We have identified a Drosophila homolog of vertebrate classic cadherins. A monoclonal antibody to Drosophila alpha-catenin (D alpha-catenin) copurifies a 150-kDa glycoprotein (gp150) along with the alpha-catenin. To further characterize this protein, we generated monoclonal antibodies to gp150 and isolated its cDNAs using the antibodies. Predicted sequences of the encoded product revealed that it is a transmembrane protein with similarity to vertebrate classic cadherins, and so we designated this molecule DE-cadherin. The extracellular domain has six cadherin-specific repeats, although the first repeat seems to be cleaved off upon maturation, and the cytoplasmic domain shows significant identity to that of vertebrate classic cadherins. DE-cadherin is distinguishable from its vertebrate counterparts by a large insertion with local sequence similarity to Fat, laminin A chain, Slit, and neurexin I at the proximal region of the extracellular domain. Despite such differences, DE-cadherin is functionally similar to vertebrate classic cadherins. For example, it is associated with alpha-catenin and beta-catenin (Armadillo), and protected from trypsin digestion only in the presence of Ca2+, as is the case for many of classic cadherins. Transfection of S2 cells with the DE-cadherin cDNA enhances their Ca(2+)-dependent cell aggregation. Antibodies to this molecule inhibited aggregation of not only the transfectants but also early embryonic cells. DE-cadherin is concentrated at the apical poles of epithelial cell-cell junctions. All these results suggest that DE-cadherin is a homolog of vertebrate classic cadherins and that the vertebrate and invertebrate share common mechanisms for regulation of cell-cell adhesion.