Interactions of platelet integrin α_IIb and β₃ transmembrane domains in mammalian cell membranes and their role in integrin activation

Clustering and occupancy of platelet integrin α_IIbβ ₃ (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin α and β transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, wedevelop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin α_IIbβ₃ TMD-tails via specific TMD interactions in mammalian cell membranes and in lipid bicelles. Furthermore, charge reversal mutations at α_IIb(R995)β₃(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the β₃ tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin α_IIbβ₃ TMD-tails in mammalian cell membranes and document their role in integrin signaling.