The sequential addition of ribosomal proteins during the formation of the small ribosomal subunit in Friend erythroleukemia cells.

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 crossreactivity between rat and mouse ribosomes [F. Noll and H. Bielka (1970) Mol. Gen. Genet. 106, 106-113]. The identification of the proteins was achieved by two independent immunological techniques: the passive haemagglutination 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, S14, S18, S20, S23/24 and S25) are present in the '80-S' preribosome. Only two 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. The results obtained provide evidence for the sequential addition of proteins during the formation of the small ribosomal subunit in Friend erythroleukemia cells.