A Fos protein containing the Jun leucine zipper forms a homodimer which binds to the AP1 binding site.
The TPA (12-O-tetradecanoyl-phorbol-13-acetate) responsive element (TRE) is recognized by the inducible transcription factor AP1, a heterodimeric complex of Fos- and Jun-protein subunits, which each contain a specific structure known as the leucine zipper through which they interact. Studies using site-directed mutagenesis have shown that a basic region adjacent to the leucine zipper in Fos is crucial for the interaction of the Fos-Jun complex with the TRE, and probably represents a site of interaction with DNA. The functionally crucial amino acids in this region are almost completely conserved between Fos and Jun (refs 6, 7 and 11; M.N. and R.M., unpublished results), indicating the formation of a nearly symmetrical DNA-binding site in the Fos-Jun complex. Whereas Jun can form a homodimeric protein complex which binds to the TRE, Fos is unable to do so. The Fos-Jun heterodimer, however, possesses at least a 30-fold-higher affinity for the TRE than does the Jun-Jun homodimer, indicating cooperative binding. Because Fos cannot form a homodimer it is not known whether Fos specifically recognizes part of the TRE or has a different role in the binding of the Fos-Jun complex to DNA. Here we report that exchanging the leucine zipper in Fos with that of Jun generates a protein (termed psi-Fos) that can form a complex with Fos. This Fos-psi-Fos complex, and to a lesser extent a homodimeric psi-Fos complex, exhibits specific binding to the TRE. This finding strongly supports the hypothesis that Fos and Jun form a nearly symmetrical DNA-binding site that interacts with the palindromic TRE.
Neuberg
M
M
Institut für Molekularbiologie und Tumorforschung, Phillipps-Universität Marburg, FRG.
Adamkiewicz
J
J
Hunter
J B
JB
Müller
R
R
D001665:Binding Sites; D004268:DNA-Binding Proteins / Q000378:metabolism*; D066298:In Vitro Techniques; D046911:Macromolecular Substances; D009841:Oligonucleotides / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D011994:Recombinant Proteins / Q000378:metabolism; D012045:Regulatory Sequences, Nucleic Acid; D013329:Structure-Activity Relationship; D014157:Transcription Factors / Q000378:metabolism*
D004268:DNA-Binding Proteins; D046911:Macromolecular Substances; D009841:Oligonucleotides; D011518:Proto-Oncogene Proteins; D016760:Proto-Oncogene Proteins c-fos; D016755:Proto-Oncogene Proteins c-jun; D011994:Recombinant Proteins; D014157:Transcription Factors