Surface membranes isolated from mouse fibroblasts contain RNA organized in ribosome-like particles. These surface membranes can incorporate radioactive amino acids into material insoluble in hot trichloroacetic acid. Characterization of the radioactive products indicated that they are large polypeptides. Most requirements and inhibitors of the system were similar to those of the microsomal system of the endoplasmic reticulum. Further comparisons were made with microsomal and pH 5 fractions prepared from L-cells by standard procedures and by two methods attempting to simulate the conditions necessary for isolating surface membranes. The amount of L-leucine incorporated by the surface membranes per mg of RNA was comparable to the values obtained by the microsomal and pH 5 fractions prepared by standard procedures. In contrast, the activity of the two other microsomal fractions was less than 10 per cent of this amount. The most striking difference noted was in the products formed by the surface membranes and by the microsomal systems. These products were demonstrated to be quite different by electrophoresis on acrylamide gels. It is suggested that the surface membranes participate actively in protein synthesis in vivo, and that, because of this location, the protein-synthesizing mechanism may be able to perform special functions more efficiently
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