Cadherins are essential cell surface proteins that mediate cell-cell adhesion in all soft tissues. They play critical roles in development and morphogenesis, by directing tissue-specific cell segregation and sorting. A major, unresolved question concerns the molecular basis of cadherin adhesion and binding selectivity. There have been several attempts to elucidate the structural basis of cadherin function from crystal structures, but despite numerous biochemical and structural studies, the identities of the functional adhesive domains and the mechanisms of cadherin binding remain controversial. In this work, we propose to directly address this issue with experimental approaches that directly probe the adhesive function of these proteins. Molecular force probes provide the means to achieve this. Both the surface force apparatus (SFA) and single molecule dynamic force spectroscopy (DFS) provide a powerful, complementary set of tools for uniquely determining the molecular mechanisms of cadherin adhesion and selectivity. By directly measuring the distance-dependent cadherin interaction potentials with the SFA, we will obtain unique information regarding the cadherin docking alignments, and hence, the protein regions that mediate binding. This information, which can't be directly obtained by any other current techniques, will facilitate the identification of the adhesive ectodomain segments, and thereby directly test several key hypotheses concerning the identities of the functional cadherin domains. On the other hand, the proposed DFS measurements will reveal subtle details of single cadherin bonds that may not be evident from the population-averaged, SFA measurements, but may nevertheless control cadherin recognition. These latter measurements will test the hypothesis that differences in the characteristics of cadherin bonds determine cadherin specificity. We will directly test models for cadherin binding and recognition suggested by these force measurements, by determining the impact of cadherin structure variations on both the interprotein potentials and the cadherin bond strengths. The latter will provide a direct link between cadherin architecture and function. Together, these proposed investigations will provide a comprehensive analysis of cadherin function at an unprecedented level of detail, revealing both the underlying adhesive mechanisms and their structural origins.
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