Mapping early conformational changes in αIIb and β3 during biogenesis reveals a potential mechanism for αIIbβ3 adopting its bent conformation

Current evidence supports a model in which the low-affinity state of the platelet integrin αIIbβ3 results from αIIbβ3 adopting a bent conformation. To assess αIIbβ3 biogenesis and how αIIbβ3 initially adopts the bent conformation, we mapped the conformational states occupied by αIIb and β3 during biogenesis using conformation-specific monoclonal antibodies (mAbs). We found that αIIbβ3 complex formation was not limited by the availability of either free pro-αIIb or free β3, suggesting that other molecules, perhaps chaperones, control complex formation. Five β3-specific, ligand-induced binding site (LIBS) mAbs reacted with much or all free β3 but not with β3 when in complex with mature αIIb, suggesting that β3 adopts its mature conformation only after complex formation. Conversely, 2 αIIb-specific LIBS mAbs directed against the αIIb Calf-2 region adjacent to the membrane reacted with only minor fractions of free pro-αIIb, raising the possibility that pro-αIIb adopts a bent conformation early in biogenesis. Our data suggest a working model in which pro-αIIb adopts a bent conformation soon after synthesis, and then β3 assumes its bent conformation by virtue of its interaction with the bent pro-αIIb.