G proteins of the G12 family are activated via thromboxane A2 and thrombin receptors in human platelets.

Abstract

Using subtype-specific antisera, we were able to identify the recently described alpha subunits of G12 and G13 in platelet membranes as 43-kDa proteins. Activation of the thromboxane A2 and the thrombin receptors in platelet membranes led to increased incorporation of the photoreactive GTP analogue [alpha-32P]GTP azidoanilide into immunoprecipitated alpha 12 and alpha 13, indicating that both receptors couple to G12 and G13. In addition, both activated receptors were demonstrated to couple to one or more members of the Gq family. In the absence of receptor agonists, incorporation of [alpha-32P]GTP azidoanilide into alpha 12 and alpha 13 was low over a long time period (up to 45 min) due to an obviously low basal nucleotide exchange rate, whereas an agonist-stimulated photolabeling of alpha 12 and alpha 13 could be observed after 4-8 min and reached a maximum after 30-45 min. Effective activation of G12 and G13 via the thromboxane A2 and the thrombin receptors was not dependent on the presence of GDP. Our results provide evidence that G12 and G13 play a functional role in transmembrane signal transduction and suggest that both proteins are involved in pathways leading to platelet activation.

The galanin receptor type 2 initiates multiple signaling pathways in small cell lung cancer cells by coupling to Gq, Gi and G12 proteins

“…Thus, GALR2 has to be added to the ever growing list of heptahelical receptors with multiple G-protein coupling potential (Gudermann et al, , 1997. Two other G-protein-coupled receptors involved in the regulation of cell growth and di erentiation, the thrombin receptor and the neurokinin-1 receptor, are characterized by the same broad G-protein coupling pattern (Barr et al, 1997;O ermanns et al, 1994). However, to our knowledge the present study is the ®rst to directly demonstrate the activation of members of three distinct G-protein families, i.e.…”

Thromboxane A2 Signalling in Humans: a ‘Tail’ of Two Receptors

“…Whereas many of the latter reported studies have implicated various G protein α subunits in mediating TP activation [45][46][47][48][49][50][51][52][53][54][55] and the latter studies [33,60] mobilization by TPα and TPβ indicating that the TP isoforms may mediate opening of L-type channels in a Gα 12 dependent signalling mechanism [61]. Taken together, these studies investigating the signalling by the TP isoforms indicate that both TPα and TPβ couple to both the G q and G 12 family with no apparent isoform specific differences in their signalling behaviour.…”

Coordinated Signaling through Both G12/13 and Gi Pathways Is Sufficient to Activate GPIIb/IIIa in Human Platelets

“…ADP binds to the G q 1 -coupled P2Y1 and the G i -coupled P2Y12 receptors, and signaling through both of these pathways is necessary for ADPinduced GPIIb/IIIa activation (8, 10 -12), although ADP does not cause dense granule secretion in aspirin-treated human platelets (13). Thromboxane A 2 binds to the TP␣ and TP␤ receptor subtypes that activate both G q (14,15) and G 12/13 signaling (16). Thromboxane receptor stimulation causes both platelet aggregation and dense granule secretion but depends upon secreted contents to provide G i signaling.…”