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Mol. Cell. Biol., Mar 1997, 1459-1468, Vol 17, No. 3
GD Spotts, SV Patel, Q Xiao and SR Hann
The c-myc gene has been implicated in multiple cellular processes including
proliferation, differentiation, and apoptosis. In addition to the
full-length c-Myc 1 and 2 proteins, we have found that human, murine, and
avian cells express smaller c-Myc proteins arising from translational
initiation at conserved downstream AUG codons. These c- Myc short (c-Myc S)
proteins lack most of the N-terminal transactivation domain but retain the
C-terminal protein dimerization and DNA binding domains. As with
full-length c-Myc proteins, the c-Myc S proteins appear to be localized to
the nucleus, are relatively unstable, and are phosphorylated. Significant
levels of c-Myc S, often approaching the levels of full-length c-Myc, are
transiently observed during the rapid growth phase of several different
types of cells. Optimization of the upstream initiation codons resulted in
greatly reduced synthesis of the c-Myc S proteins, suggesting that a "leaky
scanning" mechanism leads to the translation of these proteins. In some
hematopoietic tumor cell lines having altered c-myc genes, the c-Myc S
proteins are constitutively expressed at levels equivalent to that of
full-length c-Myc. As predicted, the c-Myc S proteins are unable to
activate transcription and inhibited transactivation by full-length c- Myc
proteins, suggesting a dominant-negative inhibitory function. While these
transcriptional inhibitors would not be expected to function as full-length
c-Myc, the occurrence of tumors which express constitutive high levels of
c-Myc S and their transient synthesis during rapid cell growth suggest that
these proteins do not interfere with the growth- promoting functions of
full-length c-Myc.
Copyright © 1997, American Society for Microbiology
Identification of downstream-initiated c-Myc proteins which are dominant-negative inhibitors of transactivation by full-length c-Myc proteins
Department of Cell Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2175, USA.
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