Summary
Platelet membrane glycoprotein Ib (GPIb) functions as receptors for thrombin and von
Willebrand factor (vWF) in the presence of ristocetin. To precisely locate the domains
on GPIb interacting with vWF and thrombin, we prepared several peptides that have
amino acid sequences analogous to that ol the GPIb α-chain and examined their effects
on ristocetin-induced (vWFdependent) and thrombin-induced platelet aggregations. A
peptide extending from residues Asp235 to Lys262 showed the strongest inhibitory effect
on ristocetin-induced platelet agglutination, and a group of overlapping peptides
composed of 24-28 amino acid residues representing sequences extending from Phe216
to Asp274 was found to inhibit platelet aggregation induced by thrombin. Other peptides
did not inhibit platelet aggregations. Moreover the binding to platelets of the monoclonal
anti-GPIb antibody (TM60) which had been shown to inhibit both ristocetin- and thrombin-induced
platelet aggregations was strongly inhibited by a peptide extending from Asp249 to
Asp274. These data demonstrate that the vWF-hinding domain exists in a small region
between residues Asp235 and Lys262; the thrombin-interacting domain, in contrast,
is located between residues Phe216 and Ala274, with a possible center of interaction
in the sequence from Phe216 to Thr240 on the GPIb α-chain, and thrombin binding requires
a relatively strict conformation in this domain.
Keywords
Platelet glycoprotein Ib - von Willebrand factor - Thrombin - Synthetic peptides