Modulation of the Brd4/P-TEFb Interaction by the Human T-Lymphotropic Virus Type 1 Tax Protein

Abstract

Positive transcription elongation factor (P-TEFb), which is composed of CDK9 and cyclin T1, plays an important role in cellular and viral gene expression. Our lab has recently demonstrated that P-TEFb is required for Tax transactivation of the viral long terminal repeat (LTR). P-TEFb is found in two major complexes: the inactive form, which is associated with inhibitory subunits 7SK snRNA and HEXIM1, and the active form, which is associated with, at least in part, Brd4. In this study, we analyzed the effect of Brd4 on human T-lymphotropic virus type 1 (HTLV-1) transcription. Overexpression of Brd4 repressed Tax transactivation of the HTLV-1 LTR in a dose-dependent manner. In vitro binding studies suggest that Tax and Brd4 compete for binding to P-TEFb through direct interaction with cyclin T1. Tax interacts with cyclin T1 amino acids 426 to 533, which overlaps the region responsible for Brd4 binding. In vivo, overexpression of Tax decreased the amount of 7SK snRNA associated with P-TEFb and stimulates serine 2 phosphorylation of the RNA polymerase II carboxyl-terminal domain, suggesting that Tax regulates the functionality of P-TEFb. Our results suggest the possibility that Tax may compete and functionally substitute for Brd4 in P-TEFb regulation.Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL) (37, 50, 51), chronic diseases such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) (13, 32, 35), HTLV-1-associated arthropathy (7,22,34,45), uveitis (31), and Sjögren's syndrome (44). The transactivator Tax protein, encoded by the pX region of HTLV-1, is essential for viral replication, transformation, and gene regulation (2,4,10,15,21,33,42). Three highly conserved 21-bp repeat elements located within the long terminal repeat (LTR), Tax-responsive elements (TRE), are critical for Tax-mediated viral transcriptional activation (5,9,40,41,50). Association of Tax with CREB on the HTLV-1 LTR (1, 43, 52) facilitates recruitment of cellular cofactors CBP (14, 17, 23, 52), p300 (16, 52), and PCAF (19), thereby mediating the activation of HTLV-1 gene expression. Tax also regulates the binding of histone deacetylases, a repressor of transactivation, from the HTLV-1-LTR promoter (25, 27).P-TEFb is a protein kinase involved in RNA polymerase (Pol) II elongation of many genes in mammalian cells (39). P-TEFb is composed of catalytic subunit CDK9 and either regulatory subunit cyclin T1, -T2, or -K (11, 36). The transcriptional activity of P-TEFb depends on the CDK9 kinase activity, which hyperphosphorylates the Ser 2 residue of the carboxylterminal domain (CTD) of the largest subunit of RNA Pol II (28, 53). P-TEFb also phosphorylates negative elongation factors DSIF and NELF, which cooperatively inhibit RNA Pol II processivity, to stimulate the elongation of transcription (3,12,38,39). In a previous study, our lab found that CDK9 was an essential factor for transactivation of the HTLV-1 LTR by Tax (54). Our results further suggested that P-TEFb was recruited to the...

The Interaction of the Gammaherpesvirus 68 orf73 Protein with Cellular BET Proteins Affects the Activation of Cell Cycle Promoters

“…A peptide derived from the C-terminal domain of Brd4 L can inhibit Tat-mediated transcription (7). Similarly, HTLV-I Tax and Brd4 compete for binding to pTEF (10). In this work, we demonstrated that MHV-68, the only available small-animal model for gamma-2 herpesvirus infection and pathogenesis, also interacts with the BET proteins Brd2, Brd3, and Brd4 via its orf73 protein, the LANA-1 homolog.…”

“…There are a number of different ways in which pTEFb can be recruited to transcription complexes, and the recruitment via the chromatin-bound activator Brd4 is one of them (46). HTLV-1 has recently been shown to modulate the Brd4/pTEFb interaction through the interaction of its Tax protein with the cyclin T1 subunit of pTEFb, thereby possibly competing with the Brd4/pTEFb interaction (10). Similarly, Brd4…”

“…Cellular BET proteins are important interaction partners for three families of DNA viruses, papillomaviruses, gamma-2 herpesviruses, cytomegalovirus, and EBV, as well as two human retroviruses, HIV-1 and human T-lymphotropic virus type I (HTLV-1) (7,10,31,34,37,40,41,45,47,54,58,63,64). Papillomaviruses target Brd4 L via their E2 proteins (63), and the gamma-2 herpesvirus KSHV has so far been shown to interact with Brd2, Brd3, and Brd4 via its LANA-1 protein (45,65).…”

The Brd4 Extraterminal Domain Confers Transcription Activation Independent of pTEFb by Recruiting Multiple Proteins, Including NSD3

“…Several different viruses target the individual BET proteins for a variety of purposes but often to regulate viral and cellular transcription (4,7,31,37,41,45,57,60). The papillomavirus E2 proteins bind to Brd4, and some utilize this interaction in tethering the viral genomes to mitotic chromosomes (1,3,57,59).…”

Recruitment of cdk9 to the Immediate-Early Viral Transcriptosomes during Human Cytomegalovirus Infection Requires Efficient Binding to Cyclin T1, a Threshold Level of IE2 86, and Active Transcription

“…In addition, cdk9 plays an integral role in p53 phosphorylation during Kaposi's sarcoma-associated herpesvirus infection via cdk9 interaction with a virus-encoded cyclin (11). P-TEFb activity is also modulated through Brd4 during human T-cell leukemia virus infection (12,55).…”