Maintenance of cellular levels of G-proteins: different efficiencies of αs and αo synthesis in GH3 cells

Abstract

G-proteins couple membrane-bound receptors to intracellular effectors. Each cell has a characteristic complement of G-protein α, β and γ subunits that partly determines the cell's response to external signals. Very little is known about the mechanisms that set and maintain cellular levels of G-proteins or about potential points of regulation. We have assayed the steady-state levels of mRNA and protein for two types of G-protein subunits, αs and αo, in rat brain, heart and GH3 cells, and found that in all these cases, it takes 9- to 20-fold more mRNA to produce a given amount of αs protein than to produce the same amount of αo protein. Such a situation could arise from a relatively rapid rate of αs protein degradation, requiring rapid protein synthesis to compensate, or from relatively inefficient translation of αs mRNA compared with αo mRNA. The latter appears to be the case in GH3 cells. These cells contain 94 times more mRNA for αs than for αo, yet the rate of αs protein synthesis is only 9 times greater than αo protein synthesis. The degradation rates of the two proteins are similar (13 h for αs and 18 h for αo). To begin to define the mechanism that accounts for the fact that it takes more mRNA to synthesize a given amount of αs than αo, we asked whether there is a pool of αs mRNA that does not participate in protein synthesis. We found that virtually all αs and αo mRNA is associated with ribosomes. Therefore, all the mRNA is likely to be capable of directing protein synthesis. Since the rate-limiting step in protein synthesis is usually binding of the ribosome to mRNA at initiation, our results suggest that the relatively slow rate of αs protein synthesis is regulated by a mechanism that acts beyond initiation at peptide elongation and/or termination.