Biosynthesis of Serum Proteins and Ferritin by Free and Attached Ribosomes of Rat Liver

Abstract

Polysomes of rat liver appear to be either attached to the membrane of the endoplasmic reticulum or free in the cytoplasm. It has been postulated that attached ribosomes are engaged in the synthesis of secretory proteins while free ribosomes make nonexportable proteins. Experimental support for this hypothesis has been obtained by studying the site of synthesis of serum proteins, an example of secretory proteins, and of ferritin, an example of a nonexportable protein. Two to three minutes after an intravenous injection of 3H-leucine into the rat there is equal and maximum incorporation, per mg of RNA, into trichloracetic acid-insoluble proteins bound to both free and attached ribosomes. The newly formed protein attached to the ribosomes was released by treatment with spermine and puromycin. About 80% of the newly formed serum protein recovered by this method was obtained from the attached ribosomes. The radioactive ferritin, however, was found to occur in the proteins released from the free ribosomes. Similar results were obtained by incubating free and attached polysomes (microsomes) in vitro with radioactive amino acids and under conditions for protein synthesis. Again, over 80% of the protein synthesized by attached ribosomes was found to be serum proteins, and the free ribosomes made 7 to 20 times more ferritin than did attached ribosomes. The radioactive proteins were detected and isolated by precipitation with specific antisera and the specificity of the methods checked by a combination of immunoelectrophoresis and autoradiography.