1. Cardiovascular Research Institute and Daiichi Research Center,
2. Department of Medicine, and
3. Department of Cellular and Molecular Pharmacology, University of California, San Francisco, 505 Parnassus Avenue, San Francisco, California 94143-0130 , USA

Correspondence to: Shaun R. Coughlin^1,2,3 Correspondence and requests for materials should be addressed to S.R.C. (e-mail: Email: coughlin@cvrimail.ucsf.edu).

Abstract

Identification of the mechanisms by which the coagulation protease thrombin activates platelets is critical for understanding haemostasis and thrombosis. Thrombin activates cells at least in part by cleaving protease-activated G-protein-coupled receptors (PARs)¹. PAR3 and PAR4 are thrombin receptors expressed in mouse platelets^{2, 3}. Inhibition of thrombin binding to mPAR3 (ref. 4) and knockout of the mPAR3 gene³ inhibited mouse platelet activation at low but not high concentrations of thrombin. Thus PAR3 is important for thrombin signalling in mouse platelets. Expression of human PAR3 in heterologous expression systems reliably resulted in responsiveness to thrombin². Curiously, despite its importance for the activation of mouse platelets by thrombin^{3, 4}, mouse PAR3 (mPAR3) did not lead to thrombin signalling even when overexpressed. We now report that mPAR3 and mPAR4 interact in a novel way: mPAR3 does not itself mediate transmembrane signalling but instead functions as a cofactor for the cleavage and activation of mPAR4 by thrombin. This establishes a paradigm for cofactor-assisted PAR activation and for a G-protein-coupled receptor's acting as an accessory molecule to present ligand to another receptor.