文摘
The cytosolic protein Ric-8A acts as a guanine nucleotide exchange factor for G¦Á 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¦Á proteins in vivo. The mechanism whereby Ric-8 regulates G¦Á content, however, has not been fully understood. Here we show that Ric-8 Astabilizes G¦Ái2 and G¦Áq by preventing their ubiquitination. Ric-8A interacts with and stabilizes G¦Ái2, G¦Áq, G¦Á12, but not G¦Ás, when expressed in COS-7 cells. The protein levels of G¦Ái2 and G¦Áq appear to be controlled via the ubiquitin-proteasome degradation pathway, because these G¦Á subunits undergo polyubiquitination and are stabilized with the proteasome inhibitor MG132. The ubiquitination of G¦Ái2 and G¦Áq is suppressed by expression of Ric-8A. The suppression likely requires Ric-8A interaction with these G¦Á proteins; the C-terminal truncation of G¦Áq and G¦Ái2 completely abrogates their interaction with Ric-8A, their stabilization by Ric-8A, and Ric-8A-mediated inhibition of G¦Á ubiquitination.