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Molecular and Cellular Biology, January 2000, p. 496-506, Vol. 20, No. 2
Department of Molecular Genetics, Weizmann
Institute of Science, Rehovot 76100, Israel
Received 1 June 1999/Returned for modification 27 July
1999/Accepted 19 October 1999
Death-associated protein 5 (DAP5) (also named p97 and NAT1) is a
member of the translation initiation factor 4G (eIF4G) family that
lacks the eIF4E binding site. It was previously implicated in
apoptosis, based on the finding that a dominant negative
fragment of the protein protected against cell death. Here we address
its function and two distinct levels of regulation during
apoptosis that affect the protein both at translational and
posttranslational levels. DAP5 protein was found to be cleaved at a
single caspase cleavage site at position 790, in response to activated
Fas or p53, yielding a C-terminal truncated protein of 86 kDa that is capable of generating complexes with eIF4A and eIF3. Interestingly, while the overall translation rate in apoptotic cells was
reduced by 60 to 70%, in accordance with the simultaneous degradation of the two major mediators of cap-dependent translation, eIF4GI and
eIF4GII, the translation rate of DAP5 protein was selectively maintained. An internal ribosome entry site (IRES) element capable of
directing the translation of a reporter gene when subcloned into a
bicistronic vector was identified in the 5' untranslated region of DAP5
mRNA. While cap-dependent translation from this transfected vector
was reduced during Fas-induced apoptosis, the translation via
the DAP5 IRES was selectively maintained. Addition of recombinant
DAP5/p97 or DAP5/p86 to cell-free systems enhanced preferentially the
translation through the DAP5 IRES, whereas neutralization of the
endogenous DAP5 in reticulocyte lysates by adding a dominant negative
DAP5 fragment interfered with this translation. The DAP5/p86
apoptotic form was more potent than DAP5/p97 in these
functional assays. Altogether, the data suggest that DAP5 is a
caspase-activated translation factor which mediates cap-independent
translation at least from its own IRES, thus generating a positive
feedback loop responsible for the continuous translation of DAP5 during apoptosis.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
A Novel Form of DAP5 Protein Accumulates in Apoptotic Cells as
a Result of Caspase Cleavage and Internal Ribosome Entry
Site-Mediated Translation
*
Corresponding author. Mailing address: Department of
Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel. Phone: 972-8-9342428. Fax: 972-8-9344108. E-mail:
lvkimchi{at}weizmann.weizmann.ac.il.
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