The αL I (inserted or interactive) domain of integrin αLβ2 undergoes conformational changes upon activation. Recent studies show that the isolated, activated αL I domain is sufficient for strong ligand binding, suggesting the β2 subunit to be only indirectly involved. It has been unclear whether the activity of the αL I domain is regulated by the β2 subunit. In this study, we demonstrate that swapping the disulfide-linked CPNKEKEC sequence (residues 169-176) in the β2 I domain with a corresponding β3 sequence, or mutating Lys174 to Thr, constitutively activates αLβ2 binding to ICAM-1. These mutants do not require Mn2+ for ICAM-1 binding and are insensitive to the inhibitory effect of Ca2+. We have also localized a component of the mAb 24 epitope (a reporter of β2 integrin activation) in the CPNKEKEC sequence. Glu173 and Glu175 of the β2 I domain are identified as critical for mAb 24 binding. Because the epitope is highly expressed upon β2 integrin activation, it is likely that the CPNKEKEC sequence is exposed or undergoes conformational changes upon activation. Deletion of the αL I domain did not eliminate the mAb 24 epitope. This confirms that the αL I domain is not critical for mAb 24 binding, and indicates that mAb 24 detects a change expressed in part in the β2 subunit I domain. These results suggest that the CPNKEKEC sequence of the β2 I domain is involved in regulating the αL I domain.
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