Autonomous conformational regulation of β integrin and the conformation-dependent property of HPA-1a alloantibodies.

Integrin α/β heterodimer adopts a compact bent conformation in the resting state, and upon activation undergoes a large-scale conformational rearrangement. During the inside-out activation, signals impinging on the cytoplasmic tail of β subunit induce the α/β separation at the transmembrane and cytoplasmic domains, leading to the extended conformation of the ectodomain with the separated leg and the opening headpiece that is required for the high-affinity ligand binding. It remains enigmatic which integrin subunit drives the bent-to-extended conformational rearrangement in the inside-out activation. The β integrins, including αβ and αβ, are the prototypes for understanding integrin structural regulation. The Leu33Pro polymorphism located at the β PSI domain defines the human platelet-specific alloantigen (HPA) 1a/b, which provokes the alloimmune response leading to clinically important bleeding disorders. Some, but not all, anti-HPA-1a alloantibodies can distinguish the αβ from αβ and affect their functions with unknown mechanisms. Here we designed a single-chain β subunit that mimics a separation of α/β heterodimer on inside-out activation. Our crystallographic and functional studies show that the single-chain β integrin folds into a bent conformation in solution but spontaneously extends on the cell surface. This demonstrates that the β subunit autonomously drives the membrane-dependent conformational rearrangement during integrin activation. Using the single-chain β integrin, we identified the conformation-dependent property of anti-HPA-1a alloantibodies, which enables them to differently recognize the β in the bent state vs. the extended state and in the complex with α vs. α This study provides deeper understandings of integrin conformational activation on the cell surface.