Modulation of CD11b/CD18 adhesive activity by its extracellular, membrane-proximal regions

The integrin receptor CD11b/CD18 is normally kept in a low adhesive state and can be activated by many different agents. However, the mechanism underlying receptor activation is not yet fully understood. We hypothesized that the extracellular, membrane-proximal regions of CD11b/CD18 are critically involved in modulation of its adhesive functions. To test our hypothesis, we perturbed the extracellular, membrane-proximal regions of individual CD11b and CD18 subunits and studied their effect on ligand binding, receptor clustering, and lipid raft association. We report here three major findings: 1) perturbation of the extracellular, membrane-proximal region of either subunit leads to enhanced adhesion, caused by changes in receptor conformation, but not the state of receptor clustering or lipid raft association; 2) the CD11b subunit plays a more important role in confining the receptor in an inactive state; and 3) upon modification of the extracellular, membrane-proximal region, the mutant CD11b/CD18 acquires the ability to respond to stimulation by "inside-out" signaling. Our results suggest that the extracellular, membrane-proximal region of the receptor plays an important role in integrin activation and therefore could be targeted by certain cell surface proteins as a conduit to control the integrin "inside-out" signaling process.