Four-cylinder bundles

Both the twin-wedge complex and the unpolymerized analog exhibited enantio-tropic hexagonal columnar LC phases, based on a repeating four-cylinder bundle pattern, as confirmed by X-ray diffraction (Fig. 7.8). 

Figure 3.6 Four-helix bundles frequently occur as domains in a proteins. The arrangement of the a helices is such that adjacent helices in the amino acid sequence are also adjacent in the three-dimensional structure. Some side chains from all four helices are buried in the middle of the bundle, where they form a hydrophobic core, (a) Schematic representation of the path of the polypeptide chain in a four-helrx-bundle domain. Red cylinders are a helices, (b) Schematic view of a projection down the bundle axis. Large circles represent the main chain of the a helices small circles are side chains. Green circles are the buried hydrophobic side chains red circles are side chains that are exposed on the surface of the bundle, which are mainly hydrophilic.

All but one of the above structures have four helices in the bundle, with + 1,+ 1,+ 1 connections. For the up and down topology on a cylinder, handedness can be defined by whether the chain turns to the right or to the left at the end of the first structure element (whether it is a helix or a /3 strand). With an even number of helices, reversing N to C direction of the chain also reverses handedness of the topology for an odd number of helices or strands handedness is invariant to chain reversal. For + 1, + 1, +1 topologies in general, handedness is not... 

These four structures then form the second major subgrouping of antiparallel a domains, which we will call Greek key helix bundles (see Fig. 73). The helix elements lie on an approximate cylinder (see Fig. 89d for an end view), with 0 to 45° right-handed twist relative to the cylinder axis they are connected with a Greek key topology which can have either a counterclockwise (globins) or a clockwise (thermolysin d2 and T4 lysozyme d2) swirl when viewed from the outside. 

The acetylcholine receptor has four subunits (a, /3, y, and 5, with Mr 52, 56, 63, and 66 x 103, respectively). The complete receptor includes two copies of the a subunit, and one of each of the others. The overall structure, as visualized by electron microscopy, resembles a cylindrical bundle of five, approximately parallel rods, with a water-filled channel along the axis of the cylinder (fig. SI. 12). This assembly projects about 70 A into the synaptic cleft on one side of the membrane and about 40 A into the intracellular... 

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