How P-TEFb gets out of the reservoir: a dephosphorylation, a dissociation, and a reassembly on whichever scaffold is waiting
also called P-TEFb release · 7SK snRNP disassembly · free versus sequestered P-TEFb
Cells keep a large stock of the elongation enzyme locked away and let it out when they need to switch genes on fast. Getting it out is not one step: the enzyme is chemically untagged, the particle falls apart, the two halves of the enzyme separate, and then they are rebuilt on whichever scaffold grabs them first.
The same thing, for a scientist
Release is a sequence, not an event. (1) **Untagging.** CDK9 T-loop phosphorylation is required for capture; stress-induced release is caused by dephosphorylation of P-TEFb, not by a change in 7SK [E1]. (2) **Handover of the RNA.** hnRNPs Q, R, A1 and A2 take over 7SK as P-TEFb and HEXIM1 leave, and knocking down hnRNP A1 and A2 attenuates the release, so the RNA does not sit empty [E2]. (3) **Monomerisation.** The released CDK9/cyclin T1 heterodimer dissociates completely into monomers under stress [E3]. (4) **Reassembly on a scaffold.** Brd4 or the SEC recruits the monomers and rebuilds active P-TEFb, with T186 autophosphorylation induced by that binding; the same route operates on exit from nocodazole arrest into early G1 [E3]. (5) **Feedback.** Release induces HEXIM1 transcription from an unannotated proximal promoter, and that induction itself requires the SEC subunits AFF4 and ELL2 [E4]. A directed extraction mechanism exists but only one protein is known to do it: HIV-1 Tat displaces HEXIM from 7SK stem-loop 1, exploiting the local destabilisation HEXIM itself creates, and AFF1's cyclin T1-binding sequence raises Tat's affinity for cyclin T1 to make the extraction efficient [E5][E6]. **No cellular protein has been established to extract P-TEFb from the reservoir the way Tat does** - the single candidate, DDX6, rests on one unreplicated paper and is carried at weak confidence in `gene-ddx6` - **and nothing in this pathway has been measured in a leukaemia cell.**
Why 'the SEC extracts P-TEFb from 7SK' is not supported as written
It is an appealing sentence and this graph should not assert it. What is established is that the SEC is a destination: Brd4 and the SEC capture released P-TEFb monomers and reassemble them 3. What is not established is that the SEC causes release. The one well-replicated directed extraction on record is Tat's, and Tat works by binding 7SK RNA directly at stem-loop 1 to displace HEXIM 5 - a capability no SEC subunit has been shown to have. One cellular candidate exists, the helicase DDX6, on the strength of a single unreplicated paper (gene-ddx6).
The nearest thing to a cellular extraction mechanism is thermodynamic rather than enzymatic: SEC components concentrate P-TEFb out of the soluble HEXIM1-containing pool by multivalent condensation 7. That is a real proposal and it is treated in mechanism-sec-condensates-extract-ptefb-from-the-reservoir, with the reasons to be careful about condensate evidence.
The homeostat is the reason single interventions here tend to fail
Three independent negative feedbacks operate on this axis:
- Release induces HEXIM1 transcription, which rebuilds the reservoir 4.
- HEXIM2 compensates quantitatively for HEXIM1 loss to hold bound P-TEFb constant 8.
- Dismantling the core (LARP7 or 7SK knockdown) lowers total P-TEFb protein after transiently raising the free pool 9.
Any agent that perturbs the partition is therefore acting against a system built to restore it, and the sign of its effect depends on the timescale observed. Every compound known to release P-TEFb - HMBA, SAHA, JQ1 - produces a transient rise followed by reassembly 10.
| mechanism class | regulated complex disassembly and reassembly |
|---|---|
| only directed extractor known | HIV-1 Tat (well replicated); DDX6 proposed as a cellular equivalent in one unreplicated paper (PMID 27679741) |
| measured in leukaemia | false |
steps
[ "CDK9 T186 dephosphorylation removes the capture tag", "HEXIM1/2 and P-TEFb leave 7SK; hnRNPs Q/R/A1/A2 take over the RNA", "CDK9 and cyclin T1 dissociate into monomers", "Brd4 or the SEC recruits the monomers and reassembles active P-TEFb with T186 autophosphorylation", "release induces HEXIM1 transcription via an AFF4/ELL2-dependent proximal promoter, rebuilding the reservoir" ]
homeostatic feedbacks
[ "HEXIM1 transcriptional autoinduction", "HEXIM2 quantitative compensation", "core loss lowers total P-TEFb" ]
related entries
[ "protein-7sk-snrnp", "gene-cdk9", "gene-hexim1", "mechanism-sec-elongation-hijack", "localization-nucleolar-retention-versus-sec-droplets-tat-comparator", "gene-ddx6" ]
- Every known way of releasing P-TEFb is transient and is followed by reassembly driven by HEXIM1 autoinduction, so a single dose moves the partition and then the cell moves it back.
- Dismantling the reservoir lowers total P-TEFb on a longer timescale than it raises the free pool, so the same intervention has opposite signs at different times.
- The release mechanism is global. Nothing in it is specific to a fusion-driven gene, so an agent acting here changes elongation everywhere in the cell.
- Is the free-versus-sequestered P-TEFb ratio different in KMT2A-rearranged blasts? The measurement is a glycerol gradient and has never been run in this disease.
- Does any cellular protein extract P-TEFb from 7SK the way Tat does? The single candidate, DDX6, is unreplicated, so twenty-five years after 7SK was assigned a function Tat remains the only well-supported example.
- Does an intact KMT2A fusion alter the HEXIM1 autoinduction loop, given that the loop runs through AFF4 and ELL2 - two proteins the fusion competes for?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
CDK9 T-loop phosphorylation is required for the 7SK/P-TEFb interaction, and stress-induced disruption is caused by dephosphorylation of P-TEFb rather than by a prohibitive change in 7SK.
Phosphorylated positive transcription elongation factor b (P-TEFb) is tagged for inhibition through association with 7SK snRNA. · Chen R, Yang Z, Zhou Q · J Biol Chem 2004What this evidence cannot show
Reconstituted in vitro system plus in vivo binding assays in HeLa; the responsible kinase was unidentified.
Evidence that argues against this (1)
- The phosphorylation state is also a consequence of complex membership, since T186 autophosphorylation follows rebinding to Brd4 or the SEC, so the causal arrow is not one-directional. Zhou K et al. 2022
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
hnRNPs Q, R, A1 and A2 are major 7SK-associated proteins whose association increases when P-TEFb and HEXIM1 are released, and siRNA knockdown of hnRNP A1 and A2 attenuates the transcription-dependent dissociation of the P-TEFb-HEXIM1-7SK complex.
The transcription-dependent dissociation of P-TEFb-HEXIM1-7SK RNA relies upon formation of hnRNP-7SK RNA complexes. · Barrandon C, Bonnet F, Nguyen VT, Labas V, Bensaude O · Mol Cell Biol 2007What this evidence cannot show
Proteomics plus siRNA in human cell lines; the trapping model is inferred from association kinetics.
Evidence that argues against this (1)
- An alternative reading is that hnRNP loading follows release rather than driving it; the knockdown result constrains but does not settle the order. Guo C et al. 2020
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
The CDK9/cyclin T1 heterodimer released from the 7SK snRNP dissociates completely into monomers in response to stress; Brd4 or the SEC then recruits the monomers and reassembles core P-TEFb, and binding of monomeric dephosphorylated CDK9 to Brd4 or the SEC induces autophosphorylation of CDK9 T186. The same mechanism operates on nocodazole-released entry into early G1.
Disrupting the Cdk9/Cyclin T1 heterodimer of 7SK snRNP for the Brd4 and AFF1/4 guided reconstitution of active P-TEFb. · Zhou K, Zhuang S, Liu F, Chen Y, Li Y, Wang S, Li Y, Wen H, Lin X, Wang J, Huang Y, He C, et al · Nucleic Acids Res 2022What this evidence cannot show
Cell-line biochemistry with stress and cell-cycle synchronisation; monomerisation is inferred from fractionation and binding rather than from a single-molecule measurement. No leukaemia cell.
Evidence that argues against this (1)
- Other work treats the CDK9-cyclin T-AFF1 unit as transferred intact between complexes of the P-TEFb network, which is difficult to reconcile with complete monomerisation and indicates the two models describe different conditions. Lu H et al. 2014
- E4established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Release of P-TEFb from the 7SK snRNP increases HEXIM1 synthesis from an unannotated proximal promoter carrying P-TEFb-sensitive poised Pol II, and the induction requires recruitment of the SEC subunits AFF4 and ELL2.
Release of positive transcription elongation factor b (P-TEFb) from 7SK small nuclear ribonucleoprotein (snRNP) activates hexamethylene bisacetamide-inducible protein (HEXIM1) transcription. · Liu P, Xiang Y, Fujinaga K, Bartholomeeusen K, Nilson KA, Price DH, Peterlin BM · J Biol Chem 2014What this evidence cannot show
HeLa cells; driven with P-TEFb-releasing compounds rather than a physiological signal, and HEXIM2 is not induced.
Evidence that argues against this (1)
- If the feedback runs through the SEC, then a cell whose SEC is occupied by a KMT2A fusion might feed back differently - a prediction nobody has tested. Liu P et al. 2014
- E5strong 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, with the conformational plasticity of 7SK permitting three base-pair configurations at a 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, not of the assembled particle. A viral protein, not a cellular one.
Evidence that argues against this (1)
- A separate purification finds Tat forming a stable complex with a HEXIM-lacking 7SK snRNP rather than simply displacing HEXIM and leaving, so 'displacement' may understate what Tat does to the particle. Sobhian B et al. 2010
- E6established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
By increasing Tat's affinity for cyclin T1, AFF1 is required for Tat's extraction of P-TEFb from the 7SK snRNP and for formation of Tat-SECs; the AFF1 M60A/L61A substitution in the cyclin T1-binding sequence abolishes the ability 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 requirement is for HIV transactivation, not for any cellular gene.
Evidence that argues against this (1)
- The region of AFF1 that carries this function is absent from KMT2A::AFF1, so whatever extraction capability AFF1 confers, the fusion does not carry it. UniProt Consortium 2026
- E7moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
SEC components compartmentalise and concentrate P-TEFb by liquid-liquid phase separation away from the soluble inactive HEXIM1-containing P-TEFb complex, with ENL or its intrinsically disordered region sufficient to initiate droplet formation and AFF4 fluidising the droplets; fusion of ENL with MLL boosts SEC phase separation.
ENL initiates multivalent phase separation of the super elongation complex (SEC) in controlling rapid transcriptional activation. · Guo C, Che Z, Yue J, Xie P, Hao S, Xie W, Luo Z, Lin C · Sci Adv 2020What this evidence cannot show
Largely overexpression imaging with droplet assays. See mechanism-sec-condensates-extract-ptefb-from-the-reservoir for why the two commonest condensate assays cannot separate phase separation from binding to clustered sites.
Evidence that argues against this (1)
- At physiological expression, low-complexity-domain interactions that control transcription form dynamic hubs without detectable phase separation, so a punctate appearance in an overexpression system is weak evidence about the endogenous protein. Chong S et al. 2018
- E8established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
HEXIM2 compensates functionally and quantitatively for HEXIM1 loss to maintain a constant level of 7SK/HEXIM-bound P-TEFb.
Compensatory contributions of HEXIM1 and HEXIM2 in maintaining the balance of active and inactive positive transcription elongation factor b complexes for control of transcription. · Yik JH, Chen R, Pezda AC, Zhou Q · J Biol Chem 2005What this evidence cannot show
Knockdown in HeLa; measured as the bound fraction, not as a phenotype.
Evidence that argues against this (1)
- In MOLM-13 the buffering is plainly incomplete, since HEXIM1 knockdown raises c-Myc and growth, so the compensation is partial in at least one KMT2A-rearranged cell. Yik JH et al. 2005
- E9established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Knockdown of LARP7 or 7SK initially increases free P-TEFb and ultimately decreases total P-TEFb protein levels, so free P-TEFb is set by a balance between release from the large form and loss of total P-TEFb.
LARP7 is a stable component of the 7SK snRNP while P-TEFb, HEXIM1 and hnRNP A1 are reversibly associated. · Krueger BJ, Jeronimo C, Roy BB, Bouchard A, Barrandon C, Byers SA, Searcey CE, Cooper JJ, Bensaude O, Cohen EA, Coulombe B, Price DH · Nucleic Acids Res 2008What this evidence cannot show
siRNA in human cell lines; timescales are those of the experiment and may not map onto a drug exposure.
Evidence that argues against this (1)
- An independent group reports the same knockdown as increasing Pol II transcription, which is the early-timepoint reading of the same manipulation. Markert A et al. 2008
- E10established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
HMBA, SAHA and other HDAC inhibitors, and the BET inhibitor JQ1, all cause a rapid, transient release of free P-TEFb from the 7SK snRNP followed by reassembly, with HEXIM1 induction as a consequence; the effects of JQ1 and of HMBA or SAHA on the P-TEFb equilibrium are cooperative.
Bromodomain and extra-terminal (BET) bromodomain inhibition activate transcription via transient release of positive transcription elongation factor b (P-TEFb) from 7SK small nuclear ribonucleoprotein. · Bartholomeeusen K, Xiang Y, Fujinaga K, Peterlin BM · J Biol Chem 2012What this evidence cannot show
HeLa and Jurkat in an HIV-latency framework; the transcriptional readout is HIV.
Evidence that argues against this (1)
- In cells with very low P-TEFb the same agents do almost nothing until P-TEFb is first raised, so the size of the release depends on the pool, which has never been measured in a KMT2A-rearranged blast. Bartholomeeusen K et al. 2013
the disassembly/reassembly cycle that sets the free P-TEFb pool
T186 phosphorylation is the capture tag; dephosphorylation releases
- related to →Step 3 — The hijack: the fusion drags the super elongation complex to paused RNA polymerase II
the SEC is a destination for released P-TEFb; it has not been shown to cause release
Brd4 reassembles released CDK9 and cyclin T1 monomers into active P-TEFb
the only cellular candidate for a directed extraction step
the only cellular candidate for a directed extraction step
gliotoxin-LARP7 is a chemical instance of extraction from the reservoir, in an HIV-latency system