A new crystal structure, Ca2+ dependence and mutational analysis reveal molecular details of E-cadherin homoassociation

Electron microscopy of ECADCOMP, a recombinant E-cadherin ectodomain pentam erized by the assembly domain of cartilage oligomeric matrix protein, has b een used to analyze the role of cis-dimerization and trans-interaction in t he hemophilic association of this cell adhesion molecule. The Ca2+ dependen cy of both interactions was investigated. Low Ca2+ concentrations (50 mu M) stabilized the rod-like structure of E-cadherin. At medium Ca2+ concentrat ion (500 mu M), two adjacent ectodomains in a pentamer formed cis-dimers. A t high Ca2+ concentration (>1 mM), two cis-dimers from different pentamers formed a trans-interaction. The X-ray structure of an N-terminal domain pai r of E-cadherin revealed two molecules per asymmetric unit in an intertwist ed X-shaped arrangement with closest contacts in the Ca2+-binding region be tween domains 1 and 2, Contrary to previous data, Trp2 was docked in the hy drophobic cavity of its own molecule, and was therefore not involved in cis -dimerization of two molecules. This was supported further by W2A and A80I (a residue involved in the hydrophobic cavity surrounding Trp2) mutations i n ECADCOMP which both led to abrogation of the trans- but not the cis-inter action. Structural and biochemical data suggest a link between Ca2+ binding in the millimolar range and Trp2 docking, both events being essential for the trans-association.