Human T-cell leukemia virus type 1 Tax shuttles between functionally discrete subcellular targets

Area of Science:

• Molecular Biology
• Virology
• Cell Biology

Background:

• Human T-cell leukemia virus type 1 (HTLV-1) Tax protein exhibits pleiotropic functions.
• Tax localizes to distinct nuclear speckled structures (TSS).
• TSS partially overlap with active transcription hot spots.

Purpose of the Study:

• To investigate the role of transcription hot spots as subtargets for HTLV-1 Tax function.
• To determine if Tax localization within TSS influences its transcriptional activity.
• To elucidate the mechanisms regulating Tax's subcellular localization and function.

Main Methods:

• Construction and expression of fusion proteins (Tat-Tax, Rev-Tax) to redirect Tax localization.
• Analysis of protein localization using microscopy.
• Assessment of transcriptional activation of HIV-1 and HTLV-1 LTRs.
• NF-kappaB activation assays.
• Site-directed mutagenesis of nuclear localization and export signals.
• Heterokaryon fusion assays to study protein shuttling.

Main Results:

• Tat-Tax fusion protein localized to transcription subsites and activated both HIV-1 and HTLV-1 LTRs.
• Rev-Tax fusion protein showed predominantly Rev-like localization (nucleolar/cytoplasmic) with reduced TSS localization.
• Rev-Tax exhibited a 10-fold decrease in HTLV-1 LTR activation but retained NF-kappaB activation.
• Mutation of the nuclear localization signal in Rev-Tax restored TSS localization and HTLV-1 LTR activation.
• Tax shuttles between the nucleus and cytoplasm, suggesting compartmental regulation of its functions.

Conclusions:

• Targeting HTLV-1 Tax to transcription hot spots is compatible with its transcriptional regulatory functions.
• NF-kappaB-dependent Tax function does not require localization to transcription hot spots, suggesting a cytoplasmic role.
• Subcellular localization, regulated by protein shuttling, is a key mechanism controlling Tax's pleiotropic activities.