Short Isoform of POU Factor Brn-3b Can Form a Heterodimer with Brn-3a That Is Inactive for Octamer Motif Binding (*)
- From the Institut für Molekularbiologie und Tumorforschung, Philipps Universität Marburg, Emil-Mannkopff-Strasse 2, D-35037 Marburg, Germany
- ¶ To whom correspondence should be addressed. Tel.: 49-6421-28-6775; Fax: 49-6421-28-5390; moeroey{at}imt.uni-marburg.de.
Abstract
The POU proteins Brn-3a and Brn-3b belong to a family of DNA binding transcription factors that share stretches of extensive homology. Both Brn-3a and Brn-3b are expressed as shorter and longer isoforms. The long form of Brn-3a is able to oncogenically transform primary fibroblasts. By contrast, the short form of Brn-3b (Brn-3b(s)) cannot transform fibroblasts but is able to specifically inhibit the transforming activity of Brn-3a(l). Moreover, Brn-3a(l) can act as a transcriptional transactivator, while Brn-3b(s) is not only unable to do so but in addition specifically inhibits the transactivating activity of Brn-3a(l). Here, we show that the opposite and antagonistic activities of Brn-3a(l) and Brn-3b(s) proteins are due to their different DNA binding properties; Brn-3a(l) but not Brn-3b(s) can form stable complexes with several octamer-related target DNA sequences. The presence of Brn-3b(s) completely inhibits the binding of Brn-3a(l) to DNA by preventing the formation of Brn-3a(l)-DNA complexes as well as by disrupting preformed complexes. Experiments with GST fusion proteins and in vitro binding studies suggest that the inhibition of Brn-3a(l) activity by Brn-3b(s) occurs via direct interaction of the two transcription factors in solution. Therefore, we hypothesize that Brn-3b(s) can act as a direct antagonist of Brn-3a(l) by inhibiting its DNA binding through the formation of an inactive hetero-oligomeric complex.
Footnotes
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↵§ Supported by a fellowship from the Graduiertenkolleg “Zell-und Tumorbiologie.” Present address: National Institute for Medical Research, The Ridgeway, Mill Hill, London, NW7 1AA, United Kingdom.
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↵* This work was supported in part by Deutsche Forschungsgemeinschaft Grant SFB 215-D10 and a grant from the Mildred Scheel Stiftung für Krebsforschung (to T. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵1 The abbreviations used are:
- PCR
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polymerase chain reaction
- GST
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glutathione S-transferase
- PAGE
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polyacrylamide gel electrophoresis
- bp
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base pair(s)
- CRH
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corticotropin-releasing hormone.
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- Received August 24, 1995.
- Revision received September 26, 1995.
- © 1995 by The American Society for Biochemistry and Molecular Biology, Inc.
