Crystal structure of GCN4-pI(Q)I, a trimeric coiled coil with buried polarresidues

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-hole s" arrangement. Most naturally occurring coiled coils, however, also contai n buried polar residues, as do the cores of the majority of naturally occur ring globular proteins. Two common buried polar residues in both dimeric an d trimeric coiled coils are asparagine and glutamine. In dimeric coiled coi ls, buried asparagine, but not glutamine, residues have been shown to confe r 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- pI(Q)I, is a discrete, trimeric coiled coil with a lower stability than GCN4-pII. The crystal structure det ermined to 1.8 Angstrom shows that GCN4-pI(Q)I is a trimeric coiled coil wi th a chloride ion coordinated by one buried glutamine residue from each mon omer. (C) 1998 Academic Press.