Competition for G[beta][gamma] dimers mediates a specific cross-talk between stimulatory and inhibitory G protein [alpha] subunits of the adenylyl cyclase in cardiomyocytes

• Published in: Naunyn-Schmiedeberg's archives of pharmacology 2013-06, Vol.386 (6), p.459
• Main Authors: Hippe, Hans-jörg, Lüdde, Mark, Schnoes, Katrin, Novakovic, Ana, Lutz, Susanne, Katus, Hugo A, Niroomand, Feraydoon, Nürnberg, Bernd, Frey, Norbert, Wieland, Thomas
• Format: Article
• Language: English
• ISSN: 0028-1298; 1432-1912
• DOI: 10.1007/s00210-013-0876-x

Heterotrimeric G proteins are key regulators of signaling pathways in mammalian cells. Beyond G protein-coupled receptors, the amount and mutual ratio of specific G protein [alpha], [beta], and γ subunits determine the G protein signaling. However, little is known about mechanisms that regulate the concentration and composition of G protein subunits at the plasma membrane. Here, we show a novel cross-talk between stimulatory and inhibitory G protein [alpha] subunits (G[alpha]) that is mediated by G protein [beta]γ dimers and controls the abundance of specific G[alpha] subunits at the plasma membrane. Firstly, we observed in heart tissue from constitutively G[alpha]i2- and G[alpha]i3-deficient mice that the loss of G[alpha]i2 and G[alpha]i3 was accompanied by a slight increase in the protein content of the nontargeted G[alpha]i isoform. Therefore, we analyzed whether overexpression of selected G[alpha] subunits conversely impairs endogenous G protein [alpha] and [beta] subunit levels in cardiomyocytes. Integration of overexpressed G[alpha]i2 subunits into heterotrimeric G proteins was verified by co-immunoprecipitation. Adenoviral expression of increasing amounts of G[alpha]i2 led to a reduction of G[alpha]i3 (up to 90 %) and G[alpha]s (up to 75 %) protein levels. Likewise, increasing amounts of adenovirally expressed G[alpha]s resulted in a linear 75 % decrease in both G[alpha]i2 and G[alpha]i3 protein levels. In contrast, overexpression of either G[alpha]i or G[alpha]s isoform did not influence the amount of G[alpha]o and G[alpha]q, both of which are not involved in the regulation of adenylyl cyclase activity. The mRNA expression of the disappearing endogenous G[alpha] subunits was not affected, indicating a posttranslational mechanism. Interestingly, the amount of endogenous G protein [beta]γ dimers was not altered by any G[alpha] overexpression. However, the increase of G[beta]γ level by adenoviral expression prevented the loss of endogenous G[alpha]s and G[alpha]i3 in G[alpha]i2 overexpressing cardiomyocytes. Thus, our results provide evidence for a novel mechanism cross-regulating adenylyl cyclase-modulating G[alpha]i isoforms and G[alpha]s proteins. The G[alpha] subunits apparently compete for a limited amount of G[beta]γ dimers, which are required for G protein heterotrimer formation at the plasma membrane.