Characterization of the rough endoplasmic reticulum ribosome-binding activity

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Abstract
THE rough endoplasmic reticulum membranes of mammalian cells contain specific ribosome-binding sites1. A purification to apparent homogeneity of a negatively charged protein (ERpl80) of relative molecular mass 180,000 (180 K) was reported which was proposed to function as a rough endoplasmic reticulum ribosome receptor2. We report here that ribosome-binding site activity quantitatively solubilized from rough endoplasmic reticulum membranes does not cofractionate with ERpl80. By contrast, ribosome-binding site activity fractionates as a much smaller, positively charged protein.