Differentiation of the two forms of GPIb functioning as receptors for alpha-thrombin and vonWillebrand factor: Ca2+ responses of protease-treated human platelets activated with alpha-thrombin and the tethered ligand peptide

Previous results have shown that both GPIb and the seven transmembrane domain receptor (STDR) are required for optimal thrombin-induced platelet activation (Greco et al., 1996). Limited degradation (similar to 10%) of GPIb and the STDR by elastase reduced the Ca2+ response to 0.5 nM alpha-thrombin by only 10% whereas Serratia marcescens metalloprotease reduced the Ca2+ response by 80% and fully abrogated high-affinity thrombin binding and aggregation. vWF/ristocefin-induced agglutination was only slightly reduced (20%) while Ca2+ and aggregation responses to higher thrombin concentrations were retained. At increasing elastase and Serratia protease concentrations, degradation of the STDR proceeded from the amino-terminal domain, but Ca2+ responses to the tethered ligand peptide SFLLRNPNDKYEPF were not affected by either protease. These results show that both putative thrombin receptors are susceptible to protease degradation and suggest that Serratia protease is able to differentiate the GPIb-mediated events associated with thrombin activation from those associated with ristocetin-induced agglutination.