AP-1 transcription factors. (a) Structural model of the AP-1 heterodimer of Fos (purple) and Jun (green), bound to DNA. (b) Gel from an electrophoretic mobility shift assay using a ³²P-end-labeled DNA fragment containing the AP-1–binding site sequence. The DNA was mixed with Fos, FosC (a fragment of Fos), or Jun (all of which would form a homodimer), or with a mixture of Fos and Jun or a mixture of FosC and Jun (both of which would form the two types of homodimer and the heterodimer). Reactions were analyzed by polyacrylamide gel electrophoresis, then autoradiography. Binding of protein dimers to the DNA causes the complex to migrate more slowly through the gel, resulting in distinct bands. The radioactive signal at the bottom of the gel is unbound DNA.