Figure 2. Common types of helical bundles.

Generally, helical bundles form via relatively well-defined and recurring motifs. The classical diverging straight-helix bundle shown in A) is from the structure E. coli PhoQ sensor domain (PDB code 3BQ8). The bundle shown in B) (from bacteriophytochrome chromophore binding domain, PDB code 2O9C) also diverges, but the individual helices are bent into circular arcs and do not wrap around each other like in coiled coils. This motif can be thought of as a first-order generalization of the classical straight-helix diverging bundle. Shown in C) is the left-handed coiled-coil domain from the yeast transcription factor GCN4 (PDB code 2ZTA). The helical wheel diagram on the left illustrates the seven-residue heptad repeat commonly characteristic of left-handed coiled coils. Positions a and d are in the core and often occupied with hydrophobic amino acids, e and g are along the interface, often forming salt bridges and polar interactions, and b, c, and f adjust to the environment. Although significantly less common, right-handed coiled coils also occur, such as the example shown in D) from the human VASP tetramerization domain (PDB code 1USE). Boxed insets represent an alternative view of each bundle.