Last modified September 9, 2013
This subsection of the ‘Sequence annotation (Features)’ section denotes the positions of regions of coiled coil within the protein.
Coiled coils are built by two or more alpha-helices that wind around each other to form a supercoil. There can be two, three or four helices in the bundle and they might either run in the same (parallel) or in the opposite (antiparallel) directions.
In essence coiled coils are built of sequence elements of three and four residues whose hydrophobicity pattern and residue composition is compatible with the structure of amphipathic alpha-helices. The alternating three and four residue sequence elements constitute heptad repeats in which the amino-acids are designated a’,’b’,’c’,’d’,’e’,’f’
and ‘g’. Residues in positions ‘a’ and ‘d’ are generally hydrophobic and form a zig-zag pattern of knobs and holes that interlock with a similar pattern on another strand to form a tight-fitting hydrophobic core. Of the remaining residues, ‘b’, ‘c’, and ‘f’ tend to be charged. Therefore the formation of heptad repeat depends on the physical properties of hydrophobicity and charge that are required at a particular position, not on a specific amino acid.
Examples of proteins with coiled coil regions include myosins, tropomyosins and intermediate filaments.
In UniProtKB, we annotate coiled coils predicted using the COILS program of Lupas. The minimum size for predicted coiled coils is 28 amino acids. Where two coiled coils are separated by a distance less than 25 amino acids they are merged to form a single coiled coil. Coiled coils are usually annotated with the ‘Potential’ qualifier even when they are proven experimentally since their precise borders have not been determined in most cases.
Examples: O15697, Q9Y2D8, O97593
Coiled coils and Leucine-zippers
Leucine-zippers constitute a subtype of coiled coil in which the amino acid leucine is predominant at the ‘d’ position of the heptad repeat. They may be shorter than 28 amino acids. When a paper reports the existence of a leucine-zipper, we usually annotate this as a coiled coil unless the proposed leucine-zipper is proven to be important for the function of the protein. In that case, we annotate the leucine-zipper in the ‘Domain’ of the ‘Sequence annotation (Features)’ section .
Examples: P18289, P15407
Related keyword: Coiled coil