Disruption of the beta3 663-687 disulfide bridge confers constitutive activity to beta3 integrins

The platelet fibrinogen receptor, integrin alphaIIbbeta3, is a noncovalent heterodimer of glycoproteins IIb and IIIa. This work was aimed at elucidating the role played by the carboxy-terminal extracellular, trans-membrane, and cytoplasmic regions of the glycoprotein beta3 in the formation of functional complexes with alpha subunits. Progressive carboxy-terminal deletions of beta3 revealed that surface exposure of alphaIIbbeta3 or alphavbeta3 could not occur in the absence of the transmembrane domain of beta3. In contrast, internal deletions 616 to 690 of the carboxy-terminal regions of the beta3 ectodomain led to surface exposure of constitu tive active receptors in CHO cells, as indicated by the enhanced rate of cell adhesion to immobilized ligands and spontaneous binding to soluble fibrinogen or activation-dependent antibody PAC-1. The functional analysis of cysteine mutations within the 616 to 690 region of beta3 or chimeric beta3-beta7 subunits revealed that disruption of the C663-C687 disulfide bridge endows constitutive activity to the alphaIIbbeta3 receptor. It is concluded that the carboxy-terminal tail of the beta3 ectodomain, so-called beta tail domain (betaTD), is not essential for cell surface expression of beta3 receptors. However, a basal, nonactivated, low ligand-affinity state of the beta3 integrins demands a normal conformation of this domain.