The cytosolic protein Ric-8A acts as a guanine nucleotide exchange factor for G alpha subunits of the Gi, Gq, and G12/13 classes of heterotrimeric G protein in vitro, and is also known to increase the amounts of these G alpha proteins in vivo. The mechanism whereby Ric-8 regulates G alpha content, however, has not been fully understood. Here we show that Ric-8 Astabilizes G alpha i2 and G alpha q by preventing their ubiquitination. Ric-8A interacts with and stabilizes G alpha i2, G alpha q, G alpha 12, but not G alpha s, when expressed in COS-7 cells. The protein levels of G alpha i2 and G alpha q appear to be controlled via the ubiquitin-proteasome degradation pathway, because these G alpha subunits undergo polyubiquitination and are stabilized with the proteasome inhibitor MG132. The ubiquitination of G alpha i2 and G alpha q is suppressed by expression of Ric-8A. The suppression likely requires Ric-8A interaction with these G alpha proteins: the C-terminal truncation of G alpha q and G alpha i2 completely abrogates their interaction with Ric-8A, their stabilization by Ric-8A, and Ric-8A-mediated inhibition of G alpha ubiquitination. (C) 2013 Elsevier Inc. All rights reserved.