Integrity of FOS B leucine zipper is essential for its interaction with JUN proteins.

fos B encodes a nuclear protein with 70% homology to c-fos, whose expression is transiently induced during the G0/G1 transition. Immunoprecipitation studies demonstrated that FOS B protein forms a complex in vitro with c-JUN, JUN B, and JUN D. We have mutated some of the leucines of the 'leucine zipper' present in the FOS B protein and determined their effect in the interaction with JUN proteins and their binding to an AP-1 containing sequence. The exchange of either leucine 1, 3, or 5 of the leucine repeat of FOS B to a proline dramatically inhibits its association with JUN proteins. However, a more conserved substitution to isoleucine has only a 50% inhibition. These results demonstrate that any major alteration in the alpha-helical structure of the 'leucine zipper' completely inhibits the interaction of FOS B with any of the three JUN proteins.