Proteasome involvement in the degradation of the Gq family of Gα subunits

Abstract

Metabolically unstable proteins are involved in a multitude of regulatory networks, including those that control cell signaling, the cell cycle and in many responses to physiological stress. In the present study, we have determined the stability and characterized the degradation process of some members of the Gq class of heterotrimeric G proteins. Pulse-chase experiments in HEK293 cells indicated a rapid turnover of endogenously expressed Gαq and overexpressed Gαq and Gα6 subunits. Pretreatment with proteasome inhibitors attenuated the degradation of both G alpha subunits. In contrast, pretreatment of cells with inhibitors of lysosomal proteases and nonproteasomal cysteine proteases had very little effect on the stability of the proteins. Significantly, the turnover of these proteins is not affected by transient activation of their associated receptors. Fractionation studies showed that the rates of Gαq and Gαq degradation are accelerated in the cytosol. In fact, we show that a mutant Gαq which lacks its palmitoyl modification site, and which is localized almost entirely in the cytoplasm, has a marked increase in the rate of degradation. Taken together, these results suggest that the Gq class proteins are degraded through the proteasome pathway and that cellular localization and/or other protein interactions determine their stability. © 2005 FEBS.