The island domain in BRI1 corresponds to a large insertion in the regular repeat-structure between LRRs 21 and 22. The resulting 70-residue segment forms a small domain that folds back into the interior of the LRR superhelix, where it makes extensive polar and hydrophobic interactions with LRRs 13-25. The domain fold is characterized by an anti-parallel beta-sheet, which is sandwiched between the LRR core and a 3-10 helix and stabilized by a disulphide bridge. The insertion of a folded domain into the LRR repeat has not been observed in other LRR receptor structures, and is probably an adaptation to the challenge of sensing a small steroid ligand (rather than larger ligands, such as proteins, nucleic acids, or lipids.

1: Structural basis of steroid hormone perception by the receptor kinase BRI1. Hothorn M, Belkhadir Y, Dreux M, Dabi T, Noel JP, Wilson IA, Chory J; Nature. 2011;474:467-471. PMID:21666665