Dd. Hu et al., AN ALLOSTERIC CA2-SITE ON THE BETA-3 INTEGRINS THAT REGULATES THE DISSOCIATION RATE FOR RGD LIGANDS( BINDING), The Journal of biological chemistry, 271(36), 1996, pp. 21745-21751
Here we use a model RGD-containing ligand to study how Ca2+ and Mg2+ r
egulate ligand binding to beta 3-integrins, Fab-9, an antibody that co
ntains an optimized RGD loop in its antigen binding site (Barbas, C. F
., Languino, L., and Smith, J. W. (1993) Proc. Natl. Acad. Sci. U.S.A.
90, 10003-10007), was used as the model ligand. Across a physiologic
range of Mg2+, Fab-9 bound to both alpha v beta 3 and alpha IIb beta 3
with a monophasic binding isotherm. Across the same range of Ca2+, th
e binding of Fab-9 to the beta 3-integrins was biphasic. Low concentra
tions of Ca2+ (mu M) promoted the binding of Fab-9. Higher concentrati
ons of Ca2+ (mM) blocked Fab-9 binding. These data suggest that Ca2+ b
inds to two distinct classes of sites on the beta 3-integrins, with th
e low affinity Ca2+ binding site(s) being an inhibitory site. We desig
nate this inhibitory site(s) as the I site. Further biochemical charac
terization showed that the I site has the following characteristics: 1
) it is specific for Ca2+; 2) it is allosteric to the ligand binding s
ite; 3) its occupation increases the dissociation rate between integri
n and RGD ligand; and 4) occupation of the I site can induce cellular
deadhesion.