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Analysis of Heterophilic and Homophilic Interactions of Cadherins Using the c-Jun/c-Fos Dimerization Domains*

Cadherin-mediated cell-cell adhesion is initiated by cis dimerization of cadherin ectodomains at the cell surface followed by an antiparallel trans interaction of dimers on opposing cells. To resolve open questions concerning the molecular details and specificity of cis andtrans interactions, ectodomains of E- and P-cadherin were analyzed by chemical cross-linking and by electron microscopy. At the high intrinsic concentration created by artificial oligomerization the N-terminal cadherin (CAD)-domain of P-cadherin are forming ring-likecis dimers. At 2 mm Ca2+-associated rings involving two cis dimers indicate transcontacts in electron micrographs. cis and transinteractions were further analyzed by heterodimerization of the ectodomains of E-cadherin (ECAD) and P-cadherin (PCAD) through the leucine zipper domains of c-Jun and c-Fos. ECADJun/ECADFos dimers predominantly form ring-like cis dimers at 1 mmCa2+ and double-ringed trans contacts above 2 mm Ca2+. The Ca2+-dependent tetrameric transcontacts of ECADJun/ECADFos dimers are also detectable after chemical cross-linking. Only cis contacts but no transinteractions are observed for heterodimers of ECADFos and the Trp-2 to Ala mutant ECADW2AJun arguing for a decisive role of Trp-2 intrans but not cis interaction. Neithercis nor trans interaction was found for heterodimers of ECADJun and PCADFos suggesting that specificity for homophilic interactions already exists at the level of cisdimerization.