Cadherins are calcium dependent cell adhesion molecules. The interaction between cadherin molecules is important in establishing cell polarity, tissue identity and tissue boundary. This study focused on the mechanism of cadherin mediated cell-cell interaction and binding specificity. First, the bond properties were studied with the micropipette manipulation method.The results revealed that cadherin binding exhibits biphasic kinetics. Further studies with domain deletion mutants showed that the EC1 domain of cadherin extracellular region is responsible for the first phase of the biphasic kinetic profile. The affinity and dissociation rate of EC1 bond were then extracted by modeling the first phase. Second, the differences between cadherin homophilic and heterophilic interactions were investigated. The measurements showed that the properties of both heterophilic and homophilic bonds are similar. Surprisingly, Rac1 activation triggered by cadherin adhesion showed a greater response to homophilic than to heterophilic binding by Xenopus cleavage stage cadherin (C-cadherin) and by canine epithelial cadherin (E-cadherin). Finally, mutations were introduced in C-cadherin to study the structural basis of cadherin dependent cell sorting specificity. Three mutations were chosen according to the structure and sequence difference between C- and N- (neural) cadherin. One mutation, the S78A, completely switched the C-cadherin sorting specificity. More interestingly, the three mutations influenced both cadherin affinity and Rac1 activation. The consequent changes in affinity correlated both with sorting and with the changes in Rac1 activation. The results showed a direct link between affinity and Rac1 activation, and identified a subset of residues in C-cadherin that have a significant impact on binding specificity.
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