Toroid repeat proteins

[1][2] In the case when the N- and C-terminal repeats lie in close physical contact in a tandem repeat domain, the result is a topologically compact, closed structure.

Such domains typically display a high rotational symmetry (unlike open repeats that only have translational symmetries), and assume a wheel-like shape.

Because of the limitations of this structure, the number of individual repeats is not arbitrary.

Closed solenoids frequently function as protein-protein interaction modules: it is possible that all repeats must be present to form the ligand-binding site if it is located at the centre or axis of the domain "wheel".

[4] The following major sub-classes of toroid repeat proteins can be found:[1][2]