Role of glycoprotein V in the formation of the platelet high-affinity thrombin-binding site

Blood. 1997 Jun 15;89(12):4355-63.

Abstract

The glycoprotein (GP) Ib-IX-V complex contains a high-affinity binding site for thrombin on the platelet surface with a poorly defined role in platelet activation by this agonist. Four polypeptides comprise the complex: GP Ib alpha, GP Ib beta, GP IX, and GP V. The site within the complex that binds thrombin has been localized to a 45-kD region at the amino terminus of GP Ib alpha, which also contains the site through which the complex interacts with von Willebrand factor. A GP Ib-IX complex that lacks GP V can be efficiently expressed on the surface of transfected cells. We examined the ability of L cells expressing the GP Ib-IX complex (L2H cells) to bind thrombin at high affinity, and found no increase over the level of thrombin binding to control L cells. Because it is one of the few substrates for thrombin on the platelet surface, GP V has also been implicated as possibly participating in thrombin's actions on the platelet. To examine the role of GP V in forming the high-affinity thrombin-binding site, we compared the binding of thrombin to L2H cells versus cells that express the entire GP Ib-IX-V complex (L2H/V cells). Surface expression of GP Ib alpha was equivalent in these two stable cell lines. Thrombin binding to L2H/V cells was detectable at 0.25 nmol/L thrombin and reached a plateau at 1 nmol/L. No binding to L2H cells was detectable at these concentrations. Comparable results were obtained when thrombin binding to L2H cells transiently expressing GP V was compared with its binding to sham-transfected L2H cells. Again, only cells transiently expressing GP V bound thrombin specifically. As with the platelet polypeptide, thrombin cleaved GP V from the surface of L2H/V cells. To test whether GP V cleavage was required for enhancing thrombin binding to the complex, we tested the binding of enzymatically inactive D-phenylalanyl-L-prolyl-L-arginine chloromethylketone (PPACK)-thrombin to L2H and L2H/V cells. Like native thrombin, PPACK-thrombin at 1 nmol/L bound only to L2H/V cells, indicating that GP V cleavage is not a prerequisite for the formation of the high-affinity thrombin receptor. These data provide the first indication of a physiologic function for GP V, and suggest that formation of the high-affinity thrombin receptor on the platelet surface has complex allosteric requirements.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Allosteric Regulation
  • Amino Acid Chloromethyl Ketones / metabolism
  • Blood Platelets / metabolism*
  • Cell Line
  • Humans
  • Macromolecular Substances
  • Models, Molecular
  • Platelet Glycoprotein GPIb-IX Complex / metabolism*
  • Platelet Glycoprotein GPIb-IX Complex / physiology*
  • Protein Binding
  • Receptors, Thrombin / metabolism*
  • Thrombin / metabolism*

Substances

  • Amino Acid Chloromethyl Ketones
  • Macromolecular Substances
  • Platelet Glycoprotein GPIb-IX Complex
  • Receptors, Thrombin
  • Thrombin
  • phenylalanyl-prolyl-arginine-chloromethyl ketone