Integrin receptor GPIIb/IIIa bound state conformation of the fibrinogen gamma-chain C-terminal peptide 400-411: NMR and transfer NOE studies

The C-terminal dodecapeptide from human fibrinogen gamma-chain, residues 400-411, HHLG-GAKQAGDV (gamma 12), is known to inhibit fibrinogen-mediated platelet cell aggregation via competitive interactions with platelet glycoprotein integrin receptor GPIIb/IIIa. NMR studies of gamma 12 in the presence of purified GPIIb/lIIa (230 kDa) demonstrate that two gamma 12 binding states (gamma 12-I and gamma 12-II) are present on the integrin receptor. The N-terminal sequence HHLG is crucial to formation of gamma 12 state I since in a shorter gamma-chain octapeptide, GAKQAGDV, gamma 12-I is not observed. Addition of the hexapeptide GRGDSP to the gamma 12-receptor preparation effectively removes the gamma 12-I population, suggesting either that gamma 12 and GRGDSP share one binding site or that their binding sites are allosterically linked. Distance geometry calculations using transfer NOEs from gamma 12-I (gamma/12-II shows practically no NOEs) indicate the presence of helix conformation when bound to the receptor. Line broadening and chemical shift changes relative to free gamma 12 suggest that gamma 12 interacts with GPIIb/IIIa primarily through N-terminal residues H400 to Q407.