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Interaction of G_q and Kir3, G Protein-Coupled Inwardly Rectifying Potassium Channels

Departments of Anatomy & Cell Biology (T. Ka., P.Z., C.V.F., Y.N.) and Pharmacology (T. Ko., S.N.), College of Medicine, University of Illinois at Chicago, Illinois

Activation of substance P receptors, which are coupled to G_q, inhibits the Kir3.1/3.2 channels, resulting in neuronal excitation. We have shown previously that this channel inactivation is not caused by reduction of the phosphatidylinositol 4,5-bisphosphate level in membrane. Moreover, G_q immunoprecipitates with Kir3.2 (J Physiol 564:489–500, 2005), suggesting that G_q interacts with Kir3.2. Positive immunoprecipitation, however, does not necessarily indicate direct interaction between the two proteins. Here, the glutathione transferase pull-down assay was used to investigate interaction between G_q and the K⁺ channels. We found that G_q interacted with N termini of Kir3.1, Kir3.2, and Kir3.4. However, G_q did not interact with the C terminus of any Kir3 or with the C or N terminus of Kir2.1. TRPC6 is regulated by the signal initiated by G_q. Immunoprecipitation, however, showed that G_q did not interact with TRPC6. Thus, the interaction between G_q and the Kir3 N terminus is quite specific. This interaction occurred in the presence of GDP or GDP-AlF^–₄. The G_q binding could take place somewhere between residues 51 to 90 of Kir3.2; perhaps the segment between 81 to 90 residues is crucial. G, which is known to bind to N terminus of Kir3, did not compete with G_q for the binding, suggesting that these two binding regions are different. These findings agree with the hypothesis (J Physiol 564:489–500, 2005) that the signal to inactivate the Kir3 channel could be mainly transmitted directly from G_q to Kir3.

Address correspondence to: Shigehiro Nakajima, 835 South Wolcott Avenue, M/C 868, Dept. of Pharmacology, University of Illinois College of Medicine, Chicago, IL 60612. E-mail: shign{at}uic.edu

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