Two classic cadherin-related molecules with no cadherin extracellular repeats in the cephalochordate amphioxus: distinct adhesive specificities and possible involvement in the development of multicell-layered structures

We previously reported the existence of Bb-cadherin, a molecule related to classic cadherin, in the cephalochordate amphioxus (Branchiostoma belcheri). The structure of Bb-cadherin is unique in that it lacks the cadherin extracellular repeats, although its cytoplasmic domain shows close similarities to those of typical classic cadherins. The extracellular region of Bb-cadherin consists of laminin globular domains and a cysteine-rich EGF-like domain that are similar to domains in nonchordate classic cadherins. In this study, we identified a second amphioxus cadherin. It was designated Bb2-cadherin (Bb2C) while the previously reported cadherin has been renamed Bb1-cadherin (Bb1C). Bb2C is very similar to Bb1C in its overall structure and amino acid sequence. Genomic BLAST searches and phylogenetic analyses suggested that these two amphioxus genes have been generated through a gene duplication that occurred after separation of the cephalochordates from the other animals. They also bear distinct adhesive specificities. Immunohistochemical analyses showed that Bb1C and Bb2C, together with β-catenin, appear to function as adherens junction constituents in the epithelia of different germ layers of the amphioxus embryo. Differential expression of the two cadherins was also observed in the developing, multicell-layered notochord. These observations suggest that, despite their unique structures, the functions and developmental roles of Bb1C and Bb2C are comparable to those of the classic cadherins characterized to date in other animal groups, such as the vertebrate E- and N-cadherins and the Drosophila DE- and DN-cadherins. The possible involvement of Bb1C and Bb2C in the development of multicell-layered structures characteristic of the cephalochordate body plan is presented.