Single molecule atomic force microscopy was used to characterize structure,
binding strength (unbinding force), and binding kinetics of a classical ca
dherin, vascular endothelial (VE)-cadherin, secreted by transfected Chinese
hamster ovary cells as cis-dimerized full-length external domain fused to
Fc-portion of human IgC, In physiological buffer, the external domain of VE
-cadherin dimers is a approximate to 20-nm-long rod-shaped molecule that co
llapses and dissociates into monomers (V-shaped structures) in the absence
of Ca2+ Trans-interaction of dimers is a low-affinity reaction (K-D = 10(-3
-)10(-5) M, k(off) = 1.8 s(-1), k(on) = 10(3)-10(5) M-1 s(-1)) with relativ
ely low unbinding force (35-55 pN at retrace velocities of 200-4,000 nm.s(-
1)), Higher order unbinding forces, that increase with interaction time, in
dicate association of cadherins into complexes with cumulative binding stre
ngth. These observations favor a model by which the inherently weak unit bi
nding strength and affinity of cadherin trans-interaction requires clusteri
ng and cytoskeletal immobilization for amplification. Binding is regulated
by low-affinity Ca2+ binding sites (KD = 1.15 mM) with high cooperativity (
Hill coefficient of 5.04), Local changes of free extracellular Ca2+ in the
narrow intercellular space may be of physiological importance to facilitate
rapid remodeling of intercellular adhesion and communication.