Effects of I domain deletion on the function of the β2 integrin lymphocyte function-associated antigen-1

A subset of integrin alpha subunits contain an I domain, which is important for ligand binding. We have deleted the I domain from the beta 2 integrin lymphocyte function-asssociated antigen-1 (LFA-1) and expressed the resulting non-I domain-containing integrin (Delta I-LFA-1) in an LFA-l-deficient T cell line. Delta I-LFA-1 showed no recognition of LFA-1 ligands,confirming the essential role of the I domain in ligand binding. Except for I domain monoclonal antibodies (mAbs), Delta I-LFA-1 was recognized by a panel of anti-LFA-l mAbs similarly to wild-type LFA-1. However, Delta I-LFA-1 had enhanced expression of seven mAb epitopes that are associated with beta 2 integrin activation, suggesting that it exhibited an "active" conformation. Ln keeping with this characteristic, Delta I-LFA-1 induced constitutive activation of alpha 4 beta 1 and alpha 5 beta 1, suggesting intracellular signaling to these integrins. This "cross-talk" was not due to an effect on beta 1 integrin affinity. However, the enhanced activity was susceptible to inhibition by cytochalasin D, indicating a role for the cytoskeleton, and also correlated with clustering of beta 1 integrins. Thus, removal of the I domain from LFA-1 created an integrin with the hallmarks of a constitutively active receptor mediating signals into the cell. These findings suggest a key role for the I domain in controlling integrin activity.