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. 2016 Jan 15;5:e11861. doi: 10.7554/eLife.11861

Figure 1. Transitions in periodicity caused by insertions of one to six residues into the heptad repeat.

Figure 1.

The green area marks the estimated boundaries of periodicities accessible to α-helical coiled coils. It is centered around the periodicity of unperturbed α-helices, about 3.63 residues per turn. Higher values than 3.63 lead to right-handed and lower values to left-handed supercoiling. The effects of consecutive insertions of stammers (3 residues) or stutters (4 residues) into a heptad pattern are shown by blue and green lines, respectively. The red lines correspond to the insertion of 1 to 6 residues into the heptad periodicity and their progressive delocalization over neighboring heptads. For example, an insertion of 4 residues is accommodated as 11 residues over 3 turns (11/3), when delocalized over one heptad, or as 18/5, when delocalized over two. Insertions of 1 or 5 residues have to be delocalized over two heptads, resulting in periodicities of 15/4 or 19/5 (which could also be brought about by consecutive stutters – following the green line from 7/2 over 11/3 over 15/4 to 19/5). Insertions of 3 can be accommodated as 10/3, at the very edge of the green area, although in the known examples the α-helices are distorted due to the strong left-handed supercoiling which could be avoided by further delocalization. For insertions of 2 or 6 residues (dashed lines) a strong delocalization would be required to reach the green lawn of accessible periodicities. However, for all constructs in this paper, this is not observed. Via the formation of β-layers these insertions sustain the heptad periodicity as unperturbed as possible.

DOI: http://dx.doi.org/10.7554/eLife.11861.003