Role of the alphaI domain in ligand binding by integrin alphaEbeta7.

The I domain of integrin alphaE was modeled on the crystal structure of that in CD11b and mutated to produce an open (high affinity) or closed (low affinity) conformation. K562 transfectants expressing mutant alphaE and wild-type beta7 were tested for adhesion to E-cadherin-Fc. Downward displacement of the C terminus of the alphaI domain with a disulfide bridge enhanced adhesion and Mn(2+) dependency. Adhesion greatly exceeded that observed using wild type integrin under similar conditions. The closed integrin gave poor adhesion which was greatly improved by PMA-induced clustering. Blocking beta7 function with a betaI domain-specific antibody inhibited the wild-type but not the locked open integrin. Isolated open alphaI domain expressed on K562 cells showed strong Mn(2+)-dependent adhesion to E-cadherin, whereas the wild-type version was ineffective. alphaEbeta7 was shown to bind to monomeric E-cadherin but to only one component of dimeric E-cadherin. Finally, we report that M290, a function-blocking antibody, bound to a conformation-sensitive epitope near the rim of the alphaI domain MIDAS and recognized wild-type and closed alphaI domain but not the open conformation. The results broadly support the paradigm for affinity regulation by conformational change that has been established for beta2 integrins. Nevertheless, for alphaE, the fully open conformation may represent an extreme situation that does not occur physiologically.