Galpha12 and galpha13 are phosphorylated during platelet activation

Area of Science:

• Cellular signaling
• Molecular biology
• Biochemistry

Background:

• G-proteins G12 and G13 are widely expressed, but their functions remain unclear.
• These G-proteins are activated in platelet membranes by receptors that trigger platelet aggregation.

Purpose of the Study:

• To investigate if G protein alpha subunits Galpha12 and Galpha13 can be phosphorylated under physiological conditions in human platelets.
• To elucidate the signaling pathways involved in Galpha12 and Galpha13 phosphorylation.

Main Methods:

• Used intact human platelets stimulated with thrombin and U46619 (a thromboxane A2 receptor agonist).
• Investigated phosphorylation using phorbol 12-myristate 13-acetate and protein kinase C inhibitors (calphostin C).
• Reconstituted phosphorylation in COS-7 cells co-transfected with Galpha12/Galpha13 and various protein kinase C isoforms.

Main Results:

• Thrombin and U46619 activation of human platelets led to rapid phosphorylation of Galpha12 and Galpha13.
• This phosphorylation was independent of glycoprotein IIb/IIIa activation.
• Thrombin-induced phosphorylation was inhibited by calphostin C, implicating protein kinase C.
• Protein kinase C beta, delta, and epsilon isoforms were most effective in phosphorylating Galpha12 and Galpha13 in a phorbol 12-myristate 13-acetate-dependent manner.

Conclusions:

• Demonstrated that Galpha12 and Galpha13 are phosphorylated in vivo within human platelets.
• Established that protein kinase C is involved in the phosphorylation of Galpha12 and Galpha13.
• Suggests a potential regulatory role for G protein phosphorylation in platelet function.