A MECHANISM FOR DIVALENT-CATION REGULATION OF BETA-3-INTEGRINS

Integrins alpha(IIb)beta3 and alpha(V)beta3 mediate numerous cell-matr ix and cell-cell contacts. Both integrins contain multiple divalent ca tion-binding motifs that regulate ligand binding. Here, we elucidate a major difference in the regulation of alpha(IIb)beta3 and alpha(V)bet a3 by divalent ions. Fibrinogen binding to alpha(IIb)beta3 in Ca2+-con taining buffer is rapid, with an apparent association rate constant (k 1app) of 8.2 x 10(5) M-1 s-1, but Ca2+ does not support association be tween fibrinogen and alpha(V)beta3. Interestingly, Mn2+ supports fibri nogen binding to both integrins, albeit with a relatively slow associa tion rate (k1app = 10(4) M-1 s-1). This influence of divalent ions on ligand association rates accounts for the opposite divalent ion requir ements for platelet aggregation and tumor cell adhesion to fibrinogen. Furthermore, the regulation of fibrinogen binding to alpha(V)beta3 is complex when both Ca2+ and Mn2+ are present. Physiological concentrat ions of Ca2+ completely ablated adhesion. Kinetic analysis demonstrate d that Ca2+ is a mixed-type inhibitor of Mn2+-supported fibrinogen bin ding to alpha(V)beta3. Consequently, the data presented here suggest a mechanism in which two separate cation-binding sites regulate ligand binding to beta3-integrins.