title: Role of glycoprotein V in the formation of the platelet high-affinity thrombin-binding site.
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.
authors.affiliation: Department of Internal Medicine, Baylor College of Medicine and Veterans Affairs Medical Center, Houston, TX 77030, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Dong
authors.forename: G
authors.initials: G
authors.lastname: Sae-Tung
authors.forename: J A
authors.initials: JA
authors.lastname: López
mesh_terms: D000494:Allosteric Regulation; D000590:Amino Acid Chloromethyl Ketones / Q000378:metabolism; D001792:Blood Platelets / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D046911:Macromolecular Substances; D008958:Models, Molecular; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism* / Q000502:physiology*; D011485:Protein Binding; D018179:Receptors, Thrombin / Q000378:metabolism*; D013917:Thrombin / Q000378:metabolism*
chemical_list: D000590:Amino Acid Chloromethyl Ketones; D046911:Macromolecular Substances; D019038:Platelet Glycoprotein GPIb-IX Complex; D018179:Receptors, Thrombin; D013917:Thrombin; C040308:phenylalanyl-prolyl-arginine-chloromethyl ketone