Solenoid protein domain

As a "rule of thumb", short repetitive sequences (e.g. those below the length of 10 amino acids) may be intrinsically disordered, and not part of any folded protein domains.

Examples of disordered repetitive sequences include the 7-mer peptide repeats found in the RPB1 subunit of RNA polymerase II,[2] or the tandem beta-catenin or axin binding linear motifs in APC (adenomatous polyposis coli).

A clear advantage of this topology is that both the N- and C-terminal ends are free to add new repeats and folds, or even remove existing ones during evolution without any gross impact on the structural stability of the entire domain.

Whenever a linear solenoid domain structure participates in protein-protein interactions, frequently at least 3 or more repetitive subunits form the ligand-binding sites.

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".

Even if these domains are perfectly capable of folding on their own, some of them might bind together and assume a rigidly fixed orientation in the full protein.

This elegantly explains why individual constituents of this supradomain block are incapable of ligand binding, while their proper assembly endows them with a novel function.