Differential signaling by the thromboxane receptor isoforms via the novel GTP-binding protein, Gh

Thromboxane A(2) acts via G protein-coupled receptors; two splice variants of the thromboxane A(2) receptor (TP alpha and TP beta) have been cloned. It is unknown whether they differ in their capacity to activate intracellular signaling pathways. Recently, a high molecular weight G protein, G(h), that can also function as a tissue transglutaminase, has been described. We investigated whether G(h) functions as a signaling protein in association with thromboxane receptors, First, we sought G(h) expression in cells known to express TPs, Reverse transcription-polymerase chain reaction and immunoblotting demonstrated G(h) expression in platelets, megakaryocytic cell lines, and endothelial and vascular smooth muscle cells. Second, immunoprecipitation of both TP alpha and TP beta in transfected COS-7 cells resulted in the co-immunoprecipitation of G(h), indicating that TPs may associate G(h) in vice. Finally, agonist activation of TP alpha, but not of TP beta, resulted in stimulation of phospholipase C-mediated inositol phosphate production in cells cotransfected with G(h), By contrast, agonist activation of both TP iso forms resulted in G(h)-mediated inositol phosphate signaling. G(h) is expressed in platelets and vascular cells and may associate with both TP isoforms. However, stimulation of TP isoforms results in differential activation of downstream signaling pathways via this novel G protein.