Identification of Ca2(+)-dependent cell adhesion molecules in Xenopus by the use of interspecies homology

Ca2(+)-dependent cell adhesion molecules (CAMs) are transmembrane glycoproteins structurally and functionally related in mammalian and avian species. This suggests that Ca2(+)-dependent CAMs consist of an evolutionary conserved gene family. Antibodies or cDNA probes specific either to the extracellular part or the cytoplasmic domain of uvomorulin were compared for their ability to detect corresponding molecules in Xenopus. Only antibodies directed against the evolutionary highly conserved cytoplasmic domain afforded a clear membrane staining on sections of Xenopus embryos or on cultured Xenopus epithelial cells. However, these antibodies recognized different polypeptides of 156, 140 and 128 kDa in immunoblots prepared from cell lysates of epithelial, neural, muscle and embryonic tissues. In concordance with the antibody analysis, signals in Northern hybridizations were only obtained when the cDNA probe encoding the cytoplasmic domain of uvomorulin was used. Here again, this cDNA probe revealed different mRNA species of 4.3, 4.1, 3.8 and 3.2 kb in the studied cell types. These results provide further direct evidence that the Ca2(+)-dependent CAMs are evolutionary conserved. The variety of polypeptides and transcripts observed in Xenopus indicates that several members of this gene family were detected by the use of probes specific to conserved sequences. More important, with this approach we also identified members of this gene family in the early stages of Xenopus development. Since these proteins were present in mature eggs but not in oocytes, we assume a maternal store of Ca2(+)-dependent CAM RNAs whose translation might be initiated during egg maturation.