title: GpIbα interacts exclusively with exosite II of thrombin.
abstract: Activation of platelets by the serine protease thrombin is a critical event in haemostasis. This process involves the binding of thrombin to glycoprotein Ibα (GpIbα) and cleavage of protease-activated receptors (PARs). The N-terminal extracellular domain of GpIbα contains an acidic peptide stretch that has been identified as the main thrombin binding site, and both anion binding exosites of thrombin have been implicated in GpIbα binding, but it remains unclear how they are involved. This issue is of critical importance for the mechanism of platelet activation by thrombin. If both exosites bind to GpIbα, thrombin could potentially act as a platelet adhesion molecule or receptor dimerisation trigger. Alternatively, if only a single site is involved, GpIbα may serve as a cofactor for PAR-1 activation by thrombin. To determine the involvement of thrombin's two exosites in GpIbα binding, we employed the complementary methods of mutational analysis, binding studies, X-ray crystallography and NMR spectroscopy. Our results indicate that the peptide corresponding to the C-terminal portion of GpIbα and the entire extracellular domain bind exclusively to thrombin's exosite II. The interaction of thrombin with GpIbα thus serves to recruit thrombin activity to the platelet surface while leaving exosite I free for PAR-1 recognition.
authors: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 0XY, United Kingdom.
authors: Bernhard C
authors: BC
authors: Lechtenberg
authors: MRC Laboratory for Molecular Biology, Cambridge CB2 0QH, United Kingdom.
authors: Stefan M V
authors: SM
authors: Freund
authors: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 0XY, United Kingdom. Electronic address: jah52@cam.ac.uk.
authors: James A
authors: JA
authors: Huntington
mesh_terms: D000590:Amino Acid Chloromethyl Ketones / Q000737:chemistry; D000991:Antithrombins / Q000737:chemistry; D001665:Binding Sites; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D044463:Receptor, PAR-1 / Q000378:metabolism; D013917:Thrombin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D000590:Amino Acid Chloromethyl Ketones; D000991:Antithrombins; D019038:Platelet Glycoprotein GPIb-IX Complex; D044463:Receptor, PAR-1; C094990:adhesion receptor; D013917:Thrombin; C040308:phenylalanyl-prolyl-arginine-chloromethyl ketone
keywords: GpIbα; LRR; NMR; PAR; PEG; PPACK; SPR; TM; TROSY; crystallography; d-phenylalanyl-l-prolyl-l-arginine chloromethyl ketone; exosite; glycoprotein Ibα; haemostasis; leucine-rich repeat; platelet; polyethylene glycol; protease-activated receptor; surface plasmon resonance; thrombomodulin; transverse relaxation optimised spectroscopy