Biosynthesis of Nonglobin Protein by Membrane-bound Ribosomes in Reticulocytes

Abstract

Abstract Both free and membrane-bound reticulocyte ribosomes synthesize protein in the intact cell and the reticulocyte cell-free system. The soluble proteins synthesized by these two classes of ribosomes in the cell-free system were isolated and analyzed by ion exchange and gel chromatography and electrophoresis. In the cell-free system free reticulocyte ribosomes incorporate 70% of 14C-valine into the α and β chains of globin and the remainder into two non-globin protein fractions which are separable by Sephadex gel chromatography. In contrast, membrane-bound ribosomes incorporate only 11% of the radioactivity into globin subunits but synthesize primarily high molecular weight nonglobin proteins. The nonglobin proteins are resolved into multiple discrete subunits by electrophoresis in the presence of mercaptoethanol and urea. In the intact reticulocyte 14% of radioactive amino acids is incorporated into nonglobin proteins of the same character as the nonglobin proteins synthesized in the cell-free system. The data indicate that free ribosomes are primarily engaged in globin synthesis while membrane-bound ribosomes preferentially synthesize high molecular weight nonglobin proteins.