CLONING OF NOVEL ISOFORMS OF THE HUMAN GLI2 ONCOGENE AND THEIR ACTIVITIES TO ENHANCE TAX-DEPENDENT TRANSCRIPTION OF THE HUMAN T-CELL LEUKEMIA-VIRUS TYPE-1 GENOME

The expression of human T-cell leukemia virus type 1 (HTLV-1) is activ ated by interaction of a viral transactivator protein, Tax, and cellul ar transcription factor, CREB (cyclic AMP response element binding pro tein), which bind to a 21-bp enhancer in the long terminal repeats (LT R), THP (Tax-helping protein) was previously determined to enhance the transactivation by Tax protein. Here we report novel forms of the hum an homolog of a member of the Gli oncogene family, Gli2 (also termed G li2/THP), an extended form of a zinc finger protein, THP, which was de scribed previously. Four possible isoforms (hGli2 alpha, beta, gamma, and delta) are formed by combinations of two independent alternative s plicings, and all the isoforms could bind to a DNA motif, TRE2S, in th e LTR, The longer isoforms, alpha and beta, were abundantly expressed in various cell lines including HTLV-1-infected T-cell lines. Fusion p roteins of the hGli2 isoforms with the DNA-binding domain of Gal4 acti vated transcription when the reporter contained a Gal4-binding site an d one copy of the 21-bp sequence, to which CREB binds. This activation was observed only in the presence of Tax. The 21-bp sequence in the r eporter was also essential for the activation. These results suggest t hat simultaneous binding of hGli2 and CREB to the respective sites in the reporter seems to be critical for Tax protein to activate transcri ption. Consequently, it is probable that the LTR can be regulated by t wo independent signals through hGli2 and CREB, since the LTR contains the 21-bp and TRE2S sequences in the vicinity.