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© The Rockefeller University Press, 0021-9525/1999//745 $5.00
The Journal of Cell Biology, Volume 144, Number 4, , 1999 745-754


Regular Articles

Activation of G12/G13 Results in Shape Change and Rho/Rho-Kinase–mediated Myosin Light Chain Phosphorylation in Mouse Platelets



Birgit Klages*, Ursula Brandt*, Melvin I. Simon{ddagger}, Günter Schultz*, and Stefan Offermanns*

* Institut für Pharmakologie, Universitätsklinikum Benjamin Franklin, Freie Universität Berlin, 14195 Berlin, Germany; and {ddagger} Division of Biology, California Institute of Technology, Pasadena, California 91125

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the {alpha}-subunit of the heterotrimeric G protein Gq do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A2 (TXA2) or thrombin receptors. In contrast to thrombin, the TXA2 mimetic U46619 led to the selective activation of G12 and G13 in G{alpha}q-deficient platelets indicating that these G proteins mediate TXA2 receptor-induced shape change. TXA2 receptor-mediated activation of G12/G13 resulted in tyrosine phosphorylation of pp72syk and stimulation of pp60c-src as well as in phosphorylation of myosin light chain (MLC) in G{alpha}q-deficient platelets. Both MLC phosphorylation and shape change induced through G12/G13 in the absence of G{alpha}q were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G12/G13 couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase–mediated regulation of MLC phosphorylation. We provide evidence that G12/G13-mediated Rho/Rho-kinase–dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.

Key Words: platelet • platelet shape change • G protein • Rho-kinase • myosin light chain phosphorylation



Abbreviations used in this paper: MLC, myosin light chain; MLCK, myosin light chain kinase; GEF, guanine nucleotide exchange factor; TXA2, thromboxane A2.



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