The cysteine-rich repeats in the stalk region of integrin β subunits appear to convey signals impinging on the cytoplasmic domains to the ligand-binding headpiece of integrins. We have examined the functional properties of mAbs to the stalk region and mapped their epitopes, providing a structure-function map. Among a panel of 14 mAbs to the β2 subunit, one, KIM127, preferentially bound to αLβ2 that was activated by mutations in the cytoplasmic domains, and by Mn2+. KIM127 also bound preferentially to the free β2 subunit compared with resting αLβ2. Activating β2 mutations also greatly enhanced binding of KIM127 to integrins αMβ2 and αXβ2. Thus, the KIM127 epitope is shielded by the α subunit, and becomes reexposed upon receptor activation. Three other mAbs, CBR LFA-1/2, MEM48, and KIM185, activated αLβ2 and bound equally well to resting and activated αLβ2, differentially recognized resting αMβ2 and αXβ2, and bound fully to activated αMβ2 and αXβ2. The KIM127 epitope localizes within cysteine-rich repeat 2, to residues 504, 506, and 508. By contrast, the two activating mAbs CBR LFA-1/2 and MEM48 bind to overlapping epitopes involving residues 534, 536, 541, 543, and 546 in cysteine-rich repeat 3, and the activating mAb KIM185 maps near the end of cysteine-rich repeat 4. The nonactivating mAbs, 6.7 and CBR LFA-1/7, map more N-terminal, to subregions 344–432 and 432–487, respectively. We thus define five different β2 stalk subregions, mAb binding to which correlates with effect on activation, and define regions in an interface that becomes exposed upon integrin activation.
