Competition for Gβγ dimers mediates a specific cross-talk between stimulatory and inhibitory G protein α subunits of the adenylyl cyclase in cardiomyocytes

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 gamma 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 gamma 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 gamma dimers was not altered by any G alpha overexpression. However, the increase of G beta gamma 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 gamma dimers, which are required for G protein heterotrimer formation at the plasma membrane.