Self-assembly of the vascular endothelial cadherin ectodomain in a Ca2+-dependent hexameric structure

Vascular endothelial cadherin (VE-cadherin) is a transmembrane protein essential for endothelial cell monolayer integrity (Gulino, D., Delachanal, E., Concord, E., Genoux, Y., Morand, B., Valiron, M. O., Sulpice, E., Scaife, R., Alemany, M., and Vernet, T. (1998) J. Biol. Chem. 273, 29786-29793). This molecule belongs to the cadherin family of cell-cell adhesion receptors, for which molecular details of homotypic interactions are still lacking. In this study, a recombinant fragment encompassing the four N-terminal modules of VE-cadherin (VE-EC1-4) was shown to associate, in solution, as a stable Ca(2+)-dependent oligomeric structure. Cross-linking experiments combined with mass spectrometry demonstrated that this oligomer is a hexamer. Gel filtration chromatography experiments and analytical ultracentrifugation analyses revealed the existence of an equilibrium between the hexameric and monomeric species of VE-EC1-4. The concentration at which 50% of VE-EC1-4 is in its hexameric form was estimated as 1 microm. The dimensions of the hexamer, measured by cryoelectron microscopy to be 233 +/- 10 x 77 +/- 7 A, are comparable to the thickness of adherens endothelial cell-cell junctions. Altogether, the results allow us to propose a novel homotypic interaction model for the class II VE-cadherin, in which six molecules of cadherin form a hexamer.