Thrombin orchestrates cellular events after injury to the vascular system and extravasation of blood into surrounding tissues. The pathophysiological response to thrombin is mediated by proteaseactivated receptor-1 (PAR-1), a seven-transmembrane G proteincoupled receptor expressed in the nervous system that is identical to the thrombin receptor in platelets, fibroblasts, and endothelial cells. Once activated by thrombin, PAR-1 induces rapid and dramatic changes in cell morphology, notably the retraction of growth cones, axons, and dendrites in neurons and processes in astrocytes. The signal is conveyed by a series of localized ATP-dependent reactions directed to the actin cytoskeleton. How cells meet the dynamic and localized energy demands during signal transmission is unknown. Using the yeast two-hybrid system, we identified an interaction between PAR-1 cytoplasmic tail and the brain isoform of creatine kinase, a key ATP-generating enzyme that regulates ATP within subcellular compartments. The interaction was confirmed in vitro and in vivo. Reducing creatine kinase levels or its ATP-generating potential inhibited PAR-1-mediated cellular shape changes as well as a PAR-1 signaling pathway involving the activation of RhoA, a small G protein that relays signals to the cytoskeleton. Thrombin-stimulated intracellular calcium release was not affected. Our results suggest that creatine kinase is bound to PAR-1 where it may be poised to provide bursts of site-specific high-energy phosphate necessary for efficient receptor signal transduction during cytoskeletal reorganization. P rotease activated receptor-1 (PAR-1) mediates the cellular responses to thrombin during blood coagulation, cell proliferation, vascular permeability changes, tumor metastasis, and nervous system injury (1-3). PAR-1 is a seven-transmembrane G protein-coupled receptor with a novel activation mechanism. Proteolysis at a thrombin cleavage site in the extracellular amino terminus exposes a new amino terminus containing the peptide ligand SFLLRN, which binds intramolecularly to initiate intracellular signals (4). Although originally detected in platelets, endothelial cells, and fibroblasts, PAR-1 is also expressed in the nervous system in a developmentally regulated manner and by specific subpopulations of neurons and astrocytes that are especially vulnerable to neurodegeneration and ischemic injury (1,5,6).In most cells expressing PAR-1, activation of the receptor transmits signals to the actin cytoskeleton that profoundly alter cell shape. Platelets, for example, convert from a spherical to disk shape and extend filopodia (7), endothelial cells contract (8), neurons retract axons, and astrocytes resorb processes and flatten their cell bodies (9-11). These signals also regulate changes in actin-related cell motility observed in neurons (10), fibroblasts (12), and tumor cells (3). The morphological response is mediated by a key signaling pathway that uses serine͞ threonine kinases, G␣12͞13, RhoA, and myosin light chain kinase; actomyosin contraction ...