We are also grateful for helpful discussions with Kenneth Rock, Jay Hoofnagle, Margaret Koziel, and Reed Wickner. REFERENCES 1. transactivation function of HBX. The inhibition of protein breakdown by proteasomes may account for the multiple actions of HBX and may be an important feature of HBV infection, possibly in helping stabilize viral gene products and suppressing antigen presentation. Hepatitis B virus (HBV) has a unique fourth L-Ascorbyl 6-palmitate open reading frame (ORF) coding for a protein known as hepatitis B virus X (HBX) (for reviews, see references 3, 10 and 55). The HBX gene is well conserved among the mammalian hepadnaviruses and codes for a 16.5-kDa protein, which has been detected in both the nucleus and cytoplasm. HBX mRNA (0.7 kb) has been detected in infected liver, but the protein has not been easy to detect. However, this protein must be expressed in vivo because antibodies against HBX have been detected in infected individuals. The HBX gene has been shown to be essential for the establishment of HBV infection in vivo (4, 57). Its gene product also activates a variety of viral and BMP6 cellular promoters in diverse cell types. Recently, HBX has been shown to participate in signal transduction pathways, in particular the activation of the Ras/Raf pathway (1, 9). Furthermore, components of the basal transcription complex such as CREB/ATF2 (31) and TATA binding protein (36, 37), p53 (51, 52), ERCC3 (a general transcription factor involved in nucleotide excision repair) (53), RPB5 (a common subunit of RNA polymerases) (5), XAP-1/UVDDB (a DNA repair protein) (26, 27), and a cell senescence-associated protein (47) have been reported to be potential cellular targets of HBX. Although many of these findings may explain the biological functions of HBX, definitive functional evidence supporting these claims is lacking. Recently HBX expression has been linked to the induction L-Ascorbyl 6-palmitate of apoptosis (6, 46). Finally, the demonstration of the oncogenic potential of the HBX gene in a transgenic mouse model suggests that HBX may contribute to the pathogenesis of HBV-associated hepatocellular carcinoma (23). Using the yeast two-hybrid system, we previously identified a putative target of HBX as the subunit of the 20S proteasome (PSMA7; initially referred to as XAPC7) and demonstrated that this interaction is functionally important for the transactivation function of HBX (22). This interaction has also been independently reported by two other groups (12, 45). In addition, during further analyses of the HBX-interacting clones from the two-hybrid screen, we found that the PSMC1 subunit of the 19S proteasome complex also interacts specifically with HBX (56). The proteasome complex is responsible for the majority of nonlysosomal protein degradation in eukaryotic cells (for reviews, see references 8 and 21). The 26S proteasome is an 2,000-kDa, multisubunit, ATP-dependent proteolytic complex. It consists of the 19S cap complex, which is required for the recognition and degradation of ubiquitinated proteins, and the 700-kDa 20S proteasome core, where proteins are degraded (for reviews, see references 8 and 28). The 20S particle can also interact with another multisubunit complex, PA28, which is induced by gamma interferon and functions to activate peptide hydrolysis by the 20S proteasome (18, 30). The proteasome functions in diverse cellular processes ranging from cell differentiation, cell cycle control, signal transduction, stress response, transcriptional activation, DNA repair, apoptosis, and antigen presentation (for reviews, see references 8 and 21). In view of the pleiotropic effects of HBX on signal transduction, transcription, cell proliferation, and transformation, we reasoned that the L-Ascorbyl 6-palmitate interaction between HBX and the proteasome may possibly L-Ascorbyl 6-palmitate result in alterations in proteasome function and contribute to the observed effects.
