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. 1993 Jul;67(7):4436–4441. doi: 10.1128/jvi.67.7.4436-4441.1993

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

J D Fontes 1, J M Strawhecker 1, N D Bills 1, R E Lewis 1, S H Hinrichs 1
PMCID: PMC237821  PMID: 8510230

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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