Modulation of α-Thrombin Function by Distinct Interactions with Platelet Glycoprotein Ibα

Abstract

Thrombin bound to platelets contributes to stop bleeding and, in pathological conditions, may cause vascular thrombosis. We have determined the structure of platelet glycoprotein Ibalpha (GpIbalpha) bound to thrombin at 2.3 angstrom resolution and defined two sites in GpIbalpha that bind to exosite II and exosite I of two distinct alpha-thrombin molecules, respectively. GpIbalpha occupancy may be sequential, as the site binding to alpha-thrombin exosite I appears to be cryptic in the unoccupied receptor but exposed when a first thrombin molecule is bound through exosite II. These interactions may modulate alpha-thrombin function by mediating GpIbalpha clustering and cleavage of protease-activated receptors, which promote platelet activation, while limiting fibrinogen clotting through blockade of exosite I.

Thrombin-dependent intravascular leukocyte trafficking regulated by fibrin and the platelet receptors GPIb and PAR4

“…D277N mutation (h-GPIba D277N ). Aspartic acid residue 277 was chosen for mutation as crystallographic 23,24 and functional 24 studies have revealed that the carboxyl group of this residue in human GPIba is essential for measurable a-thrombin interaction.…”

“…Exosite II is the major heparin-binding site 20 , but can also interact with highly sulfated polysaccharides and g 0 -fibrinogen 21 . These exosites are also important for a-thrombin binding to GPIba, the principal thrombin-binding site on platelets [22][23][24] . The importance of the GPIba/exosite I and II interaction in promoting thrombin binding to GPIba has been well defined 22,25,26 .…”

“…The importance of the GPIba/exosite I and II interaction in promoting thrombin binding to GPIba has been well defined 22,25,26 . However, despite more than 30 years of investigation and two published crystal structures 23,24 , the precise pathophysiological function of the thrombin-GPIba interaction remains unclear.…”

Platelet Interactions in Thrombosis

“…Independent structural analyses of complexes of a GPIba fragment and athrombin reveal the possibility of two distinct interactions (18,19). Both structures involve predominantly the C-terminal region of GPIba (disulphide-knot or the anionic/sulphated region).…”

“…The 1 -282 sequence contains noncontiguous binding sites for the von Willebrand factor A1-domain, for a conserved insert (or 'I') domain in the aM subunit of the leukocyte integrin, aMb2 (Mac-1), and for Pselectin expressed on activated platelets and endothelial cells (1,4). This region of GPIba also binds a-thrombin (18,19), thrombospondin (20), kininogen, and clotting factors XI/XIIa (1). In contrast, collagen binds to GPV (3).…”