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 soluble ligands (fibrinogen [Fg] and PAC1). Ligand binding to integrins initiates outside-in signaling that leads to cellular responses such as cell spreading and focal adhesion formation. A point mutation in the beta(3) cytoplasmic domain (S-752 --> P) is associated with blocked inside-out (alpha(IIb)beta(3) signaling in a variant Glanzmann's thrombasthenia. This mutation was introduced into beta(3) and cotransfected into Chinese hamster ovary cells with a chimeric alpha subunit consisting of the alpha(IIb) extracellular and transmembrane domains and the alpha(6B) cytoplasmic domain. The substitution of the alpha(IIb) cytoplasmic domain with that of alpha(6) led to activation of (alpha(IIb)beta(3) to bind PAC1, mimicking inside-out signaling. This effect was reversed by the S-752 --> P mutation, indicating a disruption of inside-out signaling by the mutation. In addition, transfectants expressing this beta(3) variant showed reduced alpha(IIb)beta(3)-mediated cell spreading on immobilized Fg, focal adhesion, and fibrin clot retraction, suggesting an impairment in outside-in alpha(IIb)beta(3) Signaling. Therefore, a single point mutation in the beta(3) cytoplasmic domain impaired bidirectional signaling through alpha(IIb)beta(3). (C) 1994 by The American Society of Hematology.