The platelet integrin α(IIb)β3 has an endogenous thiol isomerase activity

Abstract

Integrins are cysteine-rich heterodimeric cell-surface adhesion molecules that alter their affinity for ligands in response to cellular activation. The molecular mechanisms involved in this activation of integrins are not understood. Treatment with the thiol-reducing agent, dithiothreitol, can induce an activation-like state in many integrins suggesting that cysteine-cysteine dithiol bonds are important for the receptor's tertiary structure and may be involved in activation-induced conformational changes. Here we demonstrate that the platelet-specific integrin, α(IIb)β3, contains an endogenous thiol isomerase activity, predicted from the presence of the tetrapeptide motif, CXXC, in each of the cysteine-rich repeats of the β3 polypeptide. This motif comprises the active site in enzymes involved in disulfide exchange reactions, including protein-disulfide isomerase (EC 5.3.4.1) and thioredoxin. Intrinsic thiol isomerase activity is also observed in the related integrin, α(v)β3, which shares a common β-subunit. Thiol isomerase activity within α(IIb)β3 is time-dependent and saturable, and is inhibited by the protein-disulfide isomerase inhibitor, bacitracin. Furthermore, this activity is calcium-sensitive and is regulated in the EDTA-stabilized conformation of the integrin. This novel demonstration of an enzymatic activity associated with an integrin subunit suggests that altered thiol bonding within the integrin or its substrates may be locally modified during α(IIb)β3 activation.