The Polysomal Proteins of L Cells

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Abstract
Three groups of proteins can be clearly discriminated in the total protein of L cell polysomes by selective labelling in the presence of low doses of actinomycin D and two‐dimensional polyacrylamide/dodecylsulfate gel electrophoresis followed by autoradiography: (a) structural ribosomal proteins which are not labelled in the presence of actinomycin D and form stained non‐radioactive spots in gels; (b) exchangeable ribosomal proteins which are labelled in the presence of actinomycin D and form stained radioactive spots; (c) non‐ribosomal proteins which are detectable only by autoradiography of gels. The large and small subunits of L cell ribosomes contain respectively 45 and 34 ribosomal proteins with molecular weights ≤ 50000; seven of the large subunit proteins and nine of the small subunit proteins are exchangeable. Most of the non‐ribosomal proteins migrate in the region of the second‐dimension gel slab corresponding to the molecular weight range 50000 –120000. Problems related to the separation of the ribosomal proteins of mammalian cells and the possible significance of the presence of non‐ribosomal proteins in polysomes are discussed.