Directed extraction of P-TEFb from the reservoir: HIV-1 Tat is the worked example, and the only cellular candidate rests on one unreplicated paper
also called Tat extraction of P-TEFb · Tat 7SK remodelling · Tat-SEC
A virus solved this problem long before anyone asked whether a leukaemia had. HIV's Tat protein grabs the off-switch RNA directly, pushes the braking protein off it, and walks away with the elongation enzyme. Only one human protein has ever been proposed to do the same, in a single unreplicated study.
The same thing, for a scientist
Tat is the paradigm and it is worth being exact about what it does, because the comparison is what makes the leukaemia gap visible. (1) Tat and HEXIM1 bind competitively to the same region of cyclin T1, and Tat binds 7SK RNA directly, displacing HEXIM from the apical stem-loop 1 by exploiting a local destabilisation that HEXIM binding itself creates [E1]. (2) Tat forms two distinct stable complexes: Tatcom1, containing core active P-TEFb, the MLL fusion partners AF9, AFF4, AFF1, ENL and ELL, and the PAF1 complex; and Tatcom2, containing CDK9, cyclin T1 and the 7SK snRNP **lacking HEXIM** [E2]. (3) AFF1 is required for efficient extraction - it raises Tat's affinity for cyclin T1 through its N-terminal cyclin T1-binding sequence, and the M60A/L61A substitution in that sequence abolishes the effect [E3]. The last point is the one that matters here: the AFF1 module Tat needs is AFF1 1-308, and **KMT2A::AFF1 begins at AFF1 residue 347** [E4]. So the closest thing biology has to a directed P-TEFb extractor requires a piece of AFF1 that the index fusion of this graph does not contain. The Sobhian purification also makes the deeper point that Tat's complex is built from **the MLL fusion partners** - the two systems are the same machine, approached from opposite ends [E2].
What transfers to KMT2A::AFF1 and what does not
Transfers: the components. Tatcom1 is AF9, AFF4, AFF1, ENL, ELL, P-TEFb and PAF1 - which is, subunit for subunit, the SEC this graph already holds 2. Any structural or biochemical fact about how those subunits fit together is usable.
Does not transfer: the extraction. Tat's capability rests on (a) direct 7SK RNA binding by an arginine-rich motif, and (b) an AFF1 N-terminal module that raises its cyclin T1 affinity. The fusion has neither. There is no KMT2A-derived arginine-rich motif with 7SK affinity on record, and the AFF1 CBS is lost at the junction 4.
This graph's localization-nucleolar-retention-versus-sec-droplets-tat-comparator uses Tat for a different purpose - a two-compartment localisation switch. This entry is the reservoir-facing half of the same comparison and should be read with it.
The honest negative, and the one candidate against it
No cellular protein has been established to extract P-TEFb from the 7SK snRNP the way Tat does. Every well-replicated cellular release mechanism is indirect - dephosphorylation of CDK9, hnRNP competition for the RNA, stress, transcriptional blockade, or concentration by a scaffold - rather than a targeted displacement of HEXIM by a specific factor 12.
There is exactly one candidate for the directed version. Mueck, Bracharz and Marschalek reported that DDX6 binds 7SK snRNA and causes the release and transfer of P-TEFb to the AF4/AF4N super elongation complex 5. If that holds it is the cellular counterpart of Tat's extraction. It is a single paper from one laboratory in Am J Blood Res, built on overexpression in HEK 293T, with no independent replication this beat could find, and it is carried at weak confidence in gene-ddx6. It is recorded here rather than omitted because a reader repeating this search next year should know it was found and weighed.
Correction note. An earlier draft of this entry, and its original id, asserted that Tat was the only worked example. That was written before the DDX6 paper was retrieved and is withdrawn. The claim that survives is narrower: Tat is the only well-replicated directed extractor, and the module it depends on is the module KMT2A::AFF1 loses.
| mechanism class | viral paradigm / comparator |
|---|---|
| extractor | HIV-1 Tat |
| route | direct binding to 7SK apical stem-loop 1, competitive with HEXIM on cyclin T1, assisted by the AFF1 N-terminal cyclin T1-binding sequence |
| transfers to kmt2a fusion | false |
| reason | the fusion has no 7SK-binding motif and lacks AFF1 1-346, which contains the cyclin T1-binding sequence |
| cellular equivalent known | one unreplicated candidate: DDX6 (PMID 27679741) - see gene-ddx6 |
complexes
{
"Tatcom1": [
"P-TEFb",
"AF9",
"AFF4",
"AFF1",
"ENL",
"ELL",
"PAF1 complex"
],
"Tatcom2": [
"CDK9",
"cyclin T1",
"7SK snRNP lacking HEXIM"
]
}related entries
[ "localization-nucleolar-retention-versus-sec-droplets-tat-comparator", "mechanism-ptefb-extraction-from-the-7sk-reservoir", "mechanism-kmt2a-aff1-cannot-be-sequestered-in-the-7sk-snrnp", "gene-ddx6" ]
- The Tat paradigm is a virus solving a problem for its own promoter. A therapeutic that mimicked its inverse would lock P-TEFb away from every promoter in the cell, not only the fusion's.
- Every structure of the AFF-family/cyclin T1 contact was solved in a Tat context, so the geometry a drug designer would work from is a viral geometry.
- Has any laboratory other than Marschalek's tested whether DDX6 binds 7SK and transfers P-TEFb? No replication was found.
- Could a KMT2A fusion extract P-TEFb in trans through a bound AFF4 or wild-type AFF1 molecule, preserving at complex level what it lost at protein level?
- Would a synthetic 7SK stem-loop-1 binder - the inverse of Tat - lock P-TEFb into the reservoir, and is that chemically accessible?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
HIV-1 Tat displaces HEXIM from the apical stem-loop 1 of 7SK; HEXIM binding locally destabilises the RNA and Tat exploits that destabilisation to bind 7SK more efficiently, with 7SK's conformational plasticity permitting three base-pair configurations at the critical remodelling site.
A structure-based mechanism for displacement of the HEXIM adapter from 7SK small nuclear RNA. · Pham VV, Gao M, Meagher JL, Smith JL, D'Souza VM · Commun Biol 2022What this evidence cannot show
Structures of isolated arginine-rich motifs with an RNA stem-loop rather than of the assembled particle; Tat subtypes G and Fin were used alongside HEXIM. No cellular leukaemia experiment.
Evidence that argues against this (1)
- A purification of Tat-associated factors finds Tat forming a stable complex with a HEXIM-lacking 7SK snRNP rather than simply displacing HEXIM and leaving, so the remodelling may be more extensive than displacement. Sobhian B et al. 2010
- E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
HIV-1 Tat forms two distinct stable complexes: Tatcom1 containing core active P-TEFb, the MLL fusion partners AF9, AFF4, AFF1, ENL and ELL, and the PAF1 complex; and Tatcom2 containing CDK9, cyclin T1 and the 7SK snRNP lacking HEXIM. Tat remodels the 7SK snRNP by interacting directly with 7SK RNA, forming a stress-resistant particle, and the MLL fusion partners and PAF1 are required for Tat transactivation.
HIV-1 Tat assembles a multifunctional transcription elongation complex and stably associates with the 7SK snRNP. · Sobhian B, Laguette N, Yatim A, Nakamura M, Levy Y, Kiernan R, Benkirane M · Mol Cell 2010What this evidence cannot show
Purification from HeLa nuclear extract with tagged Tat. The requirement for MLL fusion partners is for HIV transactivation, not for any leukaemia phenotype.
Evidence that argues against this (1)
- Tatcom2 retaining an intact CDK9/cyclin T1 heterodimer on 7SK is difficult to square with the later finding that released P-TEFb monomerises completely, so the two accounts describe different states of the system. Zhou K et al. 2022
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
AFF1 is required for Tat's extraction of P-TEFb from the 7SK snRNP: it increases Tat's affinity for cyclin T1 through its N-terminal cyclin T1-binding sequence, and the AFF1 M60A/L61A substitution in that sequence abolishes the ability of AFF1(1-308) to enhance the Tat-P-TEFb interaction.
AFF1 is a ubiquitous P-TEFb partner to enable Tat extraction of P-TEFb from 7SK snRNP and formation of SECs for HIV transactivation. · Lu H, Li Z, Xue Y, Schulze-Gahmen U, Johnson JR, Krogan NJ, Alber T, Zhou Q · Proc Natl Acad Sci U S A 2014What this evidence cannot show
HeLa co-IP and transactivation assays with overexpressed constructs. The Tat construct used for the rescue is a C22G mutant, so the measurement is of a binding-defective Tat being rescued rather than of wild-type extraction.
Evidence that argues against this (1)
- Overexpressed fragments in an extract can reconstitute an interaction that does not set the rate in a cell, and the paper does not measure the fraction of cellular P-TEFb actually moved. Schulze-Gahmen U et al. 2013
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.database
The AFF1 module required for Tat-assisted extraction (residues 1-308, minimal 38-72, with M60/L61 critical) lies entirely within AFF1 residues 1-346, which are absent from KMT2A::AFF1 - the fusion's AFF1 portion begins at residue 347 of P51825.
UniProtKB P51825 (AFF1_HUMAN), AF4/FMR2 family member 1, Homo sapiens, 1,210 aa; the fusion junction head peptide QTYSNEVH locates to residues 347-354 of the fetched canonical sequence · UniProt Consortium · Database 2026What this evidence cannot show
A sequence-position argument. It establishes that the module is absent from the fusion protein, not that no other part of the fusion could substitute for it; no experiment has tested whether a KMT2A fusion can extract P-TEFb from 7SK.
Evidence that argues against this (1)
- The fusion heterodimerises with AFF4 and wild-type AFF1 through the retained C-terminal homology domain, so a fusion-containing complex could still carry a CBS contributed in trans - which would preserve the capability at the complex level while losing it at the protein level. Lu H et al. 2014
- E5weak confidence 2 of 5: Preliminary, small, indirect, or not yet peer reviewed.paper
DDX6 binds 7SK snRNA and causes the release and transfer of P-TEFb to the AF4/AF4N super elongation complex, binds stably to AF4 and AF4N, and overexpression of DDX6 or AF4/AF4N raises cellular mRNA production several fold.
DDX6 transfers P-TEFb kinase to the AF4/AF4N (AFF1) super elongation complex. · Muck F, Bracharz S, Marschalek R · Am J Blood Res 2016What this evidence cannot show
A single paper from one laboratory in Am J Blood Res with no DOI; read from the PMC HTML page (PMC5030405) after fullTextXML returned HTTP 500 and efetch db=pmc returned no body. Overexpression in HEK 293T with recombinant GST fusions; no leukaemia transfer experiment; no independent replication found by this beat. It is included as the one cellular candidate for a directed extraction, not as an established mechanism.
Evidence that argues against this (1)
- The mainstream account of release in the same period describes complete monomerisation of the CDK9/cyclin T1 heterodimer followed by reassembly on Brd4 or the SEC, with no role for a helicase. Zhou K et al. 2022
Tat binds 7SK stem-loop 1 and displaces HEXIM to extract P-TEFb
the reservoir-facing half of the same Tat comparison
the AFF1 module Tat requires is the module the fusion loses
- analogous to →Step 3 — The hijack: the fusion drags the super elongation complex to paused RNA polymerase II
Tatcom1 is subunit-for-subunit the SEC: AF9, AFF4, AFF1, ENL, ELL, P-TEFb, PAF1