Protease-activated receptors 1 and 4 are shut off with distinct kinetics after activation by thrombin

Protease-activated receptors 1 and 4 (PAR1 and PAR4) mediate thrombin signa ling in human platelets. Whether these receptors are redundant, interact, o r serve only partially overlapping functions is unknown. We report that PAR 1 and PAR4 signal with distinct tempos, In transfected fibroblasts, PARA tr iggered substantially more phosphoinositide hydrolysis per activated recept or than PAR1 and was shut off more slowly than PAR1, Shutoff and internaliz ation of PAR1 depends upon phosphorylation of its carboxyl tail upon recept or activation. In contrast to PAR1, phosphorylation of PARA was undetectabl e, and activation-dependent internalization of PAR4 was much slower than th at seen for PAR1. Mutation of potential phosphorylation sites in the carbox yl tail of PAR1 enhanced PAR1 signaling, whereas analogous mutations in PAR 4 had no effect. Thus PAR4 signaling is shut off less rapidly than PAR1, pr obably due to differences in receptor phosphorylation, PAR1 and PARA also s ignaled with distinct tempos in platelets. PAR1 triggered a rapid and trans ient increase in intracellular calcium, whereas PAR4 triggered a more prolo nged response. Together, the tempo of these responses accounted for that tr iggered by thrombin, Thus differences in the rates at which PAR1 and PAR4 a re shut off allow thrombin to trigger intracellular signaling with distinct temporal characteristics.