Flow cytometry of BHK cells harboring mutants at position Arg261 of β3. (A) Surface expression of αIIb (CA3 antibody) and αIIbβ3 (P2 antibody) was measured in cells expressing WT αIIbβ3, αIIbβ3-261Ala, or αIIbβ3-261Lys. Fibrinogen binding was determined after artificial activation by activating antibodies PT25-2 (PT25+Fb) or anti-LIBS6 (L+Fb). Cells harboring only the vectors pCEP and pcDNA3 were used to evaluate background binding. Binding is shown as mean percentage of WT MFI ± SD (mean of at least 3 different transfections). *P < .01. (B) Surface expression of human-αvβ3 measured in cells expressing WT αvβ3, αvβ3-261Ala, or αvβ3-261Lys using 23C6 or LM609 antibodies that bind to human αvβ3 at a higher affinity than to the chimera αv(hamster)β3(human). Fibrinogen binding was measured after anti-LIBS6 activation (L+Fb). Binding is expressed as mean percentage of WT MFI ± SD (N ≥ 3).