Specific binding of integrin αvβ3 to the fibrinogen γ and αE chain C-terminal domains

Integrin alpha v beta 3, a widely distributed fibrinogen receptor, recognizes the RGD(572-574) motif in the alpha chain of human fibrinogen. However, this motif is not conserved in other species, nor is it required for alpha v beta 3-mediated fibrin clot retraction, suggesting that fibrinogen may have other alpha v beta 3 binding sites. Fibrinogen has conserved C-terminal domains in its alpha (E variant), beta, and gamma chains (designated alpha(E)C, beta C, and gamma C, respectively), but their function in cell adhesion is not known, except that alpha IIb beta 3, a platelet fibrinogen receptor, binds to the gamma C HHLGGAKQAGDV(400-411) sequence. Here we used mammalian cells expressing recombinant alpha v beta 3 to show that recombinant a(E)C and gamma C domains expressed in bacteria specifically bind to alpha v beta 3. Interaction between alpha v beta 3 and gamma C or alpha(E)C is blocked by LM609, a function-blocking anti-alpha v beta 3 mAb, and by RGD peptides. alpha v beta 3 does not require the HHLGGAKQAGDV(400-411) sequence of gamma C for binding, and alpha(E)C does not have such a sequence, indicating that the alpha v beta 3 binding sites are distinct from those of alpha IIb beta 3. A small fragment of gamma C (residues 148-226) supports alpha v beta 3 adhesion, suggesting that an alpha v beta 3 binding site is located within the gamma chain 148-226 region. We have reported that the CYDMKTTC sequence of beta 3 is responsible for the ligand specificity of alpha v beta 3. gamma C and alpha(E)C do not bind to wild-type alpha v beta 1, but do bind to the alpha v beta 1 mutant (alpha v beta 1-3-1), in which the CYDMKTTC sequence of beta 3 is substituted for the corresponding beta 1 sequence CTSEQNC. This suggests that gamma C and alpha(E)C contain determinants for fibrinogen's specificity to alpha v beta 3. These results suggest that fibrinogen has potentially significant novel alpha v beta 3 binding sites in gamma C and alpha(E)C.