Open AccessRibosomal Phosphoproteins of Mouse Myeloma Cells
Author(s) -
MARTINI Oskar H. W.,
KRUPPA Joachim
Publication year - 1979
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1979.tb12972.x
Subject(s) - ribosome , messenger rna , ribosomal protein , biology , eukaryotic ribosome , ribosomal rna , protein biosynthesis , polysome , protein subunit , centrifugation , phosphorylation , biochemistry , microbiology and biotechnology , rna , gene
Mouse myeloma cells (MPC 11) in culture, labelled with [ 32 P] i for several hours, were subjected to hypertonic initiation block by raising the salt concentration of the culture medium from 110 to 210 mM NaCl. The relative amounts of messenger‐free and bound ribosomes of the cells were estimated from ribosome profiles following sucrose density centrifugation. The initiation block caused the proportion of messenger‐bound ribosomes to decrease substantially. The extent of the decrease was larger under suboptimal (50%) than under satisfactory (25%) nutritional conditions. Analysis of the ribosomal proteins by two‐dimensional gel electrophoresis, autoradiography and radiospectrometry showed, that in the small subunit only protein S3 had incorporated 32 P and that S3 was the most strongly phosphorylated ribosomal protein in control cells. Upon salt treatment of the cells the radioactivity of S3 from both messenger‐free and bound ribosomes decreased to low levels. The lowest levels (15% of the controls) were reached when the nutritional conditions were suboptimal. The results show that there is a positive correlation between the number of ribosomes engaged in protein synthesis and the degree of phosphorylation of S3. However, phosphorylation of S3 does not seem to be a prerequisite for ribosomes to be attached to mRNA. Under all experimental conditions S3 from messenger‐bound ribosomes was about twice as strongly labelled as S3 from messenger‐free ribosomes. The large‐subunit phosphoproteins were identified as L28, the protein thought to be homologous to the Escherichia coli proteins L7/L12, and as L5, which was only weakly labelled. In contrast to S3 the radioactivity of L28 increased slightly upon the tonicity shift.