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J Virol. 1993 July; 67(7): 4436-4441

Phorbol esters modulate the phosphorylation of human T-cell leukemia virus type I Tax.

Department of Pathology, University of Nebraska Medical Center, Omaha 68198-6495.

ABSTRACT

The Tax protein from human T-cell leukemia virus type I (HTLV-I) 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-I 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 4 beta-phorbol-12 beta-myristate-13 alpha-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.

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