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fos-jun Conspiracy: implications for the cell
I M Verma1, L J Ransone, J Visvader
1Molecular Biology and Virology Laboratory, Salk Institute, San Diego, California 92138.
Abstract:
Two nuclear oncoproteins, fos and jun (AP-1), cooperate in forming a very stable heterodimeric complex that binds to the AP-1 site with increased affinity. The 'leucine zipper' domain of both fos and jun is necessary for the formation of this heterodimer. Mutations of single residues within the leucine zipper domain had no effect on protein complex formation. However, mutagenesis of the first leucine of the heptad repeat in either fos or jun basic regions and alteration of the spacing between the basic and leucine zipper domains indicate that the basic region of fos has a crucial role in determining the DNA binding affinity of the transcriptional complex. Mutations of the basic amino acids in fos protein prevent binding to TPA (phorbol ester)-responsive element (TRE) in the presence of wild-type jun protein. Thus fos protein appears to be dominant in jun-fos binding to DNA, even though fos alone cannot bind to TRE. Mutants in the basic regions of fos and jun can be exploited as dominant-negative mutants to ablate their normal cellular function.
Insights
The fos and jun oncoproteins form a stable complex (AP-1) that binds DNA. The fos protein
Area of Science:
- Molecular Biology
- Oncogenesis
- Transcription Factors
Background:
- Nuclear oncoproteins fos and jun (AP-1) form heterodimeric complexes.
- These complexes bind to the AP-1 site with high affinity.
- The leucine zipper domain is essential for heterodimer formation.
Purpose of the Study:
- To investigate the roles of the leucine zipper and basic regions in fos-jun complex formation and DNA binding.
- To determine the contribution of fos and jun proteins to the stability and DNA binding affinity of the AP-1 complex.
- To explore the potential of fos and jun mutants as dominant-negative regulators.
Main Methods:
- Site-directed mutagenesis of the leucine zipper and basic regions of fos and jun.
- Analysis of heterodimer formation and DNA binding affinity.
- Assessment of dominant-negative effects of mutant proteins.
Main Results:
- Mutations in the leucine zipper domain did not affect heterodimer formation.
- Mutagenesis of the fos basic region or alterations in domain spacing significantly impacted DNA binding affinity.
- Mutations in basic amino acids of fos prevented DNA binding, even with wild-type jun.
- Fos protein demonstrated dominance in DNA binding despite lacking intrinsic binding ability.
- Mutants in basic regions of fos and jun exhibited dominant-negative properties.
Conclusions:
- The basic region of fos plays a critical role in determining the DNA binding affinity of the AP-1 complex.
- Fos protein is dominant in the DNA binding of jun-fos heterodimers.
- Mutants in the basic regions of fos and jun can function as dominant-negative mutants to inhibit normal cellular functions.