Ubiquitination of the heterotrimeric G protein α subunits Gαi2 and Gαq is prevented by the guanine nucleotide exchange factor Ric-8A

Abstract

The cytosolic protein Ric-8A acts as a guanine nucleotide exchange factor for Gα 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 Gα proteins in vivo. The mechanism whereby Ric-8 regulates Gα content, however, has not been fully understood. Here we show that Ric-8 Astabilizes Gαi2 and Gαq by preventing their ubiquitination. Ric-8A interacts with and stabilizes Gαi2, Gαq, Gα12, but not Gαs, when expressed in COS-7 cells. The protein levels of Gαi2 and Gαq appear to be controlled via the ubiquitin–proteasome degradation pathway, because these Gα subunits undergo polyubiquitination and are stabilized with the proteasome inhibitor MG132. The ubiquitination of Gαi2 and Gαq is suppressed by expression of Ric-8A. The suppression likely requires Ric-8A interaction with these Gα proteins; the C-terminal truncation of Gαq and Gαi2 completely abrogates their interaction with Ric-8A, their stabilization by Ric-8A, and Ric-8A-mediated inhibition of Gα ubiquitination.