Investigation of the role of adjacent amino acids to the 313–320 sequence of the α_IIb subunit on platelet activation and fibrinogen binding to α_IIbβ₃

Abstract

The platelet integrin receptor α_IIbβ₃ plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands, primarily fibrinogen. We have previously shown that the synthetic peptide YMESRADRKLAEVGRVYLFL corresponding to residues 313–332 of α_IIb, is a potent inhibitor of platelet aggregation and fibrinogen binding to α_IIbβ₃, interacting with fibrinogen rather than the receptor. Furthermore, we have demonstrated that the biological activities of the above peptide are due to the sequence YMESRADR, which corresponds to residues 313–320. By using new synthetic peptide analogues we investigated the structural characteristics responsible for the biological activity of YMESRADR as well the possible influence of the adjacent amino acids on the peptide's biological potency. According to our results, the synthetic octapeptide YMESRADR, is a potent inhibitor of platelet aggregation and P-selectin expression. Furthermore, YMESRADR inhibits fibrinogen binding but it does not significantly influence the binding of PAC-1 to ADP-activated platelets. The inhibitory potency of YMESRADR was gradually diminished by deleting the YMES sequence from the amino terminus and prolonging the carboxyl terminus of this peptide with the KLAE sequence. Extension of YMESRADR towards the amino terminus with the GAPL sequence (GAPLYMESRADR) does not modify the biological activity of YMESRADR. Furthermore, extension of GAPLYMESRADR at its carboxy terminus with the KLAE sequence (GAPLYMESRADRKLAE) significantly diminished its biological potency. Substitution of E³¹⁵ with D significantly enhances antiaggregatory potency and completely abolishes the inhibitory effect on P-selectin expression. Importantly, the D³¹⁵-containing peptides inhibit to a similar extent both fibrinogen and PAC-1 binding to activated α_IIbβ₃ in contrast to the E³¹⁵-containing peptide which only inhibits fibrinogen binding. In conclusion, the present study suggests that the YMESRADR sequence 313–320 of α_IIb, is an important functional region of the insert connecting the β2 and β3 antiparallel β-strands of the W5 blade of the α_IIb subunit. Structural changes significantly modify the biological properties of this region.

• Platelet activation inhibitors
• peptides
• integrin inhibitors
• α_IIbβ₃ receptor