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J Biol Chem, Vol. 275, Issue 9, 6680-6688, March 3, 2000

Expression and Function of Calcium Binding Domain Chimeras of the Integrins _IIb and ₅^*

Susan Gidwitz^§, Suzanne Lyman, and Gilbert C. White II^¶

From the Center for Thrombosis and Hemostasis,  Department of Medicine, Division of Hematology and Oncology and ^¶ Department of Pharmacology, University of North Carolina, Chapel Hill, North Carolina 27599

To further identify amino acid domains involved in the ligand binding specificity of _IIb3, chimeras of the conserved calcium binding domains of _IIb and the  subunit of the fibronectin receptor ₅₁ were constructed. Chimeras that replaced all four calcium binding domains, replaced all but the second calcium binding domain of _IIb with those of ₅, or deleted all four calcium binding domains were synthesized but not expressed on the cell surface. Additional chimeras exchanged subsets or all of the variant amino acids in the second calcium binding domain, a region implicated in ligand binding. Cell surface expression of each second calcium binding domain mutant complexed with ₃ was observed. Each second calcium binding domain mutant was able to 1) bind to immobilized fibrinogen, 2) form fibrinogen-dependent aggregates after treatment with dithiothreitol, and 3) bind the activation-dependent antibody PAC1 after LIBS 6 treatment. Soluble fibrinogen binding studies suggested that there were only small changes in either the K_d or B_max of any mutant. We conclude that chimeras of _IIb containing the second calcium binding domain sequences of ₅ are capable of complexing with ₃, that the complexes are expressed on the cell surface, and that mutant complexes are capable of binding both immobilized and soluble fibrinogen, suggesting that the second calcium binding domain does not determine ligand binding specificity.

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