Schwarzer S, Nobles M, Tinker A. Do caveolae have a role in the fidelity and dynamics of receptor activation of G-protein-gated inwardly rectifying potassium channels?
J Biol Chem 2010;
285:27817-26. [PMID:
20562107 PMCID:
PMC2934649 DOI:
10.1074/jbc.m110.103598]
[Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
In atrial and nodal cardiac myocytes, M2 muscarinic receptors activate inhibitory G-proteins (Gi/o), which in turn stimulate G-protein-gated inwardly rectifying K+ channels through direct binding of the Gβγ subunit. Despite also releasing Gβγ, Gs-coupled receptors such as the β-adrenergic receptor are not able to prominently activate this current. An appealing hypothesis would be if components were sequestered in membrane domains such as caveolae/rafts. Using biochemical fractionation followed by Western blotting and/or radioligand binding experiments, we examined the distribution of the components in stable HEK293 and HL-1 cells, which natively express the transduction cascade. The channel, M2 muscarinic, and A1 adenosine receptors were located in noncaveolar/nonraft fractions. Giα1/2 was enriched in both caveolar/raft and noncaveolar/nonraft fractions. In contrast, Gsα was only enriched in caveolar/raft fractions. We constructed YFP-tagged caveolin-2 (YFP-Cav2) and chimeras with the M2 (M2-YFP-Cav2) and A1 (A1-YFP-Cav2) receptors. Analysis of gradient fractions showed that these receptor chimeras were now localized to caveolae-enriched fractions. Microscopy showed that M2-YFP and A1-YFP had a diffuse homogenous membrane signal. YFP-Cav2, M2-YFP-Cav2, and A1-YFP-Cav2 revealed a more punctuate pattern. Finally, we looked at the consequences for signaling. Activation via M2-YFP-Cav2 or A1-YFP-Cav2 revealed substantially slower kinetics compared with M2-YFP or A1-YFP and was reversed by the addition of methyl-β-cyclodextrin. Thus the localization of the channel signal transduction cascade in non-cholesterol rich domains substantially enhances the speed of signaling. The presence of Gsα solely in caveolae may account for signaling selectivity between Gi/o and Gs-coupled receptors.
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