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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 beta-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.

Recent cadherin studies have revealed that many cadherins and cadherin-related proteins are expressed in various tissues of different multicellular organisms. These proteins are characterized by the multiple repeats of the cadherin motif in their extracellular domains. The members of the cadherin superfamily are divided into two groups: classical cadherin type and protocadherin type. The current cadherins appear to have evolved from a protocadherin type. Recent studies have proved the cell adhesion role of classical cadherins in embryogenesis. In contrast, the biological role of protocadherins is elusive. Circumstantial evidence, however, suggests that protocadherins are involved in a variety of cell-cell interactions. Since protocadherins, and many other new cadherins as well, have unique properties, studies of these cadherins may provide insight into the structure and biological role of the cadherin superfamily.