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

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 having 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 DEAE-cellulose. 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.