The HBx protein of hepatitis B virus deregulates cell cycle by direct binding to the cyclin E/A-cdk2 complex and potentiating the cdk2 activity leading to degradation of p27kip1.

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The exact role of the X protein of hepatitis B virus (HBx), postulated to be a prime candidate for the virus's ability to deregulate cell cycle and establishing a permissive environment for hepatocarcinogenesis, has remained elusive. Our immunoprecipitation experiments revealed a cell cycle-dependent physical association of HBx with the cyclin E/A-cdk2 complex, through a relatively less conserved central region of HBx (amino acids 85-119). The complexes were formed early during the cell cycle and were sustained longer in the presence of HBx. Use of inhibitors of signaling cascades known to be modulated by HBx revealed a constitutive requirement for Src kinases for the formation of these complexes, while the Ras-Raf-MAPK and PI3K pathways seemed dispensable. The presence of HBx in the complex potentiated the protein kinase activity of cdk2 towards retinoblastoma, providing evidence for early appearance of the S phase - a key determinant of the cell cycle deregulation. The increased cdk2 activity was also resulted in increased post-translational degradation of the cdk inhibitor p27Kip1. In vitro phosphorylation studies combined with co-immunoprecipitation and pulse chase experiments suggested that the HBx presence led to a specific degradation of only the cdk2-bound p27 using the SCF and ubiquitin-proteasomal system without altering either the translational status or cdk4-bound versions of p27. The present study, by demonstrating the ability of HBx to override normal cell cycle restraints, corroborates previous studies and establishes HBx as an oncoprotein