The Tax protein from human T-cell leukemia virus type I (HTLV-1) is a 40- kDa phosphoprotein capable of activating transcription from its own long terminal repeat (LTR), as well as increasing the transcription of cellular genes. Transcriptional activation of the HTLV-1 LTR has been demonstrated via a cyclic-AMP-responsive element within the 21-bp Tax-responsive elements of the LTR. Phorbol esters also upregulate expression via the LTR. Since phosphorylation of Tax may play a role in these processes, we investigated the relative effects of kinase-stimulating agents on 32P incorporation into Tax. Our studies demonstrated that the phorbol ester 4β-phorbol-12β- myristate-13α-acetate greatly stimulated Tax phosphorylation in a time- and dose-dependent manner. In contrast, 8-bromoadenosine 3'-5'-cyclic monophosphate induced little stimulation of Tax phosphorylation. Tax phosphorylation occurred only on serine residues and was mapped to a single tryptic fragment in both Tax-producing human lymphocytes and mouse fibroblast cells.
ASJC Scopus subject areas
- Insect Science