Transcription factor AP-1 transduces environmental signals to the transcriptional machinery. To ensure a quick response yet maintain tight control over AP-1 target genes, AP-1 activity is likely to be negatively regulated in nonstimulated cells. To identify proteins that interact with the Jun subunits of AP-1 and repress its activity, we developed a novel screen for detecting protein-protein interactions that is not based on a transcriptional readout. In this system, the mammalian guanyl nucleotide exchange factor (GEF) Sos is recruited to the Saccharomyces cerevisiae plasma membrane harboring a temperature-sensitive Ras GEF, Cdc25-2, allowing growth at the nonpermissive temperature. Using the Sos recruitment system, we identified new c-Jun-interacting proteins. One of these, JDP2, heterodimerizes with c-Jun in nonstimulated cells and represses AP-1-mediated activation.