Contributes to the regulation of viral RNA transcription by interacting with host proteins involved in transcriptional activation such as ATF4, or CREB1, and by inhibiting their activity. Additionally, HBZ suppresses host NF-kappa-B-driven transcription mediated by host RELA as well as transcription of some classical NF-kappa-B target genes, including IL8, IL2RA, IRF4, VCAM1, and VEGFA. Interacts with host ATF4; this interaction inhibits viral RNA transcriptional activation by preventing ATF4 binding to Tax-responsive elements. Interacts with host CREB1; this interaction inhibits host CREB1 transcriptional activity. Interacts with host JUN, JUNB and JUND. Interacts with host EP300. Contains three nuclear localization signals NLS-1 and NLS-2, corresponding to two regions rich in basic amino acids, and NLS-3 corresponding to its DNA-binding domain. Expressed from antisense transcripts. While mRNA expression levels of HBZ-SI were similar to those of HBZ, HBZ-SI seems slightly more expressed at the protein level ex vivo. Belongs to the HTLV-1 HBZ protein family.