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J Biol Chem, Vol. 273, Issue 32, 20317-20322, August 7, 1998
From the Centre for Cardiovascular Science, Department of Clinical
Pharmacology, Royal College of Surgeons in Ireland, 123 St. Stephens
Green, Dublin 2, Ireland and All integrin
A Sequence within the Cytoplasmic Tail of GpIIb Independently
Activates Platelet Aggregation and Thromboxane Synthesis
,
,
Department of Biochemistry,
Queens University, Belfast, BT9 7BL, Northern Ireland
subunits contain a highly
conserved KXGFFKR motif in their cytoplasmic domains that
plays a crucial role in the regulation of integrin affinity for their
ligands. We show that a lipid-modified peptide corresponding to the
cytoplasmic region, 989-995, of the platelet integrin subunit
glycoprotein GpIIb (
IIb), palmitoyl-KVGFFKR (Ppep; 10 µM), but not a similarly modified scrambled peptide
(palmitoyl-FKFVRGK), can specifically induce platelet activation and
aggregation equivalent to that of strong agonists such as thrombin.
Ppep-induced aggregation is also associated with indices of platelet
activation including thromboxane A2 (TXA2)
synthesis (EC50 = 45 ± 5 µM), secretion of
-granules detected as enhanced surface expression of P-selectin (EC50 = 52 ± 8 µM), and conformational
changes in GpIIb/IIIa measured by the monoclonal antibody, PAC-1
(EC50 = 3.7 ± 1 µM). The
TXA2 receptor antagonist, SQ29548, PGE1, and
the ADP scavenger, apyrase, differentially inhibit the aggregation
response and TXA2 synthesis in response to Ppep. Similarly,
GpIIb/IIIa antagonists (RO-449883 and integrelin), which inhibit
aggregation by greater than 90%, have little effect on peptide-induced
TXA2 synthesis, suggesting that this event is independent
of fibrinogen binding to GpIIb/IIIa. Alanine-stepping of the Ppep
sequence identifies GFFK(991-994) as the critical residues in all
peptide-mediated events. We conclude that this peptide can imitate the
cytoplasmic domain of GpIIb and initiate parallel but independent
signaling pathways, one leading to ligand binding and platelet
aggregation and the other to intracellular signaling events such as
TXA2 synthesis and secretion.
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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