Toroid repeat proteins


A toroid repeat is a protein fold composed of repeating subunits, arranged in circular fashion to form a closed structure.

Structure

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, and assume a wheel-like shape. Because of the limitations of this structure, the number of individual repeats is not arbitrary. In the case of WD40 repeats the number of repeats can range from 4 to 10. Kelch repeats, beta-barrels and beta-trefoil repeats are further examples for this architecture.

Function

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". The WD40 repeat is a prime example of this function.

Classification

The following major sub-classes of toroid repeat proteins can be found:
  1. TIM barrel structures composed of eight units with alternating beta strands and alpha helices
  2. Beta barrel structures composed on a single circular beta sheet
  3. Beta propeller structures composed of beta sheets formed by individual repeat units arranged in a circle, in particular the WD40 repeat and Kelch motif families