Homophilic adhesion of E-cadherin occurs by a co-operative two-step interaction of N-terminal domains

Cluster formation of E-cadherin on the cell surface is believed to be of major importance for cell-cell adhesion. To mimic this process the extracellular part of mouse E-cadherin (ECAD) was recombinantly fused to the assembly domain of rat cartilage oligomeric matrix protein (COMP), resulting in the chimeric protein ECAD-COMP. The COMP domain formed a five-stranded alpha-helical coiled-coil, This enabled the formation of a pentameric ECAD with bundled C-termini and free N-termini. The pentameric protein construct ECAD-COMP and the monomeric ECAD were expressed in human embryonal kidney 293 cells. Electron microscopy, analytical ultracentrifugation, solid phase binding and cell attachment assays revealed that pentamers showed strong self-association and cell attachment, whereas monomers exhibited no activity. At the high internal concentration in the pentamer the N-terminal EC1 domains of two E-cadherin arms interact to form a ring-like structure. Then the paired domains interact with a corresponding pair from another pentamer. None of the four other extracellular domains of E-cadherin is involved in this interaction. Based on these results, an in vivo mechanism is proposed whereby two N-terminal domains of neighbouring E-cadherins at the cell surface first form a pair, which binds with high affinity to a similar complex on another cell. The strong dependence of homophilic interactions on C-terminal clustering points towards a regulation of E-cadherin mediated cell-cell adhesion via lateral association.