Nine integrin a subunits contain an 'inserted' or I-domain, known to involve in ligand binding. Mutation of an invariant isoleucine residue in the 1-domains of alpha L and alpha M has previously been reported to activate LFA-1 and Mac-1, respectively. In this article, we report notable differences in the regulation of adhesion of these two integrins. We find that mutation of the isoleucine residue in the proposed "socket for isoleucine" in full-length alpha L does not lead to an active LFA-1, although mutation of the equivalent residue in alpha M does convey constitutive activity to Mac-1. In addition, we observe the isolated I-domain of alpha L to be constitutively active. This challenges reports that state the aL I-domain exists in an inactive, closed conformation, and requires the presence of activating agents for ligand binding. These results shed further light on the many questions surrounding regulation of integrin activation. (c) 2005 Elsevier Inc. All rights reserved.