Studies on the mechanism of decrease in the RNA content in liver cells of fasted rats. II. The mechanism of starvation-induced decrease in RNA polymerase activity in liver

Analysis by two-dimensional polyacrylamide gel electrophoresis of liver ribosomal subunnit proteins obtained from free and membrane-bound polysomes of unfasted animals

Ribosomal proteins were analyzed by means of two-dimensional gel electrophoresis. To insure that the analysis included only that fraction of the ribosome actively participating in protein synthesis, only polysomal-bound ribosomes were used. This differs from previously reported analyses of liver ribosomal proteins. The ribosomal proteins were prepared from ribosomes of polysomal origin from membrane-bound and free polysomes. Membrane-bound and free liver polysomes were isolated from unfasted mice. The polysomes were purified on hydroxyapatite under conditions known to result in polysomes and ribosomes that are active in both endogenous and synthetic mRNA translation. Moreover, this is the first time that liver ribosomal protein was obtained and analyzed from animals that have not been starved prior to sacrifice. The puromycin-released ribosomes were dissociated into subunits and ribosomal proteins were analyzed by means of two-dimensional polyacrylamide gel electrophoresis. When 100-200 mug samples of the ribosomal subunit proteins were analyzed by two-dimensional electrophoresis, approximately 32 major proteins were detected for the 60 S ribosomal subunit and 25 major proteins for the 40 S ribosomal subunit. A total of 13 "fractional" ribosomal proteins was also detected in the ribosomal subunit profiles. No differences in number or mobility of the ribosomal proteins were found between the membrane-bound and free ribosome populations. We describe a system in which all ribosomal proteins are completely solubilized and quantitatively move from the first to the second dimension gel. Thus the total sample is separated and fractionated. This procedure elimates artifacts due to incomplete solubilization of ribosomal proteins, which is common for the transfer from the first- to second-dimension gel. Therefore, a more detailed and accurate analysis is achieved.

Protein synthesis in pancreas of fasted pigeons

The regulation of protein synthesis in the pigeon has been studied by comparing the capability of cell-free amino acid incorporating systems of membrane-bound and membrane-free polysomes prepared from fasted and fed birds. New methods were developed for isolating polysomes since techniques used for other tissues did not provide quantitative recovery of polysomal RNA. The sucrose gradient profile of polysomes from pigeon pancreas showed a predominance of trisome species. Although initiation factors are present on polysomes, it was found that polysomes in cell-free systems would not initiate protein synthesis without exogenous initiation factors. This suggested the presence of an inhibitor or regulator of protein synthesis. These studies show that fasting resulted in: (a) decreased amounts of polysomes; (b) disaggregation of polysomes to monosomes; (c) decreased capability of polysomes to synthesize nascent peptides and to initiate additional synthesis, apparently not related to concentration of initiation factors.