Measurement of interhelical electrostatic interactions in the GCN4 leucine zipper

The dimerization specificity of the bZIP transcription factors resides in the leucine zipper region. It is commonly assumed that electrostatic interactions between oppositely charged amino acid residues on different helices of the leucine zipper contribute favorably to dimerization specificity. Crystal structures of the GCN4 leucine zipper contain interhelical salt bridges between Glu20 and Lys15' and between Glu22 and Lys27'. 13C-nuclear magnetic resonance measurements of the glutamic acid pKa values at physiological ionic strength indicate that the salt bridge involving Glu22 does not contribute to stability and that the salt bridge involving Glu20 is unfavorable, relative to the corresponding situation with a neutral (protonated) Glu residue. Moreover, the substitution of Glu20 by glutamine is stabilizing. Thus, salt bridges will not necessarily contribute favorably to bZIP dimerization specificity and may indeed be unfavorable, relative to alternative neutral-charge interactions.