ACTIVATION OF THE INTEGRIN ALPHA(V)BETA(3) INVOLVES A DISCRETE CATION-BINDING SITE THAT REGULATES CONFORMATION

''Activation'' of integrins is involved in the dramatic transition of leukocytes and platelets from suspension to adhesion, The integrin alp ha(v) beta(3), is not known to take part in this sort of transition, e ven though it shares its beta subunit with alpha(IIb)beta(3), the acti vable integrin on platelets, In the context of a constitutively adhere d cell, changes in activation state may be more subtle in their effect s, but nonetheless important in regulating cell behavior, We hypothesi zed that alpha(v) beta(3) can undergo conformational changes analogous to those associated with alpha(IIb)beta(3) activation, Accordingly, w e examined alpha(v) beta(3) on the surface of M21 cells (a human melan oma cell line) and found that, like alpha(IIb)beta(3), it can undergo conformational changes upon binding of a ligand analog and can be acti vated for ligand binding and migration by a monoclonal antibody direct ed against beta(3). Modulation of the binding of this activating antib ody, AP5, ligand binding, and antibody mediated activation all are ass ociated with a discrete cation-binding site shared in both alpha(IIb)b eta(3) and alpha(v) beta(3). Based on a measured K-i, this site has an apparent K-d for calcium of approximately 20 mu M. At physiological l evels of calcium, about 40% of the total alpha(v) beta(3) on a cell's surface is in a conformation detected by AP5, The data suggest a model for both alpha(v) beta(3) and alpha(IIb)beta(3) function in which the molecule can exist in either of (at least) two conformational states, one stabilized either by AP5 or ligand binding, refractory to calcium binding, and enhanced for ligand recognition, the other stabilized by calcium binding and refractory to AP5 and ligand binding, Functional analysis suggests that AP5 activates alpha(v) beta(3) by preventing oc cupation of this calcium site, and that the activated form of alpha(v) beta(3) differs functionally from the basal form, The active form is more conducive to migration and the basal to tight adhesion.