A POINT MUTATION IN THE INTEGRIN BETA(3) CYTOPLASMIC DOMAIN (S752-P) IMPAIRS BIDIRECTIONAL SIGNALING THROUGH ALPHA-IIB-BETA-P3 (PLATELET GLYCOPROTEIN IIB-IIIA)

Agonist-induced inside-out signaling results in an increased affinity of integrin (gamma IIb beta(3) (platelet glycoprotein IIb-IIIa) for so luble ligands (fibrinogen [Fg] and PAC1). Ligand binding to integrins initiates outside-in signaling that leads to cellular responses such a s cell spreading and focal adhesion formation. A point mutation in the beta(3) cytoplasmic domain (S-752 --> P) is associated with blocked i nside-out (alpha(IIb)beta(3) signaling in a variant Glanzmann's thromb asthenia. This mutation was introduced into beta(3) and cotransfected into Chinese hamster ovary cells with a chimeric alpha subunit consist ing of the alpha(IIb) extracellular and transmembrane domains and the alpha(6B) cytoplasmic domain. The substitution of the alpha(IIb) cytop lasmic domain with that of alpha(6) led to activation of (alpha(IIb)be ta(3) to bind PAC1, mimicking inside-out signaling. This effect was re versed by the S-752 --> P mutation, indicating a disruption of inside- out signaling by the mutation. In addition, transfectants expressing t his beta(3) variant showed reduced alpha(IIb)beta(3)-mediated cell spr eading on immobilized Fg, focal adhesion, and fibrin clot retraction, suggesting an impairment in outside-in alpha(IIb)beta(3) Signaling. Th erefore, a single point mutation in the beta(3) cytoplasmic domain imp aired bidirectional signaling through alpha(IIb)beta(3). (C) 1994 by T he American Society of Hematology.