DNA binding and transactivation properties of Fos variants with homodimerization capacity.
The mammalian Fos and Fos-related proteins are unable to form homodimers and to bind DNA in the absence of a second protein, like c-Jun for example. In order to study the implications of hydrophobic point mutations in the c-Fox leucine zipper on DNA binding of the entire c-Fos protein, we have constructed and purified a set of Fos mutant proteins harboring one or several isoleucine or leucine residues in the five Fos zipper a positions. We show that a single point mutation in the hydrophobic interface of the c-Fos leucine zipper enables the c-Fos mutant protein to bind specifically to an oligonucleotide duplex harboring the TRE consensus sequence TGA(C/G)TCA. This point mutation (Thr196-->Ile) is situated in the a position of the second heptade (a2) of the Fos zipper. The introduction of additional isoleucine residues in the other a positions progressively increases the DNA binding affinity of these homodimerizing Fos zipper variants. Heterodimerization of these c-Fos variants with c-Jun reveals a complex behavior, in that the DNA binding affinity of these heterodimers does not simply increase with the number of isoleucine side chains in position a. For example, a c-Fos variant harboring a wild-type Thr in position a1 aad Ile in the four other a positions (c-Fos4I) interacts more tightly with c-Jun than a variant harboring Ile in all five a positions (c-Fos5I). The same holds true for the corresponding leucine variants, suggesting that the wild-type a1 residue of the Fox zipper (Thr162) is thermodynamically relevant for Fos-Jun heterodimer formations and DNA binding. The c-Fos4I variant forms heterodimers with c-Jun slightly better than the wild-type zipper protein, suggesting that the driving force for Fos-Jun heterodimerization is not the simple fact that the Fos protein is unable to form homodimers. These c-Fos variants were further tested for their transactivation properties in F9 and NIH3T3 cells. At low expression levels the most efficiently homodimerizing variant (c-Fos5I) activates transcription in F9 cells about 6-fold. However part of this activation may be due to the formation of heterodimers with a member of the Jun family (like JunD for example), since a wild type c-Fos expression vector confers a 3-fold activation under these conditions. In the case of the homodimerizing c-Fos variants however, this activation is abrogated at higher expression levels due to a strong inhibition of basal transcription activity.
Porte
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D
Institut de Biologie Moléculaire et Cellulaire, UPR 9002 du CNRS, F-67084 Strasbourg Cedex, France.
Oertel-Buchheit
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P
John
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M
Granger-Schnarr
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M
Schnarr
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M
D016475:3T3 Cells; D000818:Animals; D001665:Binding Sites; D004247:DNA / Q000378:metabolism*; D019281:Dimerization; D014644:Genetic Variation; D016350:Leucine Zippers; D051379:Mice; D017354:Point Mutation; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics / Q000378:metabolism*; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured
D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D004247:DNA