Background: We previously reported that the guanine tri-phosphate-binding proteins (G) beta and gamma are both localized in the nucleus, in addition to their expected cytoplasmic/plasma membrane localization. These proteins, as a heterodimeric complex, suppress glucocorticoid response element-mediated transcriptional activity of the glucocorticoid receptor through direct physical interactions between Gbeta and the glucocorticoid receptor.
Materials and methods: As Ggamma is prenylated at a cysteine residue in its C-terminal portion, and as this post-translational modification is required for many of the known Gbeta/Ggamma activities, we examined the effect of its absence or diminution on Gbeta/Ggamma-induced suppression of glucocorticoid receptor-induced transcriptional activity.
Results: In a functional reporter assay, Ggamma2C68S, which is defective at the prenylation site, was more potent than the wild-type Ggamma2 at increasing Gbeta2-induced suppression of glucocorticoid receptor transactivation. Interestingly, the enhanced green fluorescent protein fusion of this mutant Ggamma2 was localized preferentially in the nucleus, while it was absent from the plasma membrane. Lovastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor that abrogates the prenylation of Ggamma, shifted the subcellular localization of enhanced green fluorescence protein-fused Ggamma2 and Gbeta2 from the cytoplasm/plasma membrane to the nucleus and further suppressed glucocorticoid receptor-induced transcriptional activity.
Conclusions: These findings indicate that not only is the natural covalent addition of the prenyl residue to Ggamma unnecessary for the transcriptional suppression induced by Gbeta/Ggamma on the glucocorticoid receptor, but rather helps retain the Gbeta/Ggamma complex away from the nucleus decreasing its antiglucocorticoid actions.