Abstract
Abstract Phosphorylation of ribosomal proteins is studied in mouse sarcoma 180 tumor cells incubated in a nutrient medium with [32P]orthophosphate. Ribosomal proteins are phosphorylated to form phosphoserine and phosphothreonine residues. The pattern of ribosomal protein phosphorylation in mouse sarcoma 180 cells is strikingly similar to that in rabbit reticulocytes in the following respects: (a) The ribosomal phosphoproteins from both cells comigrate when they are fractionated according to their molecular weights by electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. It is concluded that the phosphoproteins in these ribosomes have the same molecular weights and are presumably homologous proteins related by evolution. (b) Common differences between the phosphorylation patterns of polysomal and single ribosomes occur in both cells. (c) A polyphosphorylated protein (molecular weight 70,000) is associated with the native 44 S subribosomal particles in both cells. This polyphosphorylated protein is eluted from the 44 S subunits in conditions commonly employed for extraction of initiation factors. These similarities are considered to prove that the phosphoproteins are true ribosome constituents rather than contaminants. Furthermore, the functions of these phosphoryl groups have apparently been retained during mammalian evolution and are not specifically related to differentiation or to neoplasia.
Highlights
Polyphosphorylated protein is associated with the native 44 S subribosomal particles in both cells
This phosphoprotein routinely contains at least 50% of the radioactivity in the reticulocyte 44 S subunits; it is absent from polysomes and single ribosomes; and it is extracted from the small subunits when they are sedimented into a sucrose gradient which contains a high ionic strength buffer [1]
Comparison of Ribosomal Phosphoproteins 180 Cells and in Rabbit Reticulocytes.-These similar phosphoproteins occur in ribosome in Mouse Sarcoma studies show that preparations from mouse sarcoma 180 cells and from rabbit reticulocytes
Summary
Phosphorylation of ribosomal proteins is studied in mouse sarcoma 180 tumor cells incubated in a nutrient medium with [32P]orthophosphate. The pattern of ribosomal protein phosphorylation in mouse sarcoma 180 cells is strikingly similar to that in rabbit reticulocytes in the following respects: (a) The ribosomal phosphoproteins from both cells comigrate when they are fractionated according to their molecular weights by electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. In this paper we report studies of ribosomal protein phosphorylation in mouse sarcoma 180 ascites cells. These cells incorporate [3”P]orthophosphate into phosphoserine and phosphothreonine residues of ribosomal polypeptide chains. Our data show that there are striking similarities between ribosome phosphorylation in these rapidly growing mouse tumor cells with that which occurs in differentiating rabbit reticulocytes
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