Contribution of buried lysine residues to the oligomerization specificity and stability of the Fos coiled coil

Coiled coils comprise two or more helices characterized by a heptad repeat of amino acids denoted a through g. The buried a and d positions are usually occupied by hydrophobic residues. Fos dimerizes via a coiled coil (leucine zipper) with Jun family members to form the transcription factor AP-1. Fos homodimers are relatively unstable due to unfavorable interhelical electrostatic interactions within the Fos two-stranded coiled coil. The Fos coiled coil contains two polar position a Lys residues (Lys 16 and Lys 30 of Fos-p1, a peptide corresponding to the coiled-coil domain of v-Fos). Lys 16 and Lys 30 of Fos-p1 were replaced individually and together with the unnatural amino acid norleucine (2-aminohexanoic acid), which corresponds to a deletion of the Lys epsilon-amino group. The midpoint of thermal denaturation (T-m) of Fos-p1 (10 muM) is 30degreesC at pH 7. The Lys 16-->Nle variant forms predominantly homodimers that are relatively unstable (T-m = 46degreesC). The Lys 30-->Nle variant forms a stable homotetramer (T-m = 60degreesC). The Lys 16/Lys 30-->Nle variant forms a very stable homotetramer (T-m = 80degreesC). The results show that (i) the effects of buried position a Lys residues on coiled-coil oligomerization are context dependent and (ii) electrostatic destabilization of the Fos homodimer can be mitigated by an oligomerization switch moderated by a single buried Lys residue.