X Function

Hepatitis B virus (HBV) infects 300 million people worldwide and causes liver disease and cancer. The X-protein of HBV is essential for viral infection and has been implicated in carcinogenesis, but its exact role has been enigmatic. It is known to infiltrate cell signaling pathways and to activate modest transcription from various promoters, as well as strongly activate viral replication in certain cell lines. The X-protein activates Src without interacting directly with Src. Bouchard et al. (see the Perspective by Ganem) have now discovered that this activation is mediated the activation of another kinase called Pyk. The activation of Pyk is caused by a release of calcium from intracellular stores (most likely the mitochondrion) triggered by X-protein. M. J. Bouchard, L.-H. Wang, R. J. Schneider, Calcium signaling by HBx protein in hepatitis B virus DNA replication. Science 294, 2376-2378 (2001). [Abstract] [Full Text] D. Ganem, The X files--one step closer to closure. Science 294, 2299-2300 (2001). [Full Text]