. The decisive manipulation is in the first: **disrupting the FYRN-FYRC interaction markedly decreases N320 levels and redistributes C180 to a diffuse nuclear pattern**, and the same paper notes that MLL fusion proteins of leukaemia, which lose the ability to complex with C180, have their stability conferred instead by the fusion partners [E1]. As far as this beat could establish, that is **the only published instance in the KMT2A literature of a defined module change producing a defined subnuclear change**. Every other localisation determinant claim - CXXC, AT-hooks, PHDs - either reports no positional change or maps a genomic address. It is also directly relevant, because the fusion has no C180 and therefore no clamp, so the fusion is permanently in the state the 2003 experiment created artificially. **The claim is contested.** A 2019 study reported that blocking taspase1 cleavage left MLL1 uncleaved, more degradation-resistant and markedly increased in chromatin occupancy with **subcellular localisation unchanged** [E3]. Those are different perturbations - uncleaved full-length protein versus a clamp-disrupted fragment pair - but the disagreement is real and this graph should not repeat either as settled.
Why the two studies are not necessarily in conflict, and why that does not help
The 2003 manipulation breaks the FYRN-FYRC interface so that N320 and C180 exist as separated fragments; the 2019 manipulation prevents cleavage so that they never separate at all. A protein that is never cut is a single covalently joined molecule with both halves permanently adjacent, which is functionally the most clamped state, not the least. So 'clamp broken → C180 diffuse' and 'never cut → localisation unchanged' can both be true.
That reconciliation is the reason the disagreement matters rather than the reason it does not. If both are true, then the localisation of the C-terminal half depends on physical association with the N-terminal half, whichever way that association is achieved - and the KMT2A fusion has neither route available, because the C-terminal half is on the other derivative chromosome. The prediction is that the fusion's retained N320-equivalent region, deprived of its C180 partner, should localise differently from wild-type N320. Nobody has tested that, and the readout it needs is a fraction, not a picture, because in a t(4;11) cell wild-type N320 and the fusion share every N-terminal epitope.
What the reciprocal product does to the argument
In reciprocal-positive t(4;11) patients the der(4) product AFF1::KMT2A carries the KMT2A C-terminus including both taspase1 sites, and is cleaved, releasing an MLL-C fragment sequence-identical to the wild-type one. So a reciprocal-positive cell has a source of free C180 that a reciprocal-negative cell does not.
If the clamp genuinely sets subnuclear destination, reciprocal status is a localisation variable: reciprocal-positive cells have extra C180 competing for N320 partners, reciprocal-negative cells do not. This graph already treats reciprocal status as the cleanest natural experiment in the disease for outcome; this beat adds that it is also an untested variable for protein localisation, and that an anti-MLL-C blot cannot distinguish the two C180 sources.
Attribution status of the 2003 result
Good for its own claim, weak for the fusion. The fragments and the interface mutants were studied as defined constructs, so the species giving the signal is known - that is the attributable design. But the cells are not leukaemic, the proteins are transfected, and the 'diffuse nuclear pattern' is an imaging readout with all the limits this beat documents. What the experiment does not show is that a KMT2A fusion is mislocalised; it shows that a C-terminal fragment deprived of its partner is.
The honest summary is: one measured module-to-compartment link exists in this literature, it concerns the half of the protein the fusion does not have, it is contested by a later study using a different perturbation, and the experiment that would resolve it for the fusion is a fractionation.
| module | FYRN (N320) - FYRC/SET (C180) heterodimer |
|---|---|
| measured consequence of disruption | C180 redistributes to a diffuse nuclear pattern; N320 levels markedly decrease |
| present in fusion | false |
| contested by | 30573454 (subcellular localisation unchanged when cleavage is blocked) |
| reconciliation | different perturbations - clamp disruption versus prevention of cleavage |
| untested prediction | the fusion's retained N-terminal region, deprived of C180, should fractionate differently from wild-type N320 |
- A therapeutic that stabilised uncleaved wild-type MLL1 to displace the fusion from chromatin - the strategy the 2019 study proposes - would be acting on a protein whose subnuclear behaviour is disputed, and the displacement readout is chromatin occupancy rather than protein fate.
- If free C180 from a reciprocal product competes for N320, reciprocal-positive and reciprocal-negative patients could respond differently to any such strategy, and no trial stratifies on reciprocal status.
- Does the fusion's retained N-terminal region fractionate differently from wild-type MLL-N320 in the same t(4;11) lysate?
- Does reciprocal status change the localisation of wild-type MLL-N320, by supplying or withholding free C180?
- Would a subnuclear-resolution repeat of the 2019 localisation control see the redistribution the 2003 paper reported?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
MLL is normally cleaved at two conserved sites and the resulting N-terminal p320 and C-terminal p180 fragments form a stable complex that localises to a subnuclear compartment; the FYRN domain of N320 directly interacts with the FYRC and SET domains of C180; disrupting that interaction markedly decreases N320 levels and redistributes C180 to a diffuse nuclear pattern; and MLL fusion proteins of leukaemia, which lose the ability to complex with C180, have their stability conferred instead by the fusion partners.
Proteolytic cleavage of MLL generates a complex of N- and C-terminal fragments that confers protein stability and subnuclear localization · Hsieh JJ, et al. · Molecular and cellular biology 2003What this evidence cannot show
Transfected constructs in non-leukaemic cells, with an imaging readout whose limits this beat documents at length. The experiment establishes that a C-terminal fragment deprived of its partner redistributes; it does not show that a KMT2A fusion is mislocalised, and the fusion's own position is not measured.
Evidence that argues against this (1)
- A 2019 study found that blocking taspase1 cleavage left MLL1 uncleaved, more degradation-resistant and markedly increased in chromatin occupancy with subcellular localisation UNCHANGED, so the localisation role of the cleavage-and-clamp cycle is contested. Zhao Z et al. 2019
- E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
MLL is cleaved at two conserved sites generating N320 and C180 fragments, which heterodimerise to stabilise the complex and confer its subnuclear destination; the responsible protease is taspase1, and RNAi knockdown of taspase1 results in unprocessed MLL and loss of proper HOX gene expression.
Taspase1: a threonine aspartase required for cleavage of MLL and proper HOX gene expression · Hsieh JJ, et al. · Cell 2003What this evidence cannot show
The subnuclear-destination clause is the paper's summary of the companion biochemistry rather than a result reported in this paper's own figures; the paper's own contribution is the identification of the protease.
Evidence that argues against this (1)
- Loss of proper HOX expression on taspase1 knockdown is a transcriptional phenotype that need not run through localisation, so the destination clause is not supported by the paper's functional readout. Zhao Z et al. 2019
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Taspase1-dependent cleavage destabilises MLL1 rather than maturing it: taspase1 loss leaves MLL1 uncleaved, more resistant to cycloheximide-chase degradation and markedly increased in genome-wide chromatin occupancy, with subcellular localisation unchanged, unchanged bulk H3K4 methylation and unchanged MLL/COMPASS assembly; because KMT2A breakpoints lie N-terminal to the cleavage sites the chimeras are not taspase1 substrates and are extremely stable with high chromatin occupancy.
Regulation of MLL/COMPASS stability through its proteolytic cleavage by taspase1 as a possible approach for clinical therapy of leukemia · Zhao Z, et al. · Genes & development 2019What this evidence cannot show
The perturbation is prevention of cleavage, not disruption of the FYRN-FYRC clamp, so this is not a direct replication attempt of the 2003 localisation result. 'Subcellular localisation unchanged' is a nucleus-versus-cytoplasm-grade statement in the abstract and may not have the resolution to see a subnuclear redistribution.
Evidence that argues against this (1)
- A separate group reads the same perturbation as loss of activation rather than loss of degradation - taspase1-null cells show uncleaved MLL with reduced H3 methyltransferase activity and homeotic transformations - so the direction of the cleavage mechanism itself is disputed. Hsieh JJ et al. 2003
- E4established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
The canonical t(4;11) chimera retains the KMT2A N-terminus and none of the C-terminal half, so no KMT2A fusion protein can form the N320-C180 heterodimer.
Acute mixed-lineage leukemia t(4;11)(q21;q23) generates an MLL-AF4 fusion product · Domer PH, et al. · Proceedings of the National Academy of Sciences of the United States of America 1993What this evidence cannot show
Follows from the fusion architecture rather than from a dedicated experiment; the reciprocal der(4) product, where expressed, does carry the KMT2A C-terminus and both cleavage sites.
Evidence that argues against this (1)
- In reciprocal-positive patients the der(4) product supplies a sequence-identical free MLL-C fragment, so 'the fusion cell has no C180' is true of the direct product and false of the cell. Hsieh JJ et al. 2003
- ← depends onKMT2A / MLL1 — the gene that breaks
the KMT2A subnuclear address is a property of the N320-C180 clamp, which no fusion can form
- ← related toSIAH1 — the E3 ligase that degrades AFF1, and that the reciprocal AF4::KMT2A fusion escapes
FYRN/FYRC heterodimerisation of Taspase1-cleaved AF4-MLL forms a high-molecular-weight complex PROTECTED against SIAH-mediated degradation (PMID 21233834) - same laboratory as the founding SIAH/AF4 paper, so not an independent replication (Q91 2026-09-22)
adds the localisation half of the cleavage story, and records the 2003-versus-2019 disagreement about it
the CXXC result is the contrast case - a module change with no positional consequence
the fractionation is the readout that could adjudicate the 2003-versus-2019 disagreement
in a reciprocal-positive cell the free C180 pool has two sources and no reagent separates them
- part of →KMT2A / MLL1 — the gene that breaks
the FYRN-FYRC clamp between the taspase1-generated halves of KMT2A
the one measured module-to-compartment link concerns a module the fusion has lost, which would make the fusion's compartment differ from wild-type