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Cyclin T1 Acetylation Regulates P-TEFb
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Turning a critical cofactor for HIV transcription on and off: P-TEFb is regulated by
reversible acetylation
Melanie Ott, M.D., Ph.D.
Inside each cell, RNA polymerase II does an essential job. It copies instructions from genes
into RNA, a process called transcription. HIV uses the same process to transcribe its own
genome, a key step in its life cycle. Instead of faithfully copying the entire HIV genome into
RNA, however, RNA polymerase II tends to stall and fall off soon after the start of
transcription. To solve this problem, HIV mobilizes a cellular factor that enhances
transcription elongation, the positive transcription elongation factor b. P-TEFb exists in two
forms in cells. An inactive form (large complex) comprises the core components cyclin T1 and
CDK9 and inhibitory factor Hexim1 and 7SK small nuclear (sn) RNA. An active form (small
complex) includes only cyclin T1 and CDK9. How the two complexes are established and maintained
is the subject of intense research.
We found that partitioning the two complexes critically depends on acetylation of cyclin T1.
Cyclin T1 acetylation triggers dissociation of Hexim1 and 7SK snRNA from the large complex and
activates the transcriptional activity of P-TEFb. This activation is lost in P-TEFb complexes
with cyclin T1 that cannot be acetylated. We identified four acetylation sites in cyclin T1 and
generated acetylation-specific antibodies against each site. In fractionation experiments, we
showed that acetylated cyclin T1 accumulates exclusively in the small complex, confirming that
acetylation is required for active P-TEFb.
P-TEFb is essential for transcription of HIV and most cellular genes. Accordingly, we found
that acetylation-deficient cyclin T1 mutants dominantly suppress activation of the
interleukin-8 gene, an important target of P-TEFb. However, we uncovered a surprising twist.
While basal activity of the HIV promoter requires acetylated cyclin T1, expression of the viral
Tat protein alleviates this dependency. Tat binds directly to cyclin T1 and recruits P-TEFb
directly from the nonacetylated/inactive complex to the HIV promoter instead of competing with
cellular factors for the restricted pool of acetylated/active P-TEFb. These findings support
the model that acetylation of cyclin T1 serves as an important intracellular switch that
liberates P-TEFb from its physiological inhibitors Hexim1 and 7SK snRNA, but is not required
for the cooperative action with HIV Tat.
Cho S, Schroeder S, Kaehlcke K, Kwon H-S, Pedal A, Herker E, Schnoelzer M, Ott (2009)
Acetylation of cyclin T1 regulates the equilibrium between active and inactive P-TEFb in cells.
EMBO J. 28:1407-1417.
Kwon HS, Brent MM, Getachew R, Jayakumar P, Chen L-F, Schnoelzer M, McBurney MW,
Marmorstein R, Greene WC, Ott M (2008) Human immunodeficiency virus type 1 Tat protein inhibits
the SIRT1 deacetylase and induces T-cell hyperactivation. Cell Host Microbe
3:158–167.
Kwon H-S, Ott M (2008) The ups and downs of SIRT1. Trends Biochem. Sci.
33:517–525.
Nakamura K, Nemani VM, Wallender EK, Kaehlcke K, Ott M, Edwards RH (2008) Optical reporters for
the conformation of α-synuclein reveal a specific interaction with mitochondria. J.
Neurosci. 28:12305–12317.
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A balance of cyclin T1 acetylation and deacetylation regulates
the association of P-TEFb with inhibitory factors Hexim1 and 7SK snRNA. Cyclin T1 is acetylated
by p300 at four lysines in the central portion of the protein. This acetylation dissociates
Hexim1 and 7SK RNA and activates P-TEFb. |
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