Disruption of the β3 663-687 disulfide bridge confers constitutive activity to β3 integrins

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Abstract
The platelet fibrinogen receptor, integrin αIIbβ3, 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 β3 in the formation of functional complexes with α subunits. Progressive carboxy-terminal deletions of β3 revealed that surface exposure of αIIbβ3 or αvβ3 could not occur in the absence of the transmembrane domain of β3. In contrast, internal deletions 616 to 690 of the carboxy-terminal regions of the β3 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 β3 or chimeric β3-β7 subunits revealed that disruption of the C663-C687 disulfide bridge endows constitutive activity to the αIIbβ3 receptor. It is concluded that the carboxy-terminal tail of the β3 ectodomain, so-called β tail domain (βTD), is not essential for cell surface expression of β3 receptors. However, a basal, nonactivated, low ligand-affinity state of the β3 integrins demands a normal conformation of this domain. (Blood. 2003;102:2491-2497)

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