Enhanced Activation of Tax-dependent Transcription of Human T-cell Leukemia Virus Type I (HTLV-I) Long Terminal Repeat by TORC3

Hiroshi Koga,Takayuki Ohshima,Kunitada Shimotohno

doi:10.1074/jbc.m409021200

Abstract

Tax, a protein encoded by the env-pX gene of human T-cell leukemia virus type I (HTLV-I), interacts with various host cell transcription factors. Tax activates transcription from the long terminal repeat (LTR) of HTLV-I through association with cyclic AMP-responsive element-binding protein (CREB). Here, we present evidence that transducer of regulated cyclic AMP-response element-binding protein 3 (TORC3), a co-activator of CREB, is involved in Tax-induced transcriptional activation from the HTLV-I LTR. By using a luciferase assay system, we show that TORC3 alone can enhance transcription from the HTLV-I LTR, as well as from a cellular cyclic AMP-response element (CRE). Interestingly, we find that co-expression of TORC3 and Tax dramatically increased transcriptional activation at the HTLV-I LTR. We also show by glutathione S-transferase pull-down and co-immunoprecipitation experiments that TORC3 interacts with Tax. Using deletion mutant analysis, we identify the Tax interaction domain of TORC3 as a region spanning from amino acid 1 to 103, which contains a coiled-coil domain. These results provide important clues toward understanding the molecular mechanism of Tax-dependent transcriptional activation of the HTLV-I LTR.

Highlights

The viral oncoprotein Tax is encoded by the env-pX gene (8) and plays a principal role in the regulation of the proliferation and transformation of human T-cell leukemia virus type I (HTLV-I)-infected T cells
TORC3 Activates Transcription at the HTLV-I long terminal repeat (LTR) as Well as Cellular cyclic AMP-response element (CRE)-containing Promoters—Since TORC3 is a coactivator of the transcription factor cyclic AMP-responsive element-binding protein (CREB) that enhances CREmediated transcription of target genes, we explored the effect of TORC3 on Tax-dependent transcription from the CRE-contain
Co-expression of Tax and TORC3 dramatically enhanced pTxRE-luc expression to over 5,000-fold above background (Fig. 1D). These results indicate that TORC3 alone can enhance transcription from the HTLV-I LTR via the Tax-responsive element (TxRE), Tax is able to synergize with TORC3 to further increase transcription levels

Summary

Introduction

The viral oncoprotein Tax is encoded by the env-pX gene (8) and plays a principal role in the regulation of the proliferation and transformation of HTLV-I-infected T cells. The Tax/CREB complex binds to the Tax-responsive element (TxRE) in the HTLV-I LTR, promoting viral transcription through the recruitment of co-activators such as CBP/p300 and p300/CREB-binding protein-associated factor, a process that does not require phosphorylation of CREB (20 –22). Protein complexes interacting with the flanking sequence in vivo may contain additional factors besides CREB and Tax. For example, we previously identified a cellular protein, TAXREB803/SRL300, which interacts with the 3Ј-flanking region of TxRE (27). We previously identified a cellular protein, TAXREB803/SRL300, which interacts with the 3Ј-flanking region of TxRE (27) This protein enhances Tax-dependent transcription and CREB binding to TxRE in cooperation with Tax. the precise mechanism of how Tax activation of CREB leads to transcriptional activation of the LTR remains to be elucidated. We show that TORC3 interacts with Tax directly via the N-terminal region of TORC3, which contains a coiled-coil domain

Methods

Results

Conclusion