A single amino acid in the cytoplasmic domain of the beta 2 integrin lymphocyte function-associated antigen-1 regulates avidity-dependent inside-out signaling

The leukocyte-specific beta(2) integrin lymphocyte function-associated antigen-1 (LFA-1) (alpha(L)/beta(2)) mediates activation-dependent adhesion to intercellular adhesion molecule (ICAM)-1. In leukocytes, LFA-1 requires activation by intracellular messengers to bind ICAM-1. We observed malfunctioning of LFA-1 activation in leukemic T cells and K562-transfected cells. This defective inside-out integrin activation is only restricted to beta(2) integrins, since beta(1) integrins expressed in K562 readily respond to activation signals, such as phorbol 12-myristate 13-acetate. To unravel these differences in inside-out signaling between beta(1) and beta(2) integrins, we searched for amino acids in the beta(2) cytoplasmic domain that are critical in the activation of LFA-1. We provide evidence that substitution of a single amino acid (L732R) in the beta(2) cytoplasmic DLRE motif, creating the DRRE motif, is sufficient to completely restore PMA responsiveness of LFA-1 expressed in K562. In addition, an intact TTT motif in the C-terminal domain is necessary for the acquired PMA responsiveness. We observed that restoration of the PMA response altered neither LFA-1 affinity nor the phosphorylation status of LFA-1. In contrast, strong differences were observed in the capacity of LFA-1 to form clusters, which indicates that inside-out activation of LFA-1 strongly depends on cytoskeletal induced receptor reorganization that was induced by activation of the Ca(2+)-dependent protease calpain.