These domains are in some instances better described as beta barrels rather than beta sandwiches, for example 1STP; there have a hydrophobic core in the centre of the barrel.

Up-and-down Antiparallel Beta Sheets

The simplest topology for an antiparallel beta sheet involves loops connecting adjacent strands, as shown:

Barrel structures composed entirely of up-and-down sheet are relatively uncommon e.g. soybean trypsin inhibitor, the N-terminal domain of papain, rubredoxin. azurins? plastocyanins?

The Greek Key Topology

The Greek Key topology, named after a pattern that was common on Greek pottery, is shown below. Three up-and-down beta strands connected by hairpins are followed by a longer connection to the fourth strand, which lies adjacent to the first.

Folds including the Greek key topology have been found to have 5-13 strands. Examples are given below.

1STP staphylococcal nuclease

Gamma-crystallin

Gamma-crystallin has two domains each of which is an eight- stranded beta barrel-type structure composed of two Greek keys:

In fact, the structure is more accurately described as consisting of two beta sheets, one consisting of strands 2,1,4,7 and the other of strands 6,5,8,3, as indicated in the diagram. This can be seen by examining the crystal structure of gamma-crystallin.

The Jellyroll Topology

Richardson(1979) describes the jellyroll fold as being formed by the addition of an extra "swirl" to a Greek key: