A Mechanism for Divalent Cation Regulation of &-Integrins *

Integrins axnth and avpS mediate numerous cell-matrix and cell-cell contacts. Both integrins contain multiple divalent cation-bin~ng motifs that regulate ligand binding. Here, we elucidate a major difference in the regulation of al&s and q& by divalent ions. Fibrinogen binding to a1m& in Ca2+-containing buffer is rapid, with an apparent association rate constant (kiapp) of 8.2 x lo6 M ~ s-l, but Ca2+ does not support association between fibrinogen and -os. Interestingly, Mnz+ supports fibrinogen binding to both integrins, albeit with a relatively slow association rate (klapp lo4 ~ l s-l). This influence of divalent ions on ligand association rates accounts for the opposite divalent ion requirements for platelet aggregation and tumor cell adhesion to fibrinogen. Furthermore, the regulation of fibrinogen binding to wP.9 is complex when both Ca2+ and M z+ are present. Physiological concentrations of Caa+ completely ablated adhesion. Kinetic analysis demonstrated that Ca2+ is a mixed-type inhibitor of Mn2+-supported fibrinogen binding to *&. Consequently, the data presented here suggest a mechanism in which two separate cation-binding sites regulate ligand binding to Ps-integrins.