Extending dimerization interfaces: the bZIP basic region can form a coiled coil.

We appended a rationally designed acidic amphipathic protein sequence to the N-terminus of a leucine zipper. Circular dichroism data indicate that this engineered polypeptide sequence can 'zipper' up the basic region of a bZIP monomer into a heterodimeric coiled coil. This propagation of the leucine zipper dimerization interface into the basic region can proceed for up to four heptads and stabilizes the heterodimer complex 2.5 kcal/mol or > 100-fold. The acidic nature of the extension is the most critical component of the design, suggesting that the extension is acting as a DNA mimetic. The dimerization prevents the basic region in this heterodimeric coiled coil structure from binding to DNA. Gel-shift, fluorescence and transient transfection assays indicate that the acidic extension appended to a leucine zipper can inactivate the DNA-binding and transactivation properties of the bZIP protein C/EBP. The three bZIP basic regions examined in this study dimerize with similar stability with the acidic extension, suggesting that this N-terminal extension can be used to develop dominant-negatives to other bZIP transcription factors.