Crystal structure of GCN4-pIQI, a trimeric coiled coil with buried polar residues.

Coiled coils consist of two or more alpha-helices wrapped around each other with a superhelical twist. The interfaces between helices of a coiled coil are formed by hydrophobic amino acid residues packed in a "knobs-into-holes" arrangement. Most naturally occurring coiled coils, however, also contain buried polar residues, as do the cores of the majority of naturally occurring globular proteins. Two common buried polar residues in both dimeric and trimeric coiled coils are asparagine and glutamine. In dimeric coiled coils, buried asparagine, but not glutamine, residues have been shown to confer specificity of oligomerization. We have placed a glutamine residue in the otherwise hydrophobic interior of a stable trimeric coiled coil, GCN4-pII, to study the effect of this buried polar residue in a trimeric coiled-coil environment. The resulting peptide, GCN4-pIQI, is a discrete, trimeric coiled coil with a lower stability than GCN4-pII. The crystal structure determined to 1.8 A shows that GCN4-pIQI is a trimeric coiled coil with a chloride ion coordinated by one buried glutamine residue from each monomer.