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 αIIb and β3 transmembrane domains regulate the activity of integrin αIIbβ3, we synthesized a soluble peptide corresponding to the αIIb transmembrane domain, designated αIIb-TM, and we studied its ability to affect αIIbβ3 activity in human platelets. αIIb-TM was α-helical in detergent micelles and phospholipid vesicles, readily inserted into membrane bilayers, bound to intact purified αIIbβ3, and specifically associated with the transmembrane domain of αIIb, rather than the transmembrane domains of β3, α2, and β1, other integrin subunits present in platelets. When added to suspensions of gel-filtered platelets, αIIb-TM rapidly induced platelet aggregation that was not inhibited by preincubating platelets with the prostaglandin E1 or the ADP scavenger apyrase but was prevented by the divalent cation chelator EDTA. Furthermore, αIIb-TM induced fibrinogen binding to platelets but not the binding of osteopontin, a specific ligand for platelet αvβ3. The peptide also induced fibrinogen binding to recombinant αIIbβ3 expressed by Chinese hamster ovary cells, confirming that its effect was independent of platelet signal transduction. Finally, transmission electron microscopy of purified αIIbβ3 revealed that αIIb-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.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology