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Mol. Cell. Biol., Feb 1996, 626-633, Vol 16, No. 2
M Peyton, CM Stellrecht, FJ Naya, HP Huang, PJ Samora and MJ Tsai
Using degenerate PCR cloning we have identified a novel basic helix-
loop-helix (bHLH) transcription factor, BETA3, from a hamster insulin tumor
(HIT) cell cDNA library. Sequence analysis revealed that this factor
belongs to the class B bHLH family and has the highest degree of homology
with another bHLH transcription factor recently isolated in our laboratory,
BETA2 (neuroD) (J. E. Lee, S. M. Hollenberg, L. Snider, D. L. Turner, N.
Lipnick, and H. Weintraub, Science 268:836-844, 1995; F. J. Naya, C. M. M.
Stellrecht, and M.-J. Tsai, Genes Dev. 8:1009- 1019, 1995). BETA2 is a
brain- and pancreatic-islet-specific bHLH transcription factor and is
largely responsible for the tissue-specific expression of the insulin gene.
BETA3 was found to be tissue restricted, with the highest levels of
expression in HIT, lung, kidney, and brain cells. Surprisingly, despite the
homology between BETA2 and BETA3 and its intact basic region, BETA3 is
unable to bind the insulin E box in bandshift analysis as a homodimer or as
a heterodimer with the class A bHLH factors E12, E47, or BETA1. Instead,
BETA3 inhibited both the E47 homodimer and the E47/BETA2 heterodimer
binding to the insulin E box. In addition, BETA3 greatly repressed the
BETA2/E47 induction of the insulin enhancer in HIT cells as well as the
MyoD/E47 induction of a muscle-specific E box in the myoblast cell line
C2C12. In contrast, expression of BETA3 had no significant effect on the
GAL4-VP16 transcriptional activity. Immunoprecipitation analysis
demonstrates that the mechanism of repression is via direct protein-protein
interaction, presumably by heterodimerization between BETA3 and class A
bHLH factors.
Copyright © 1996, American Society for Microbiology
BETA3, a novel helix-loop-helix protein, can act as a negative regulator of BETA2 and MyoD-responsive genes
Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
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