Human TAF(II)28 interacts with the human T cell leukemia virus type I Tax transactivator and promotes its transcriptional activity

The Tax protein encoded by human T cell leukemia virus type I transactivates the viral promoter by forming a complex with several cellular factors bound to three repeats of a specific upstream regulatory sequence. We have shown that transactivation by Tax was correlated with its ability to interact with the C-terminal moiety of the TATA box-binding protein (TBP). In the present study, the ability of Tax to interact with several human TBP-associated factors (TAF(II)s) was analyzed. We show that Tax interacts selectively with hTAF(II)28 in transfected HeLa cells. A direct interaction between Tax and hTAF(II)28 was also observed in vitro with purified proteins. In transient expression studies we show that overexpression of hTAF(II)28 significantly increased transactivation by Tax, both in the absence and in the presence of overexpressed TBP. The ability of hTAF(II)28 to potentiate transactivation correlated with the ability of Tax to interact with hTAF(II)28 and also with the ability of hTAF(II)28 to interact with TBP. Coexpression of TBP and hTAF(II)28 resulted in an additive increase in transactivation by Tax. From these observations we propose that transcriptional activation by Tax involves multiple interactions with TFIID via its TBP and hTAF(II)28 subunits.