Membrane ribosomes from which were detached from the membrane by extraction with the nonionic detergent Triton X-100 retained a protein (MBRP) with a molecular weight of 60000, which was absent from cytoplasmic ribosomes. MBRP was detected and quantified by immunological methods. When membrane ribosomes were dissociated into 50S and 30S subunits, MBRP remained associated with the 50S particle. MBRP was found both on membrane ribosomes and in the cytoplasm in roughly equal amounts. When added to Triton X-100-solubilized protoplasts, antibodies to MBRP produced immunoprecipitates which contained a complex of MBRP and three other proteins with molecular weights of 71000, 46000 and 41000. Four proteins with the same molecular weights as those of the MBRP complex were found associated with membrane ribosomes. The proteins of molecular weight 71000, 60000, 46000 and 41000 seemed to be present in stoichiometrically equivalent amounts in the complex.


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