The Sequential Addition of Ribosomal Proteins during the Formation of the Small Ribosomal Subunit in Friend Erythroleukemia Cells

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Abstract
Nucleolar 80-S and 40-S preribosomes (containing 45-S and 21-S pre-rRNA, respectively), as well as cytoplasmic ribosomes, were isolated from Friend erythroleukemia cells. The presence of structural ribosomal proteins in the isolated particles was studied by using antisera against individual rat liver small ribosomal subunit proteins. The analysis is based on the established cross-reactivity between rat and mouse ribosomes. The identification of the proteins was achieved by 2 independent immunological techniques: the passive hemagglutination test and the enzyme immunoassay of electrophoretically fractionated proteins, blotted on nitrocellulose. All 17 proteins tested are present in cytoplasmic ribosomes. A large number of proteins (S3a, S6, S7, S8, S11, S13, S14, S18, S20, S23/24 and S25) are present in the 80-S preribosome. Only 2 proteins (S3 and S21) are added during the formation of the 40-S preribosome in the nucleolus. Four proteins (S2, S19, S26 and S29) are added at later, possibly extranucleolar, stages of ribosome formation. Evidence is provided for the sequential addition of proteins during the formation of the small ribosomal subunit in Friend erythroleukemia cells.