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HBx is a hepatitis B viral protein. It is 154 amino acids long and interferes with transcription, signal transduction, cell cycle progress, protein degradation, apoptosis and chromosomal stability in the host. It forms a heterodimeric complex with its cellular target protein (HBX interacting protein: HBXIP), and this interaction dysregulates centrosome dynamics and mitotic spindle formation. It interacts with DDB1 (Damaged DNA Binding Protein 1) redirecting the ubiquitin ligase activity of the CUL4-DDB1 E3 complexes, which are intimately involved in the intracellular regulation of DNA replication and repair, transcription and signal transduction.
Although Protein X is normally absent in the Avihepadnavirus, a vestigial version has been identified in the duck hepatitis virus genome.
Although it lacks significant sequence identity with any known vertebrate proteins, it seems likely that it evolved from a DNA glycosylase.
Transgenic mice expressing the X protein in liver are more likely than the wild type to develop hepatocellular carcinoma. This is because the X protein promotes cell cycle progression while binding to and inhibiting tumor suppressor protein p53 from performing their role. Experimental observations also suggest that HBx protein increases TERT and telomerase activity, prolonging the lifespan of hepatocytes and contributing to malignant transformation.
Relation to PRMT1
In a study purifying cancerous liver cells infected with HBV, the level of expression of protein arginine methyltransferase 1 (PRMT1) was found to be associated with changes in transcription due to the methyltransferase function of PRMT1. Overexpression causes a reduction in the number of HBV genes transcribed, while conversely, underexpression causes an increase. PRMT1 was also found to be recruited by HBV DNA during the replication process to regulate the transcription process. Increased HBx expression in turn leads to an inhibition of PRMT1-mediated protein methylation, benefiting viral replication.