Specific Binding of Integrin αvβ3 to the Fibrinogen γ and αE Chain C-Terminal Domains

Abstract

Integrin αvβ3, a widely distributed fibrinogen receptor, recognizes the RGD^572-574 motif in the α chain of human fibrinogen. However, this motif is not conserved in other species, nor is it required for αvβ3-mediated fibrin clot retraction, suggesting that fibrinogen may have other αvβ3 binding sites. Fibrinogen has conserved C-terminal domains in its α (E variant), β, and γ chains (designated α_EC, βC, and γC, respectively), but their function in cell adhesion is not known, except that αIIbβ3, a platelet fibrinogen receptor, binds to the γC HHLGGAKQAGDV^400-411 sequence. Here we used mammalian cells expressing recombinant αvβ3 to show that recombinant α_EC and γC domains expressed in bacteria specifically bind to αvβ3. Interaction between αvβ3 and γC or α_EC is blocked by LM609, a function-blocking anti-αvβ3 mAb, and by RGD peptides. αvβ3 does not require the HHLGGAKQAGDV^400-411 sequence of γC for binding, and α_EC does not have such a sequence, indicating that the αvβ3 binding sites are distinct from those of αIIbβ3. A small fragment of γC (residues 148−226) supports αvβ3 adhesion, suggesting that an αvβ3 binding site is located within the γ chain 148−226 region. We have reported that the CYDMKTTC sequence of β3 is responsible for the ligand specificity of αvβ3. γC and α_EC do not bind to wild-type αvβ1, but do bind to the αvβ1 mutant (αvβ1-3-1), in which the CYDMKTTC sequence of β3 is substituted for the corresponding β1 sequence CTSEQNC. This suggests that γC and α_EC contain determinants for fibrinogen's specificity to αvβ3. These results suggest that fibrinogen has potentially significant novel αvβ3 binding sites in γC and α_EC.