Intracellular localization of the hepatitis B virus HBx protein.
The hepatitis B virus (HBV) X protein (HBx) was originally suggested to be a viral transcriptional activator, but its functional mechanisms are still unclear. In this study we have analysed the intracellular localization of HBx in transfected cells and demonstrate that its compartmentalization is dependent on overall expression levels. HBx was exclusively or predominantly localized in the nuclei in weakly expressing cells. However, elevated cellular levels correlated with its accumulation in the cytoplasm, suggesting that the capacity of HBx for nuclear compartmentalization might be limited. Cytoplasmic HBx was detected either as punctate granular staining or in dispersed, finely granular patterns. We have further analysed the detailed cytoplasmic compartmentalization, using confocal microscopy, and show no association with the endoplasmic reticulum, plasma membrane or lysosomes, but a substantial association of HBx with mitochondria. However, a major fraction of cytoplasmic HBx did not localize in mitochondria, indicating the presence of two distinctly compartmentalized cytoplasmic populations. Furthermore, high levels of HBx expression led to an abnormal mitochondrial distribution, involving clumping and organelle aggregation, which was not observed at lower expression levels. The data presented here provide novel insights into the compartmentalization of HBx and may prove important for future evaluations of its functions, both in the viral life-cycle and in the pathology of HBV-related liver disease.
National Institute for Medical Research, Division of Membrane Biology, The Ridgeway, Mill Hill, London NW7 1AA, UK1.
Frank
F
Henkler
Department of Medicine2 and Department of Histopathology5, Imperial College School of Medicine, St Mary's Campus, South Wharf Road, London W2 INY, UK.
Jonathan
J
Hoare
Division of Medical and Molecular Genetics, Guy's, King's and St Thomas's School of Medicine, London SE1 9RT, UK4.
Naushin
N
Waseem
Department of Medicine2 and Department of Histopathology5, Imperial College School of Medicine, St Mary's Campus, South Wharf Road, London W2 INY, UK.
Robert D
RD
Goldin
Department of Medicine2 and Department of Histopathology5, Imperial College School of Medicine, St Mary's Campus, South Wharf Road, London W2 INY, UK.
Michael J
MJ
McGarvey
National Institutes of Health, Hepatitis Viruses Section, LID, NIAID, Building 7, Room 206, Bethesda, MD 20892, USA3.
Rajen
R
Koshy
National Institute for Medical Research, Division of Membrane Biology, The Ridgeway, Mill Hill, London NW7 1AA, UK1.
Ian A
IA
King
D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal / Q000276:immunology; D002449:Cell Aggregation; D002460:Cell Line; D002467:Cell Nucleus / Q000737:chemistry; D003593:Cytoplasm / Q000737:chemistry; D000939:Epitopes; D005455:Fluorescent Antibody Technique; D006801:Humans; D008928:Mitochondria / Q000737:chemistry; D008969:Molecular Sequence Data; D015534:Trans-Activators / Q000032:analysis* / Q000276:immunology / Q000502:physiology; D014162:Transfection; D054334:Viral Regulatory and Accessory Proteins
D000911:Antibodies, Monoclonal; D000939:Epitopes; D015534:Trans-Activators; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein