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Molecular and Cellular Biology, September 2002, p. 6089-6099, Vol. 22, No. 17
0270-7306/02/$04.00+0 DOI: 10.1128/MCB.22.17.6089-6099.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Directs Translation by an Internal Ribosome Entry Segment That Is Most Active in Densely Growing Cells and during Apoptosis
Department of Biology, University of York, York YO10 5YW, United Kingdom,1 Molecular Development Laboratory, Murdoch Childrens Research Institute, Royal Children's Hospital, Parkville, Victoria 3052, Australia2
Received 4 February 2002/ Returned for modification 13 March 2002/ Accepted 4 June 2002
Protein kinase C
(PKC
) is a member of the PKC family of phospholipid-dependent serine/threonine kinases and is involved in cell proliferation, apoptosis, and differentiation. Previous studies have suggested that different PKC isoforms might be translationally regulated. We report here that the 395-nt-long 5' untranslated region (5' UTR) of PKC
is predicted to form very stable secondary structures with free energies (
G values) of around -170 kcal/mol. The 5' UTR of PKC
can significantly repress luciferase translation in rabbit reticulocyte lysate but does not repress luciferase translation in a number of transiently transfected cell lines. By using a bicistronic luciferase reporter, we show that the 5' UTR of PKC
contains a functional internal ribosome entry segment (IRES). The activity of the PKC
IRES is greatest in densely growing cells and during apoptosis, when total protein synthesis and levels of full-length eukaryotic initiation factor 4G are reduced. However, the IRES activity of the 5' UTR of PKC
is not enhanced during serum starvation, another condition shown to inhibit cap-dependent translation, suggesting that its potency is dependent on specific cellular conditions. Accumulating data suggest that PKC
has a function as proliferating cells reach high density and in early and later events of apoptosis. Our studies suggest a mechanism whereby PKC
synthesis can be maintained under these conditions when cap-dependent translation is inhibited.
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