Calcium-binding domains in the α-subunit of integrins contain a central loop structure. To examine the importance of the loop structure, a series of αIIb mutants containing changes to the calcium-liganding amino acids have been constructed. Significantly, none of the mutant αIIbβ3 complexes was detected on the surface of transfected cells, but mutant pro-αIIb was detected in cell lysates in complex with β3. To study the importance of the regions flanking the second calcium-binding domain for ligand-binding and ligand-binding specificity, three αIIb/α5 chimaeras containing α5 sequences flanking or flanking and including the second calcium-binding domain were constructed. The chimaera containing both α5-flanking regions was not expressed on the cell surface, but FR1 and FR2, substituting either the first or second flanking region, were expressed. FR1β3-transfected cells lost the ability to adhere to fibrinogen and to support aggregation and had minimal fibrinogen-binding ability. The heterodimer complex was less stable than the wild-type. FR2β3-transfected cells adhered to fibrinogen and bound soluble fibrinogen with higher affinity when compared with wild-type. In addition, the heterodimer complex was more stable than wild-type. These results indicate that the conformation of the second calcium-binding domain is critical for maturation of the αIIbβ3 complex and expression on the cell surface and that the surrounding sequences are critical for αIIbβ3 function.

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