Immunocytochemical studies of the interactions of cadherins and catenins in the early Xenopus embryo

Linkage of cadherins to the cytoskeleton is crucial for their adhesive function. Since α‐ and β‐catenin play a key role in this linkage, these proteins are possible targets for processes that control cell–cell adhesion. To achieve a better understanding of the regulation of cell–cell adhesion in embryonic morphogenesis, we used immunohistology to investigate how in Xenopus blastomeres catenins respond to disturbances in the expression of maternal cadherins. Overexpression of myc‐tagged maternal cadherin leads to a proportionate increase of the level of β‐catenin. The two proteins colocalize in the endoplasmic reticulum, in cytoplasmic vesicles, and along the cell membrane, indicating that the β‐catenin binds to overexpressed cadherin early in its passage to the plasma membrane. Expression of cadherin is essential for the stable presence of β‐catenin, as depletion from maternal cadherin mRNA leads to a complete loss of β‐catenin from the blastomeres. α‐catenin behaves differently. Overexpression of cadherin leaves the amount and localization of α‐catenin largely unaffected, and additional cadherin inserts itself into the membrane without a pro‐portionate rise in the level of membrane‐bound α‐catenin. However, cadherin mRNA depletion leads to a redistribution of α‐catenin from the membrane to the cytoplasm. Thus, cadherin is required to localize α‐catenin to the membrane, but the amount of α‐catenin along the mem‐brane seems to be restricted to a certain level which cannot be exceeded. The relevance of these observations for the regulation of cad‐herin‐mediated cell adhesion in the Xenopus embryo is discussed. Additionally, we demonstrate that plakoglobin, like β‐catenin an armadillo repeat protein, shows neither accumulation after overexpression nor colocalization with the overexpressed cadherin. Dev Dyn 1999;215:155–169. © 1999 Wiley‐Liss, Inc.