| Summary: | The coiled coil is a common and well-studied protein-folding motif. It is based on the seven-residue repeat abcdefg, where a and d residues are largely hydrophobic. Structurally, coiled coils comprise two or more a-helices that are brought together with the a and d residues packing in a well-defined manner to form a hydrophobic core. Interhelical electrostatic interactions are frequently observed between core-flanking g and e residues. There is debate as to whether these interactions are present solely to confer specificity, or whether they also have a role in stabilising the structures. A program, TRAWLER, was written to analyse the core-flanking interactions in a set of high-resolution structural data, and designed proteins were used to investigate the role of these interactions further. It is shown that the electrostatic interactions are stabilising in comparison to a state where the charged residues are present but not interacting. The strength of this stabilisation is strongly context dependent: pairs containing glutamic acid and lysine are more stabilising when the glutamic acid is placed at g and the lysine at e. It is proposed that this is due to the packing of these residues against the surface presented by the core a and d residues. It is noted that previous studies using different a residues in the core exhibit the opposite preference. Further designs include a histidine-based switch and a series of bi-faceted coiled coils. In the latter, coiled-coil repeats were overlaid within a sequence to produce two oligomerisation interfaces. Such sequences are seen in natural a-sheet and a-cylinder structures. Designed peptides were intended to form vertically staggered a-cylinders, leading to the formation of elongated nanotubes. The behaviours of these peptides are presented and the difficulties inherent in such designs are discussed.
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