Submitted November 17, 2006
Accepted December 20, 2006
Mapping early conformational changes in 
IIb and 
3 during biogenesis reveals a potential mechanism for IIb3 adopting its bent conformation
W Beau Mitchell*, Jihong Li, Marta Murcia, Nathalie Valentin, Peter J Newman, and Barry S Coller
Department of Pediatrics, Mount Sinai School of Medicine, New York, NY, United States
Laboratory of Blood and Vascular Biology, Rockefeller University, New York, NY, United States
Department of Physiology and Biophysics, Weill Cornell Medical College, New York, NY, United States
Laboratoire d'Immunologie, Centre Hospitalier Universitaire, Nantes, France
Blood Center of Wisconsin, Milwaukee, WI, United States
* Corresponding author; email: bmitchell{at}nybloodcenter.org.
Current evidence supports a model in which the low affinity state of the platelet integrin 
IIb
3 results from IIb3 adopting a bent conformation. To assess IIb3 biogenesis and how IIb3 initially adopts the bent conformation, we mapped the conformational states occupied by IIb and 3 during biogenesis using conformation-specific monoclonal antibodies (mAbs). We found that IIb3 complex formation was not limited by the availability of either free pro-IIb or free 3, suggesting that other molecules, perhaps chaperones, control complex formation. Five 3-specific, ligand-induced binding site (LIBS) mAbs reacted with much or all free 3 but not with 3 when in complex with mature IIb, suggesting that 3 adopts its mature conformation only after complex formation. Conversely, two IIb-specific LIBS mAbs directed against the IIb Calf-2 region adjacent to the membrane reacted with only minor fractions of free pro-IIb, raising the possibility that pro-IIb adopts a bent conformation early in biogenesis. Our data suggest a working model in which pro-IIb adopts a bent conformation soon after synthesis, and then 3 assumes its bent conformation by virtue of its interaction with the bent pro-IIb.
