The effects of glucagon, phenylephrine and insulin on the phosphorylation of cytoplasmic, mitochondrial and membrane-bound proteins of intact liver cells from starved rats

Abstract

1. The effects of glucagon, insulin and phenylephrine on the phosphorylation of cytoplasmic, mitochondrial and membrane proteins were studied in intact hepatocytes from 24 h-starved rats incubated with [32P]Pi. A rapid cell-fractionation technique was used, followed by radioautography of the proteins separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 2. Glucagon consistently caused a significant increase in the phosphorylation of four readily separable cytoplasmic phosphoproteins, of Mr 93000, 50000, 46000 and 20000, and a decrease in phosphorylation of a phosphoprotein of Mr 22000. Phosphorylation of the protein of Mr 46000 was also enhanced by both phenylephrine and insulin, and that of Mr 93000 by phenylephrine. 3. The phosphoprotein of Mr 22000 was not precipitated by boiling for 5 min, and had a mobility identical with that of similar protein whose phosphorylation is enhanced in the adipocyte by insulin [Belsham & Denton (1980) Biochem. Soc. Trans. 8, 382-383]. 4. Glucagon, but not phenylephrine or insulin, enhanced the phosphorylation of a mitochondrial protein of Mr 35000 and of four plasma- or microsomal-membrane proteins of Mr 50000, 30000, 23000 and 19000. 5. Mitochondria from glucagon-treated animals or hepatocytes phosphorylated a protein of Mr 30000 when incubated in vitro with [32P]Pi and ADP. Phosphorylation of this protein did not occur with mitochondria from control, phenylephrine- or insulin-treated cells. 6. The significance of these hormonally induced changes in protein phosphorylation is discussed.