Maintenance of cellular levels of G-proteins: different efficiencies of alpha s and alpha o synthesis in GH3 cells.
G-proteins couple membrane-bound receptors to intracellular effectors. Each cell has a characteristic complement of G-protein alpha, beta and gamma 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, alpha s and alpha o, 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 alpha s protein than to produce the same amount of alpha o protein. Such a situation could arise from a relatively rapid rate of alpha s protein degradation, requiring rapid protein synthesis to compensate, or from relatively inefficient translation of alpha s mRNA compared with alpha o mRNA. The latter appears to be the case in GH3 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. The 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 accounts 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 virtually all alpha s and alpha o 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 alpha s protein synthesis is regulated by a mechanism that acts beyond initiation at peptide elongation and/or termination.
Li
Y
Y
Cardiovascular Division, Brigham and Women's Hospital, Boston, MA, USA.
Mende
U
U
Lewis
C
C
Neer
E J
EJ
D000818:Animals; D001921:Brain / Q000378:metabolism; D002460:Cell Line; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D011132:Polyribosomes / Q000378:metabolism; D011487:Protein Conformation; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
D012333:RNA, Messenger; D019204:GTP-Binding Proteins