Importance of Potential Interhelical Salt-bridges Involving Interior Residues for Coiled-coil Stability and Quaternary Structure

Coiled coils are formed by two or more α-helices that align in a parallel or an antiparallel relative orientation. Polar interactions involving residues at the interior a and d positions are important for determining the quaternary structure of coiled coils. In the model heterodimeric coiled-coil Acid-a1-Base-a1, a buried a–d′ Asn–Asn interaction is sufficient to specify both a dimeric structure and an antiparallel relative helix orientation. Although the equivalent a–a′ interaction is found in parallel coiled coils, there is no example of an a–d′ Asn–Asn interaction in structurally characterized, naturally occurring antiparallel coiled coils. Instead, interior charged residues form interhelical salt-bridges with residues at the adjacent e or g positions. Using a model coiled-coil heterodimer, we have explored the role of a potential interhelical interaction between an Arg at an interior d position and a Glu at the adjacent g′ position. Our results demonstrate that this potentially attractive interhelical Coulombic interaction has little or no influence on helix orientation. Instead, we show that burying a single Arg residue at an interior position is sufficient to specify a dimeric state at a significantly lower thermodynamic cost than burial of two interacting Asn residues.