A Novel Mechanism of Protein Degradation
Protein degradation is a basic process in the living cell. A large fraction of onco-
and tumor suppressor proteins are regulated at the level of their stability. Therefore
investigation of the mechanisms of proteasomal protein degradation has attracted much attention.
For example the p53 level is controlled by the rate of its proteasomal degradation, a process
regulated by the Mdm2-ubiquitin-proteasome degradation pathway. We have found a novel pathway that
regulates p53 stability. This pathway is regulated by NAD(P)H: quinone oxidoreductase 1 (NQO1).
Pharmacological and genetic inhibition of NQO1 induces p53 proteasomal degradation and inhibits
p53-mediated apoptosis. NQO1 regulates degradation of p53 by a mechanism that is independent of
both Mdm2 and ubiquitin. This pathway is not exclusive for p53. A limited number of other
short-lived proteins such as p73 and ODC are regulated by NQO1. Studies have indicated that this
pathway may play a role in oncogenesis. The underlying molecular mechanism of this process of
protein destabilization is under study.
Tumor Suppressors and DNA Damage
We study the molecular basis of cell response to DNA damage, in particular double strand break
(DSB). We have characterized a novel signaling-pathway that is initiated by DSB and triggers
activation of the non-receptor tyrosine kinase c-Abl. This kinase in turn tyrosine phosphorylates
the intimately associated p73, a member of p53 tumor suppressor family. This process often
elicits apoptosis unless the damage has been repaired or the DNA fragments have been excluded.
Thus, the repair machinery and DNA fragments regulate the apoptosis pathway. The nature and
composition of this process is under study.
The Molecular Mechanisms of Virus Host-Cell Interaction
Viruses in order to propagate have to invade cells and to occupy the relevant cellular
machinery. How, a given virus with a very limited number of genes can do so is a very basic
question but the answers await understanding of the virus genes from one hand and the cellular
machineries from other. Hepatitis B virus (HBV) is a common infectious agent worldwide, hence
provides an excellent model. HBV is a liver specific virus. Our study revealed that this tropism
is determined at the level of both receptor and post-receptor levels, and identified the molecular
basis of these processes. We found that the small pX regulatory protein of HBV manipulates the
cellular transcription machinery at all possible levels, namely chromatin, enhancer and the basal
promoter binding proteins. Cellular proteins, the targets of pX were identified but the list has
not been completed.
" Synthetic Viruses" and Gene Therapy
Gene therapy has attracted much attention but proved partially successful. Viruses that are
used for DNA transduction are unsafe. Our aim is to generate non-infectious virus like particles.
The individual components are prepared in heterogonous systems and assembled in tube. These
particles are expected to be safe and their production is highly reproducible. These " synthetic
virions " may revolutionize the whole field of gene transfer technology.
Ori, A. Zauberman, A., Doitsh, G., Paran N., Oren, M. and Shaul, Y. (1998). p53 binds and
represses the HBV enhancer: an adjacent enhancer element can reverse the transcription effect
of p53. EMBO J 17, 544-553.
Haviv, I., Shamay M., Doitsh, G. and Shaul, Y. (1998). Hepatitis B virus pX targets TFIIB
in transcription coactivation. Mol. Cell. Biol. 18,1562-1569
Haviv, I., Matza, Y. and Shaul, Y. (1998). pX, the HBV encoded coactivator, suppresses the
phenotypes of TBP and TAFII250 mutants. Genes and Dev 12, 1217-1226.
Agami, R. and Shaul, Y. (1998). The kinase activity of c-Abl but not v-Abl is potentiated by
direct interaction with RFX1, a protein that binds the enhancers of several viruses and cell-
cycle regulated genes. Oncogene 16, 1779-1788.
Agami, R., Blandino, G., Oren, M. and Shaul, Y. (1999). Interaction of c-Abl and p73alpha
and their collaboration to induce apoptosis. Nature 399, 809-813.
Katan-Khaykovich, Y., Spiegel, I. and Shaul, Y. (1999). The dimerization/repression domain
of RFX1 is related to a conserved region of its yeast homologues Crt1 and Sak1: a new function
for an ancient motif. J. Mol. Biol. 294, 121-137.
Doitsh, G. and Shaul, Y. (1999). HBV transcription repression in response to genotoxic
stress is p53-dependent and abrogated by pX. Oncogene 18, 7506-7513.
Paran, N, Ori, A., Haviv, I. and Shaul, Y. (2000). A composite polyadenylation signal with
TATA box function. Mol. Cell. Biol. 20, 834-841.
Asher, G., Lotem, J., Cohen, B., Sachs, L. and Shaul, Y. (2001). Regulation of p53 stability
and p53-dependent apoptosis by NADH quinone oxidoreductase 1. Proc. Natl. Acad. Sci. USA. 98,
Barak, O., Aronheim, A. and Shaul, Y. (2001). HBV X protein targets HIV Tat-binding protein
1. Virology 283, 110-120.
Katan-Khaykovich, Y. and Shaul, Y. (2001). Nuclear import and DNA-binding activity of RFX1.
Evidence for an autoinhibitory mechanism. Eur. J. Biochem. 268, 3108-3116.
Paran, N, Geiger, B. and Shaul, Y. (2001). HBV infection of cell culture: evidence for
multivalent and cooperative attachment. EMBO J. 20, 4443-4453.
Shamay, M., Agami, R. and Shaul, Y. (2001). HBV integrants of hepatocellular carcinoma cell
lines contain an active enhancer. Oncogene 20, 6811-6819.
Shamay, M., Barak, O., Doitsh, G., Ben-Dor, I. and Shaul, Y. (2002). Hepatitis B virus pX
interacts with HBXAP, a PHD finger protein to coactivate transcription. J. Biol. Chem. 277,
Asher, G., Lotem, J., Kama, R., Sachs, L. and Shaul, Y. (2002). NQO1 stabilizes p53 through
a distinct pathway. Proc. Natl. Acad. Sci. USA. 99, 3099-3104.
Shamay, M., Barak, O. and Shaul, Y. (2002). HBXAP, a novel PHD finger protein, Pssesses
transcription repression activity. Genomics 79, 523-529.
Asher, G., Lotem, J., Sachs, L., Kahana, C. and Shaul, Y. (2002). Mdm-2 and ubiquitin-
independent p53 proteasomal degradation regulated by NQO1. Proc Natl Acad Sci U S A. 99, 13125-
Shlomai, A. and Shaul, Y. (2003). Inhibition of hepatitis B virus expression and replication
by RNA interference. Hepatology. 37, 764-770.
Doitsh, G. and Shaul, Y. (2003). A long HBV transcript encoding pX is inefficiently
exported from the nucleus. Virology. 309, 339-349.
Ben-Yehoyada, M., Ben-Dor, I. and Shaul, Y. (2003). c-Abl tyrosine kinase selectively
regulates p73 nuclear matrix association. J Biol Chem. 278, 34475-34482.
Asher, G., Lotem, J., Tsvetkov, P., Reiss, V., Sachs, L. and Shaul, Y. (2003). P53 hot-spot
mutants are resistant to ubiquitin-independent degradation by increased binding to NAD(P)H:
quinone oxidoreductase 1. Proc Natl Acad Sci U S A. 100, 15065-15070.
Doitsh, G. and Shaul, Y. (2004). Enhancer I predominance in hepatitis B virus gene expression.
Mol Cell Biol. 24, 1799-1808.
Department of Molecular Genetics
Weizmann Institute of Science
Last Updated: 14 March 2004