Coiled coil

Overview

File:GCN4 coiled coil dimer 1zik rainbow.png

Figure 1: The classic example of a coiled coil is the GCN4 leucine zipper (PDB accession code 1zik), which is a parallel, left-handed homodimer. However, many other types of coiled coil exist.

For the coiled coil shape in general, see coil.

A coiled coil is a structural motif in proteins, in which 2-7 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.

Molecular structure of coiled coils

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 them selves 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.

Biological roles of coiled coils

Role of coiled coils in HIV infection

File:Gp41 coiled coil hexamer 1aik sideview.png

Side view of the gp41 hexamer that initiates the entry of HIV into its target cell.

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.

Coiled coils as dimerization tags

Because of their specific interaction coiled coils can be used as a dimerization "tag".

History of coiled coils

The possibility of coiled coils for α-keratin was proposed by Francis Crick in 1952 as well as mathematical methods for determining their structure. Remarkably this was soon after the structure of the alpha helix was suggested in 1951 by Linus Pauling and coworkers

Software to predict coiled-coils

• STRAP contains an algorithm to predict coiled-coils from AA-sequences
• NCOILS

Additional references

• Crick FHC. (1953) "The Packing of α-Helices: Simple Coiled-Coils", Acta Cryst., 6, 689-697.
• Nishikawa K. and Scheraga HA. (1976) "Geometrical Criteria for Formation of Coiled-Coil Structures of Polypeptide Chains", Macromolecules, 9, 395-407.
• Harbury PB, Zhang T, Kim PS and Alber T. (1993) "A Switch Between Two-, Three-, and Four-Stranded Coiled Coils in GCN4 Leucine Zipper Mutants", Science, 262, 1401-1407.
• Gonzalez L, Plecs JJ and Alber T. (1996) "An engineered allosteric switch in leucine-zipper oligomerization", Nature Structural Biology, 3, 510-515.
• Harbury PB, Plecs JJ, Tidor B, Alber T and Kim PS. (1998) "High-Resolution Protein Design with Backbone Freedom", Science, 282, 1462-1467.
• Yu YB. (2002) "Coiled-coils: stability, specificity, and drug delivery potential", Adv. Drug Deliv. Rev., 54, 1113-1129.
• Burkhard P, Ivaninskii S and Lustig A. (2002) "Improving Coiled-coil Stability by Optimizing Ionic Interactions", Journal of Molecular Biology, 318, 901-910.
• Gillingham AK and Munro S. (2003) "Long coiled-coil proteins and membrane traffic.", Biochim. Biophys. Acta, 1641, 71-85.
• Mason JM and Arndt KM, (2004) "Coiled coil domains: stability, specificity, and biological implications", Chembiochem, 5, 170-6.