Hepatic Membrane Proteins Involved in Ribosome Binding: Identification by Three Procedures

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Abstract
Rat liver ribosomes, isolated from rough endoplasmic reticulum using non-ionic detergents in the presence of 25 mM KCl, were associated with non-ribosomal proteins, presumably of membranous origin. These proteins were isolated by extracting such ribosome fractions with deoxycholate or non-ionic detergents at higher concentrations of KCl. Analysis of the extracts by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed the presence of a number of discrete polypeptides having approximate MW: 166,000, 107,000; 100,000; 65,000 and 36,000. Ribosomes associated with the membrane-derived proteins reattached to degranulated membranes in vitro less well than did ribosomes from which proteins were removed. Extraction of a set of similar proteins from smooth endoplasmic reticulum by treatment with buffered 1 M urea, also interfered with ribosome reattachment. A 3rd approach to the identification of proteins associated with ribosome binding sites involved the labeling with radioactive succinic anhydride of apparently similar proteins in smooth membranes, after prior treatment of the latter, before removal of bound ribosomes, with unlabeled reagent. Certain membrane proteins may be part of the receptor sites for binding of ribosomes to the endoplasmic reticulum in rat liver.

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