Disease relevance of CCNT1

• Although overexpression of hCycT1 in NIH3T3 cells strongly enhances transcription from an integrated proviral promoter, we find that this fails to overcome all blocks to productive HIV-1 infection in murine cells [1].
• A highly specific interaction among cyclin T1, the viral protein Tat, and the transactivation response (TAR) element RNA determines the productive transcription of the human immunodeficiency virus genome [2].
• CD4-specific transgenic expression of human cyclin T1 markedly increases human immunodeficiency virus type 1 (HIV-1) production by CD4+ T lymphocytes and myeloid cells in mice transgenic for a provirus encoding a monocyte-tropic HIV-1 isolate [3].
• Interestingly, lipopolysaccharide, a component of the cell wall of gram-negative bacteria, reinduced CycT1 expression late in macrophage differentiation [4].
• These results raise the possibility that regulation of CycT1 expression may be involved in establishing latent infection in macrophages and that opportunistic infection may reactivate the virus by inducing CycT1 expression [4].

High impact information on CCNT1

• Cyclin T is a partner for CDK9, an RNAPII transcription elongation factor [5].
• It binds CycT1 via an alanine-containing heptapeptide repeat and inhibits transcriptional elongation [6].
• Interestingly, we find that the interaction between Tat and hCycT1 requires zinc as well as essential cysteine residues in both proteins [1].
• Alanine-scanning mutagenesis of the hCycT1 TRM identified residues that are critical for the interaction with Tat and others that are required specifically for binding of the complex to TAR RNA [1].
• By binding to the same surface in the cyclin T1, which together with CDK9 forms the positive transcription elongation factor b (P-TEFb) complex, Tat inhibits CIITA [7].

Chemical compound and disease context of CCNT1

• The interaction between HIV-1 Tat and human cyclin T1 requires zinc and a critical cysteine residue that is not conserved in the murine CycT1 protein [1].
• We identified granulin as a cyclin T1-interacting protein that represses expression from the HIV-1 promoter in transfected cells [8].
• Our results suggest that Cdk9/Cyclin T1 complex may be required for neuron differentiation induced by retinoic acid, because the expression level of the complex varies during differentiation, but no significant changes were observed in its expression in the astrocytoma cell line [9].
• Finally, a reciprocal exchange of the valine for the leucine at position 29 in human and equine cyclins T1, respectively, renders the human cyclin T1 active and the equine cyclin T1 inactive for Tat transactivation from EIAV [10].

Biological context of CCNT1

• Collectively, our findings add further support to the OCR strategy for functional inactivation of hetero-associated factors such as the Cdk9/cyclin T1 complex, and highlight a putative function of Cdk9 in cell growth control [11].
• Although studies on mRNA and protein expression have shown that cyclin T1 is ubiquitous in adult human tissues, no data have yet been reported regarding the expression of this protein in different cell lineages [12].
• Here we exploit fluorescence resonance energy transfer (FRET) to demonstrate that cyclin T1 physically interacts in vivo with the promyelocytic leukaemia (PML) protein within specific subnuclear compartments that are coincident with PML nuclear bodies [13].
• The human cyclin T1 (hCycT1) protein from the positive transcription elongation factor b (P-TEFb) binds the transactivator Tat and the transactivation response (TAR) RNA stem loop from human immunodeficiency virus type 1 (HIV) [14].
• In addition, increased HIV-1 transgene expression due to human cyclin T1 expression was associated with increased lipopolysaccharide-stimulated monocyte chemoattractant protein 1 production by JR-CSF mouse monocytes/macrophages in vitro [3].

Anatomical context of CCNT1

• Functional assays in HeLa cells showed that this cyclin T-binding domain (TBD) is required for the binding of Hexim1 to P-TEFb and inhibition of transcriptional activity in vivo [15].
• Induction of TAK (cyclin T1/P-TEFb) in purified resting CD4(+) T lymphocytes by combination of cytokines [16].
• These specific expression patterns are only partially justified by some well-known specialized functions of cyclin T1 in certain cell types, such as its involvement in peripheral blood lymphocytes and monocyte differentiation [12].
• The high expression level found in other tissues suggests new possible roles for cyclin T1 in cell types other than those of lymphoid tissue [12].
• CycT1 mRNA and protein levels are induced in activated human peripheral blood lymphocytes and CycT1 protein levels are induced by a post-transcriptional mechanism when human U937 promonocytic cells are stimulated to differentiate into macrophage-like cells [17].

Associations of CCNT1 with chemical compounds

• The binding affinity between the TBD and cyclin T1 was analyzed with fluorescence spectroscopy methods, using a dansyl-based fluorescence label at position G257C [15].
• The growth factor granulin interacts with cyclin T1 and modulates P-TEFb-dependent transcription [8].
• Granulin bound to the histidine-rich domain of cyclin T1, which was recently found to bind to the CTD, but not to cyclin T2 [8].
• The breast cell growth inhibitor, estrogen down regulated gene 1, modulates a novel functional interaction between estrogen receptor alpha and transcriptional elongation factor cyclin T1 [18].
• Interestingly, cyclin T1 can be re-induced with lipopolysaccharide (LPS) [19].

Physical interactions of CCNT1

• 7SK small nuclear RNA binds to and inhibits the activity of CDK9/cyclin T complexes [20].
• Previously, a histidine-rich stretch in CycT1 was found to bind the CTD of RNAPII and direct the transcriptional activity of this P-TEFb complex when tethered artificially to DNA [21].
• The addition of recombinant cyclin T1 to the selections yielded a phage display system that mirrors all binding properties of the cyclin T1-Tat-TAR complex known from cell assays and biochemical studies [22].
• Tat does so by competing with CIITA for binding to cyclin T1, a component of the transcriptional elongation complex PTEFb [23].

Enzymatic interactions of CCNT1

• Both Cdk9 and cyclin T1 showed only limited colocalization with different phosphorylated forms of RNA polymerase II [24].
• Here we report that CDK9 is ubiquitinated and degraded by the proteasome whereas cyclin T1 is stable [25].
• Here we demonstrate that a Medicago CYCLINT (CYCT) protein is a specific interactor of Ms;CDKC;1 and the interaction between these two proteins gives rise to an active kinase complex that localizes to the nucleus and phosphorylates the carboxy-terminal YSPTSPS heptapeptide repeat domain (CTD) of the largest subunit of RNA polymerase II in vitro [26].

Regulatory relationships of CCNT1

• CDK9 paired with cyclin T1 forms the human P-TEFb complex and stimulates productive transcription through phosphorylation of the RNA polymerase II C-terminal domain [25].
• Here, we report that IL-10 down-regulates cyclin T1 expression through the induction of proteasome-mediated proteolysis in human macrophages [27].

Other interactions of CCNT1

• A homology model of human cyclin T1 generated using the cyclin K structure as a template reveals that the two proteins have similar structures, as expected from their high level of sequence identity [28].
• The active form of the positive transcription elongation factor b (P-TEFb) consists of cyclin T and the kinase Cdk9 [15].
• Deletion mutants at the C-terminal region of cyclin T1 are negative for FRET with PML and fail to localize to nuclear bodies [13].
• Cyclin T1 but not cyclin T2a is induced by a post-transcriptional mechanism in PAMP-activated monocyte-derived macrophages [29].
• The positive transcriptional elongation factor b (P-TEFb), consisting of CDK9 and cyclin T, stimulates transcription by phosphorylating RNA polymerase II [30].

Analytical, diagnostic and therapeutic context of CCNT1

• GST pull-down experiments and size exclusion chromatography exhibit a mutually exclusive binding of the two effectors to cyclin T1 [15].
• Using a polyclonal antiserum raised against cyclin T1, we investigated the pattern of expression of this protein in adult human tissues by immunohistochemistry [12].
• Levels of 7SK snRNA were determined by Northern blotting and levels of the P-TEFb subunits cyclin-dependent kinase 9 and cyclin T1 were analyzed by immunoblotting [31].
• To examine the physical properties of CycT1, we purified a functional domain corresponding to residues 1-272 and found that it possesses a stably folded core, as judged by partial proteolysis and circular dichroism experiments [32].
• Using DNA microarray technology, we found that more than 20% of genes induced by PMA require cyclin T1 for their normal level of induction, and approximately 15% of genes repressed by PMA require cyclin T1 for their normal level of repression [33].

References