Isolation and Characterization of the Acidic Phosphproteins of 60-S Ribosomes from Artermia salina and Rat Liver (original) (raw)

Eucaryotic L7/L12-type proteins are present in ethanol/salt extracts (P1 protein) of ribosomes from Artemia salina and rat liver. These proteins are partially phosphorylated and occur in two forms of closely related structure: a major form eL12 havkg methionine at the N-terminal position and a minor form of eL12 (eL12') which seems slightly elongated and contains a blocked N terminus. Purification of the four different forms of this protein, eL12, eL12-P, eL12' and eL12'-P, was performed by ion-exchange chromatography on carboxymethyl-cellulose and DEAEcellulose. Using a radioimmuno assay, 1.8 copies of eL12 and 0.9 of eL12' were found on the 80-S A. salina ribosome. In ribosomes of both rat liver and A . salina, eL12 is present in a larger quantity than eL12'. 40-S and 60-S ribosomal subunits extracted with ethanol/salt were essentially free of eL12 proteins. A large pool of eL12 was found in the cytosol after removal of the ribosomes by centrifugation or molecular sieving. The proteins of rat liver and A . salina are similar with regard to their isoelectric points and molecular weights. Sedimentation equilibrium studies indicated that the isolated protein eL12 occurs as a dimer. 6 0 3 ribosomal subunits from several eucaryotic organisms contain an acidic protein with properties which resemble the protein L7/L12 from bacterial 50-S ribosomal subunits . In eucaryotes there are also two related forms of the protein. The difference between them is not due to acetylation as in Escherichia coli but rather phosphorylation .