Hepatic membrane proteins involved in ribosome binding: identification by three procedures

Rat liver ribosomes, isolated from rough-surfaced endoplasmic reticulum using non-ionic detergent in the presence of 25 mM KCl, were associated with non-ribosomal proteins, presumably of membranous origin. These proteins could be isolated by extracting such ribosome fractions with either deoxycholate or non-ionic detergents at higher concentrations of KCl. Analysis of the extracts by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate revealed the presence of a number of discrete polypeptides having the following approximate molecular weights: 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 prepared in ways which removed the proteins. Extraction of a set of similar proteins from degranulated endoplasmic reticulum by treatment with buffered 1 M urea, also interfered with ribosome reattachment. A third approach to the identification of proteins associated with ribosome attachment sites involved the labelling with radioactive succinic anhydride of apparently similar proteins in degranulated membranes, after prior treatment of the latter, before removal of bound ribosomes, with unlabelled reagent. The results indicate that certain membrane proteins may be part of the receptor sites for binding of ribosomes to the endoplasmic reticulum in rat liver.