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Mol. Cell. Biol., Dec 1995, 7106-7116, Vol 15, No. 12
I Stancovski, H Gonen, A Orian, AL Schwartz and A Ciechanover
The transcription factor c-Fos is a short-lived cellular protein. The
levels of the protein fluctuate significantly and abruptly during changing
pathophysiological conditions. Thus, it is clear that degradation of the
protein plays an important role in its tightly regulated activity. We
examined the involvement of the ubiquitin pathway in c-Fos breakdown. Using
a mutant cell line, ts20, that harbors a thermolabile ubiquitin-activating
enzyme, E1, we demonstrate that impaired function of the ubiquitin system
stabilizes c-Fos in vivo. In vitro, we reconstituted a cell-free system and
demonstrated that the protein is multiply ubiquitinated. The adducts serve
as essential intermediates for degradation by the 26S proteasome. We show
that both conjugation and degradation are significantly stimulated by c-
Jun, with which c-Fos forms the active heterodimeric transcriptional
activator AP-1. Analysis of the enzymatic cascade involved in the
conjugation process reveals that the ubiquitin-carrier protein E2-F1 and
its human homolog UbcH5, which target the tumor suppressor p53 for
degradation, are also involved in c-Fos recognition. The E2 enzyme acts
along with a novel species of ubiquitin-protein ligase, E3. This enzyme is
distinct from other known E3s, including E3 alpha/UBR1, E3 beta, and E6-AP.
We have purified the novel enzyme approximately 350-fold and demonstrated
that it is a homodimer with an apparent molecular mass of approximately 280
kDa. It contains a sulfhydryl group that is essential for its activity,
presumably for anchoring activated ubiquitin as an intermediate thioester
prior to its transfer to the substrate. Taken together, our in vivo and in
vitro studies strongly suggest that c-Fos is degraded in the cell by the
ubiquitin-proteasome proteolytic pathway in a process that requires a novel
recognition enzyme.
Copyright © 1995, American Society for Microbiology
Degradation of the proto-oncogene product c-Fos by the ubiquitin proteolytic system in vivo and in vitro: identification and characterization of the conjugating enzymes
Department of Biochemistry, Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel.
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