Summary
The platelet membrane glycoprotein lb (Gplb) has a high affinity binding site for
α-thrombin whose occupancy is thought to positively modulate the thrombin-induced
platelet activation. In this study, aimed at further characterizing the thrombin-GpIb
interaction, two thrombin anion exosites referred to as “heparin binding site” (HBS)
and “fibrino#gen recognition site” (FRS) were investigated as the possible domains
involved in Gplb binding. The role of thrombin HBS was explored by performing binding
measurements of 125I-α-thrombin to purified glycocalicin (GC), the extracytoplasmic portion of Gplb,
in the presence of heparin as well as after chemical modifications of the thrombin
heparin binding site (thrombin-HBS phosphopyridoxylation). These studies showed that
a) thrombin binding to GC could be competitively inhibited by heparin and b) the equilibrium
association constant for thrombin-GC interaction was reduced up to ten-fold by chemical
modifications at the HBS. On the other hand, the role of FRS in the thrombin-GC interaction
could be excluded by other experiments showing that GC in solution could not influence
the interaction of α-thrombin with two substrates which bind to both the catalytic
site and the fibrinogen recognition site: 1) the thrombin receptor peptide 38-60 (TR,
L38-E60) and 2) the A α-chain of fibrinogen. Altogether these results demonstrated
that GC interaction with thrombin involves the enzyme heparin binding site, whereas
the fibrinogen recognition site does not play a significant role.