Ric-8B stabilizes the α subunit of stimulatory G protein by inhibiting its ubiquitination

The α subunit of stimulatory G protein (Gα_s) activates adenylyl cyclase, which catalyzes cAMP production, and regulates many physiological aspects, such as cardiac regulation and endocrine systems. Ric-8B (resistance to inhibitors of cholinesterase 8B) has been identified as the Gα_s-binding protein; however, its role in G_s signaling remains obscure. In this study, we present evidence that Ric-8B specifically and positively regulates G_s signaling by stabilizing the Gα_s protein. An in vitro biochemical study suggested that Ric-8B does not possess guanine nucleotide exchange factor activity. However, knockdown of Ric-8B attenuated β-adrenergic agonist-induced cAMP accumulation, indicating that Ric-8B positively regulates G_s signaling. Interestingly, overexpression and knockdown of Ric-8B resulted in an increase and a decrease in the Gα_s protein, respectively, without affecting the Gα_s mRNA level.We found that the Gα_s protein is ubiquitinated and that this ubiquitination is inhibited by Ric-8B. This Ric-8B-mediated inhibition of Gα_s ubiquitination requires interaction between Ric-8B and Gα_s because Ric-8B splicing variants, which are defective for Gα_s binding, failed to inhibit the ubiquitination. Taken together, these results suggest that Ric-8B plays a critical and specific role in the control of Gα_s protein levels by modulating Gα_s ubiquitination and positively regulates G_s signaling.