Activation of platelet αIIbβ3 by an exogenous peptide corresponding to the transmembrane domain of αIIb

A transmembrane domain heterodimer, acting in concert with a membrane-proximal cytoplasmic domain clasp, is thought to maintain integrins in a low affinity state. To test whether helix-helix interactions between the alpha IIb and beta 3 transmembrane domains regulate the activity of integrin alpha IIb beta 3, we synthesized a soluble peptide corresponding to the alpha IIb transmembrane domain, designated alpha IIb-TM, and we studied its ability to affect alpha IIb beta 3 activity in human platelets. alpha IIb-TM was alpha-helical in detergent micelles and phospholipid vesicles, readily inserted into membrane bilayers, bound to intact purified alpha IIb beta 3, and specifically associated with the transmembrane domain of alpha IIb, rather than the transmembrane domains of beta 3, alpha 2, and beta 1, other integrin subunits present in platelets. When added to suspensions of gel-filtered platelets, alpha IIb-TM rapidly induced platelet aggregation that was not inhibited by preincubating platelets with the prostaglandin E-1 or the ADP scavenger apyrase but was prevented by the divalent cation chelator EDTA. Furthermore, alpha IIb-TM induced fibrinogen binding to platelets but not the binding of osteopontin, a specific ligand for platelet alpha v beta 3. The peptide also induced fibrinogen binding to recombinant alpha IIb beta 3 expressed by Chinese hamster ovary cells, confirming that its effect was independent of platelet signal transduction. Finally, transmission electron microscopy of purified alpha IIb beta 3 revealed that alpha IIb-TM shifted the integrin from a closed configuration with its stalks touching to an open configuration with separated stalks. These observations demonstrate that transmembrane domain interactions regulate integrin function in situ and that it is possible to target intra-membranous protein-protein interactions in a way that can have functional consequences.