Attractive Interhelical Electrostatic Interactions in the Proline- and Acidic-rich Region (PAR) Leucine Zipper Subfamily Preclude Heterodimerization with Other Basic Leucine Zipper Subfamilies

Basic region-leucine zipper (B-ZIP) proteins homo- or heterodimerize to bind sequence-specific double-stranded DNA. We present circular dichroism (CD) thermal denaturation data on vitellogenin promoter-binding protein (VBP), a member of the PAR subfamily of B-ZIP proteins that also includes thyroid embryonic factor, hepatocyte leukemia factor, and albumin site D-binding protein. VBP does not heterodimerize with B-ZIP domains from C/EBPα, JUND, or FOS. We describe a dominant negative protein, A-VBP, that contains the VBP leucine zipper and an acidic amphipathic protein sequence that replaces the basic region critical for DNA binding. The acidic extension forms a coiled coil structure with the VBP basic region in the VBP·A-VBP heterodimer. This new α-helical structure extends the leucine zipper N-terminally, stabilizing the complex by 2.0 kcal/mol. A-VBP abolishes DNA binding of VBP in an equimolar competition assay, but does not affect DNA binding even at 100-fold excess of CREB, C/EBPα, or FOS/JUND. Likewise, proteins containing the acidic extension appended to seven other leucine zippers do not inhibit VBP DNA binding. We show that conserved g ↔ e′ or i, i′ +5 salt bridges are sufficient to confer specificity to VBP by mutating the C/EBPα leucine zipper to contain the g ↔ e′ salt bridges that characterize VBP. A-VBP heterodimerizes with this mutant C/EBP, preventing it from binding to DNA. These conserved g ↔ e′ electrostatic interactions define the specificity of the PAR subfamily of B-ZIP proteins and preclude interaction with other B-ZIP subfamilies.