Structural determinants of the integrin transmembrane domain required for bidirectional signal transmission across the cell membrane

Studying the tight activity regulation of platelet-specific integrin α(IIb)β(3) is foundational and paramount to our understanding of integrin structure and activation. α(IIb)β(3) is essential for the aggregation and adhesion function of platelets in hemostasis and thrombosis. Structural and mutagenesis studies have previously revealed the critical role of α(IIb)β(3) transmembrane (TM) association in maintaining the inactive state. Gain-of-function TM mutations were identified and shown to destabilize the TM association leading to integrin activation. Studies using isolated TM peptides have suggested an altered membrane embedding of the β(3) TM α-helix coupled with α(IIb)β(3) activation. However, controversies remain as to whether and how the TM α-helices change their topologies in the context of full-length integrin in native cell membrane. In this study, we utilized proline scanning mutagenesis and cysteine scanning accessibility assays to analyze the structure and function correlation of the α(IIb)β(3) TM domain. Our identification of loss-of-function proline mutations in the TM domain suggests the requirement of a continuous TM α-helical structure in transmitting activation signals bidirectionally across the cell membrane, characterized by the inside-out activation for ligand binding and the outside-in signaling for cell spreading. Similar results were found for α(L)β(2) and α(5)β(1) TM domains, suggesting a generalizable mechanism. We also detected a topology change of β(3) TM α-helix within the cell membrane, but only under conditions of cell adhesion and the absence of α(IIb) association. Our data demonstrate the importance of studying the structure and function of the integrin TM domain in the native cell membrane.