A mix of
beta-sheet and
alpha helix, or rather a
superstructure. More properly called a 3-solenoid (where a
beta sandwich is a 2-solenoid), it is generally formed from three sheets joined by turns. That is, the repeating unit is:
____|-------------\___________|-------------\___________|-------------\_____
|-------------/ |-------------/ |-------------/
Strand 1 Turn 1 Strand 2 Turn 2 Strand 3
Another of these -S-T-S-T-S- units packs on top, winding round to form a large helical structure. This is naturally quite stable - although the stability is increased by ladders or stacks. A ladder is a series of residues of the same type (
asp, say) that align along the long axis of the helix. The phe rings stack nicely, forming
pi-pi interactions. Typically, the loops provide these stacking residues - so each rung of the ladder is part of a separate STSTS unit.
Examples of this fold are, notably, pectate lyases (which break down pectate) and virus tailspike proteins. This kind of structure is especially useful for the virus, as it can be extended as far as necessary by simple duplication of its gene. The regularity of the structure is also an advantage for a beetle ice binding protein. Since ice has a crystalline structure, the regular arrangement of the sidechains in a beta helix allows the protein to form a flat perfectly spaced interface for binding the ice.