MAINTENANCE OF CELLULAR-LEVELS OF G-PROTEINS - DIFFERENT EFFICIENCIESOF ALPHA(S) AND ALPHA(0) SYNTHESIS IN GH(3) CELLS

G-proteins couple membrane-bound receptors to intracellular effecters. Each cell has a characteristic complement of G-protein alpha, beta an d gamma subunits that partly determines the cell's response to externa l signals. Very little is known about the mechanisms that set and main tain cellular levels of G-proteins or about potential points of regula tion. We have assayed the steady-state levels of mRNA and;protein for two types of G-protein subunits, alpha(s) and alpha(o), in rat brain, heart and GH(3) cells, and found that in all these cases, it takes 9- to 20-fold more mRNA to produce a given amount of a(s) protein than to produce the same amount of a(o) protein. Such a situation could arise from a relatively rapid rate of a(s) protein degradation, requiring r apid protein synthesis to compensate, or from relatively inefficient t ranslation of alpha(s) mRNA compared with alpha(o) mRNA. The latter ap pears to be the case in GH(3) cells. These cells contain 94 times more mRNA for alpha(s) than for alpha(o), yet the rate of alpha(s) protein synthesis is only 9 times greater than alpha(o) protein synthesis. Th e degradation rates of the two proteins are similar (13 h for alpha(s) and 18 h for alpha(o)). To begin to define the mechanism that account s for the fact that it takes more mRNA to synthesize a given amount of alpha(s) than alpha(o), we asked whether there is a pool of alpha(s) mRNA that does not participate in protein synthesis. We found that vir tually all alpha(s) and alpha(o) mRNA is associated with ribosomes. Th erefore, all the mRNA is likely to be capable of directing protein syn thesis. Since the rate-limiting step in protein synthesis is usually b inding of the ribosome to mRNA at initiation, our results suggest that the relatively slow rate of alpha(s) protein synthesis is regulated b y a mechanism that acts beyond initiation at peptide elongation and/or termination.