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Coiled coil |
A coiled coil is a structural motif in proteins, in which 2-71 alpha-helices are coiled together like the strands of a rope (Dimers and trimers are the most common types). Many coiled coil type proteins are involved in important biological functions such as the regulation of gene expression e.g. transcription factors. Notable examples are the proteins the oncoproteins c-fos and jun, and the muscle protein tropomyosin.
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Coiled coils usually contain a repeated seven amino acid residue pattern called heptad repeats. The interacting surface between the helices often contain hydrophobic residues, such as leucine arranged in a so called leucine zipper. The most favorable way for such two helices to arrange themselves in the water-filled environment of the cytoplasm is to wrap the hydrophobic strands against each other sandwiched between the hydrophilic amino acids. It is thus the burial of hydrophobic surfaces, that provides the thermodynamic driving force for the dimerization.
The α-helices may be parallel or antiparallel, and usually adopt a left-handed super-coil (Figure 1). Although disfavored, a few right-handed coiled coils have also been observed in nature and in designed proteins.2
A key step in the entry of HIV into human cells is the exposure of a trimeric, parallel coiled coil known as gp41. The gp41 trimer is normally covered by another surface glycoprotein known as gp120, which protects it from antibodies. Upon binding to the target cell, gp120 undergoes a conformational change that exposes the gp41 trimer, whose hydrophobic N-terminal tails enter the target cell membrane. Three other helices of gp41 fold down into the grooves of the gp41 coiled coil trimer, forming a hexamer, and drawing the viral membrane and target-cell membrane close enough to fuse. The virus then enters the cell and begins its replication. Recently, inhibitors that bind in the gp41 grooves have been developed, such as Fuzeon.
Because of their specific interaction coiled coils can be used as a dimerization "tag".
The possibility of coiled coils for α-keratin was proposed by Francis Crick in 1952 as well as mathematical methods for determining their structure. 3 Remarkably this was soon after the structure of the alpha helix was suggested in 1951 by Linus Pauling and coworkers 4
| Protein secondary structure | ||
|---|---|---|
| Helices: | α-helix | 310 helix | π-helix | β-helix | Polyproline helix | Collagen helix | |
| Extended: | β-strand | Turn | Beta hairpin | Beta bulge | α-strand | |
| Supersecondary: | Coiled coil | Helix-turn-helix | EF hand | |
| Secondary structure propensities of amino acids | ||
| Helix-favoring: | Methionine | Alanine | Leucine | Glutamic acid | Glutamine | Lysine | |
| Extended-favoring: | Threonine | Isoleucine | Valine | Phenylalanine | Tyrosine | Tryptophan | |
| Disorder-favoring: | Glycine | Serine | Proline | Asparagine | Aspartic acid | |
| No preference: | Cysteine | Histidine | Arginine | |
| ←Primary structure | Tertiary structure→ | |
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