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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, CA 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 on DNA with high 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 have no effect on protein complex formation. However, results from 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 plays 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 the tumour promoter response 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 region of fos and jun can be exploited as dominant-negative mutants to ablate the normal fos cellular function.
Insights
The fos and jun proteins 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 stable heterodimers.
- The leucine zipper domain is essential for heterodimer formation.
- The AP-1 complex binds to specific DNA sequences with high affinity.
Purpose of the Study:
- To investigate the roles of fos and jun protein domains in DNA binding.
- To determine the contribution of the basic region to the transcriptional complex's DNA binding affinity.
- To explore the potential of fos and jun mutants as dominant-negative regulators.
Main Methods:
- Site-directed mutagenesis of fos and jun proteins, focusing on the leucine zipper and basic regions.
- Analysis of protein complex formation and DNA binding affinity.
- Assessment of dominant-negative effects of mutant proteins on normal fos cellular function.
Main Results:
- Mutations in the leucine zipper domain did not affect heterodimer formation.
- The basic region of fos is critical for determining DNA binding affinity.
- Mutations in fos basic amino acids prevented DNA binding, even with wild-type jun.
- Mutants in the basic regions of fos and jun can function as dominant-negative inhibitors.
Conclusions:
- Fos protein plays a dominant role in jun-fos DNA binding, despite lacking DNA binding capability alone.
- The basic region of fos is crucial for high-affinity DNA binding of the AP-1 complex.
- Engineered fos and jun mutants offer a strategy to inhibit normal fos function in cellular processes.