These are small, all beta strand domains, structurally described for the protein Internalin (InlA) and related proteins InlB, InlE, InlH from the pathogenic bacterium Listeria monocytogenes. Their function appears to be mainly structural: They are fused to the C-terminal end of leucine-rich repeats (LRR), significantly stabilising the LRR, and forming a common rigid entity with the LRR. They are themselves not involved in protein-protein-interactions but help to present the adjacent LRR-domain for this purpose. These domains belong to the family of Ig-like domains in that they consist of two sandwiched beta sheets that follow the classical connectivity of Ig-domains. The beta strands in one of the sheets is, however, much smaller than in most standard Ig-like domains, making it somewhat of an outlier [1],[2] [3].

1: Internalins from the human pathogen Listeria monocytogenes combine three distinct folds into a contiguous internalin domain. Schubert WD, Gobel G, Diepholz M, Darji A, Kloer D, Hain T, Chakraborty T, Wehland J, Domann E, Heinz DW; J Mol Biol 2001;312:783-794. PMID:11575932

2: Structure of internalin, a major invasion protein of Listeria monocytogenes, in complex with its human receptor E-cadherin. Schubert WD, Urbanke C, Ziehm T, Beier V, Machner MP, Domann E, Wehland J, Chakraborty T, Heinz DW; Cell 2002;111:825-836. PMID:12526809

3: Folding and stability of the leucine-rich repeat domain of internalin B from Listeri monocytogenes. Freiberg A, Machner MP, Pfeil W, Schubert WD, Heinz DW, Seckler R; J Mol Biol 2004;337:453-461. PMID:15003459