Accessibility of proteins in rat liver-free and membrane-bound ribosomes to lactoperoxidase-catalyzed iodination.

We have used lactoperoxidase-catalyzed iodination to study the topographical distribution of proteins in free and membrane-bound polysomes and in isolated ribosomal subunits. Free polysomes or rough microsomes were iodinated with 125I and then dissociated into 40 S and 60 S subunits by the use of puromycin/KCl. The separated subunits were iodinated with 13’1 and their protein composition analyzed by polyacrylamide gel electrophoresis. Most 40 S subunit proteins contained amino acids which were accessible to lactoperoxidase in intact polysomes. Some of these proteins appeared to be more accessible after dissociation of the polysomes into 40 S subunits. A large proportion of the 60 S ribosomal proteins of intact polysomes could not be iodinated but, after dissociation into subunits, many more ribosomal proteins became accessible to lactoperoxidase and only two proteins were detected which, although containing iodine-accepting residues, could not be iodinated in isolated subunits. We have subdivided the ribosomal proteins into four classes according to their accessibility to lactoperoxidase in both polysomes and subunits. Comparison of the results obtained with free and membrane-bound ribosomes showed that a 60 S subunit protein of free polysomes was more accessible to lactoperoxidase than its counterpart in bound ribosomes (rough microsomes). Further analysis by the use of two-dimensional pulyacrylamide gel electrophoresis allowed the identification of three 60 S subunit proteins of membrane-bound ribosomes which appear to be prntected from iodination by the presence of the microsomal membrane. These proteins may be involved in binding ribosomes to the endoplasmic reticulum.