Angulins: a family is born

The intercellular space formed at tricellular contacts, where the corners of three epithelial cells meet, is sealed by specialised structures called tricellular tight junctions (tTJs). Tricellulin and lipolysis-stimulated lipoprotein receptor (LSR) are the known protein components of tTJs, with LSR recruiting tricellulin to the tTJs. Mikio Furuse and colleagues (p. 966) now examine the cellular functions of two LSR-related proteins, immunoglobulin-like domain-containing receptor 1 (ILDR1) and ILDR2. A bootstrap analysis of all three proteins showed that they are evolutionarily conserved among vertebrates. Next, the authors demonstrated that, similar to LSR, ILDR1 and ILDR2 localise to tricellular contacts in epithelial cells and recruit tricellulin; moreover, at least one of these three proteins is expressed in each of the epithelial tissues examined, and the expression of any one of them is sufficient to recruit tricellulin. ILDR1 and ILDR2, say the authors, therefore might be found at tTJs. LSR contributes to epithelial barrier function, and the authors show that, whereas ILDR1 is involved in such functioning, the requirement for ILDR2 is less. Further analysis of ILDR1, mutation of which underlies a familial deafness at the DFNB42 locus in humans, shows that most DFNB42-associated ILDR1 mutant proteins are defective in tricellulin recruitment. Considering these findings, the authors propose grouping LSR, ILDR1 and ILDR2 together to form the angulin protein family.