The HBZ-SP1 isoform of human T-cell leukemia virus type I represses JunB activity by sequestration into nuclear bodies.
BACKGROUND
The human T-cell leukemia virus type I (HTLV-I) basic leucine-zipper factor (HBZ) has previously been shown to modulate transcriptional activity of Jun family members. The presence of a novel isoform of HBZ, termed HBZ-SP1, has recently been characterized in adult T-cell leukemia (ATL) cells and has been found to be associated with intense nuclear spots. In this study, we investigated the role of these nuclear bodies in the regulation of the transcriptional activity of JunB.


RESULTS
Using fluorescence microscopy, we found that the HBZ-SP1 protein localizes to intense dots corresponding to HBZ-NBs and to nucleoli. We analyzed the relative mobility of the EGFP-HBZ-SP1 fusion protein using fluorescence recovery after photobleaching (FRAP) analysis and found that the deletion of the ZIP domain perturbs the association of the HBZ-SP1 protein to the HBZ-NBs. These data suggested that HBZ needs cellular partners, including bZIP factors, to form HBZ-NBs. Indeed, by cotransfection experiments in COS cells, we have found that the bZIP factor JunB is able to target delocalized form of HBZ (deleted in its nuclear localization subdomains) into the HBZ-NBs. We also show that the viral protein is able to entail a redistribution of JunB into the HBZ-NBs. Moreover, by transfecting HeLa cells (known to express high level of JunB) with a vector expressing HBZ-SP1, the sequestration of JunB to the HBZ-NBs inhibited its transcriptional activity. Lastly, we analyzed the nuclear distribution of HBZ-SP1 in the presence of JunD, a Jun family member known to be activated by HBZ. In this case, no NBs were detected and the HBZ-SP1 protein was diffusely distributed throughout the nucleoplasm.


CONCLUSIONS
Our results suggest that HBZ-mediated sequestration of JunB to the HBZ-NBs may be causing the repression of JunB activity in vivo.
Hivin
Patrick
P
Laboratoire Infections Rétrovirales et Signalisation Cellulaire, CNRS/UM I UMR 5121/IFR 122, Institut de Biologie, 34000 Montpellier, France. patrick.hivin@univ-montp1.fr
Basbous
Jihane
J
Raymond
Frédéric
F
Henaff
Daniel
D
Arpin-André
Charlotte
C
Robert-Hebmann
Véronique
V
Barbeau
Benoit
B
Mesnard
Jean-Michel
JM
D000818:Animals; D050976:Basic-Leucine Zipper Transcription Factors / Q000502:physiology*; D019556:COS Cells; D002466:Cell Nucleolus / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism*; D002522:Chlorocebus aethiops; D006367:HeLa Cells; D015368:Human T-lymphotropic virus 1 / Q000472:pathogenicity*; D006801:Humans; D007120:Immunochemistry; D045586:Intranuclear Inclusion Bodies / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D020033:Protein Isoforms / Q000502:physiology; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism*; D012191:Retroviridae Proteins; D014764:Viral Proteins / Q000502:physiology*
D050976:Basic-Leucine Zipper Transcription Factors; C479751:HBZ protein, human T-cell leukemia virus type I; D020033:Protein Isoforms; D016755:Proto-Oncogene Proteins c-jun; D012191:Retroviridae Proteins; D014764:Viral Proteins