Specific binding of integrin alpha v beta 3 to the fibrinogen gamma and alpha(E) chain C-terminal domains

Integrin alpha v beta 3, a widely distributed fibrinogen receptor, recogniz es 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 ha ve 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 adhesio n is not known, except that alpha IIb beta 3, a platelet fibrinogen recepto r, binds to the gamma C HHLGGAKQAGDV(400-411) sequence. Here we used mammal ian 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 bloc ked by LM609, a function-blocking anti-alpha v beta 3 mAb, and by RGD pepti des. 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, indicatin g that the alpha v beta 3 binding sites are distinct from those of alpha II b beta 3. A small fragment of gamma C (residues 148-226) supports alpha v b eta 3 adhesion, suggesting that an alpha v beta 3 binding site is located w ithin the gamma chain 148-226 region. We have reported that the CYDMKTTC se quence 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 bi nd 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 CT SEQNC. This suggests that gamma C and alpha(E)C contain determinants for fi brinogen's specificity to alpha v beta 3. These results suggest that fibrin ogen has potentially significant novel alpha v beta 3 binding sites in gamm a C and alpha(E)C.