Alboaggregin-B: a new platelet agonist that binds to platelet membrane glycoprotein Ib

Abstract

A new protein, called alboaggregin-B (AL-B), has been isolated from Trimeresurus albolabris venom by ion-exchange chromatography. It agglutinated platelets without the need for Ca2+ or any other cofactor. The purified protein showed an apparent molecular mass on SDS-PAGE and gel filtration of about 23 kDa under nonreducing conditions. Ristocetin did not alter the binding of AL-B to platelets or affect AL-B-induced platelet agglutination. Agglutinating activity was not dependent on either proteolytic or lectin-like activity in AL-B. Binding analysis showed that AL-B bound to platelets with high affinity (Kd = 13.6 +/- 9.3 nM) at approximately 30,800 +/- 14,300 binding sites per platelet. AL-B inhibited the binding of labeled bovine von Willebrand factor (vWF) to platelets. Monoclonal antibodies against the 45-kDa N-terminal domain of platelet glycoprotein Ib inhibited the binding both of AL-B and of bovine vWF to platelets, and also inhibited platelet agglutination induced by AL-B and bovine vWF. Specific removal of the N-terminal domain of GPIb by treatment of the platelets with elastase or Serratia marcescens protease reduced the binding of labeled AL-B and bovine vWF to platelets and blocked platelet agglutination caused by both agonists. Monoclonal antibodies to glycoprotein IIb/IIIa, to bovine vWF, and to bovine serum albumin did not show any effect on the binding of AL-B to platelets. Our results indicate that the binding domain for AL-B on platelet GPIb is close to or identical with the one for vWF. This new protein may be a very useful tool for studying the interaction between platelets and vWF.

Interaction of Bruton's Tyrosine Kinase and Protein Kinase Cθ in Platelets

“…Unless otherwise specified, all platelet stimulation occurred in the presence of 1 mM EGTA. Alboaggregin A was prepared from crude T. albolabris venom as described previously (28,29) by ion exchange chromatography. Alboaggregin A was used at 3.5 g/ml, a concentration previously determined to be the EC 50 value for induction of 5-HT release (19).…”

Echicetin, a GPIb-binding snake C-type lectin from Echis carinatus, also contains a binding site for IgMκ responsible for platelet agglutination in plasma and inducing signal transduction

“…Those affecting platelets either inhibit or activate them by binding to specific receptors like glycoprotein (GP)Ib, ␣ 2 ␤ 1 , and GPVI. Those that act via GPIb to agglutinate platelets include alboaggregins, [4][5][6] flavocetin-A and -B, 7 and mamushigin. 8 Most of the inhibitory C-type lectins described so far bind to GPIb.…”

The Collagen-binding Integrin α2β1 Is a Novel Interaction Partner of the Trimeresurus flavoviridis Venom Protein Flavocetin-A

“…GPIb is a receptor for the plasma glycoprotein vWF, which initiates platelet adhesion to exposed vascular subendothelium, consequently activating platelets and leading to hemostasis (10,11). Many CLRPs block the vWF-binding site on the GPIb receptor; however, others enhance the interaction of GPIb and vWF (12)(13)(14)(15).…”