Buried polar residues in coiled-coil interfaces

Coiled coils, estimated to constitute 3-5% of the encoded residues in most genomes, are characterized by a heptad repeat, (abcdefg)(n), where the buri ed a and d positions form the interface between multiple cl-helices. Althou gh generally hydrophobic, a substantial fraction (similar to 20%) of these a- and d-position residues are polar or charged. We constructed variants of the well-characterized coiled coil GCN4-p1 with a single polar residue (As n, Gin, Ser, or Thr) at either an a or a d position. The stability and olig omeric specificity of each variant were measured, and crystal structures of coiled-coil trimers with threonine or serine at either an a or a d positio n were determined. The structures show how single polar residues in the int erface affect not only local packing, but also overall coiled-coil geometry as seen by changes in the Crick supercoil parameters and core cavity volum es.