Vol. 188, No. 2, 1992 BtOCHEMtCAL AND BtOPHYStCAL RESEARCH COMMUNtCATlONS October 30, 1992 Pages 604-610 STRUCTURE-ACTIVITY STUDIES OF THE THROMBIN RECEPTOR ACTIVATING PEPTIDE T. Sabo’, D. Gurwitz, L. Motola, P. Brodt, R. Barak and E. Elhanaty Israel Institute for Biological Research, P.O.Box 19, Ness-Ziona 70450, ISRAEL Received September 9, 1992 Cleavage of the human platelet thrombin receptor by thrombin exposes a new N-terminal which acts as a putative tethered ligand. A synthetic peptide - “SFLL” (SFLLRNPNDKYEPF), corresponding to the new N-terminal region, activates and induces platelet aggregation and serotonin secretion. We have found that the pentapeptide- SFLLR is the minimal peptide length which retains full activity in inducing [14C]serotonin secretion. Structure-activity relationship studies were performed on this pentameric peptide. Systematic replacement of all amino acids with L-Ala indicated the importance of F-2, L-3 and R-5 for activity. Further studies demonstrated that the positive charge at the N-terminus, but not at the C-terminus of the pentapeptide, is crucial for activity. o 1992 Academic pIess. lnc. The serine protease - a thrombin, plays a central role in homeostasis and thrombosis, by converting fibrinogen to fibrin and by activation and aggregation of platelets (1,2). The thrombin receptor of human platelets has recently been cloned and expressed in Xenopus oocytes by Vu et al. (3). The authors have identified a putative thrombin cleavage site at the N- terminus of the receptor - between arginine 41 and serine 42. Similar findings were reported by Rasmussen et al. (4), who cloned the thrombin receptor of the Chinese hamster. It has been further suggested that the l To whom correspondence should be addressed. 0006-291X/92 $4.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved. 604