A new type of prosome‐like particle, composed of small cytoplasmic RNA and multimers of a 21‐kDa protein, inhibits protein synthesis in vitro

A large fraction of the translationally repressed non‐globin messenger RNA in duck erythroblasts is present in non‐polyribosomal free mRNP structures which sediment in the 30–40‐S range (‘35 S’). In 0.5 M KCl, they form core complexes which show a pronounced peak at about 32 S containing mRNA and a discrete spherical RNP particle with a diameter of about 12 nm and the typical morphology of a prosome [H.‐P. Schmid et al. (1984) EMBO J. 3 , 29–34]. Buoyant density measurements and chromatography on oligo(dT)‐cellulose indicate that this particle is bound to mRNA; it can be released from the mRNA by treatment of the free mRNP fraction with SDS. This prosome‐like particle inhibits the translation of mRNA in vitro. It is composed primarily of multimers of a single 21‐kDa protein and at least one species of RNA of about 80–100 nucleotides. It is resistant to dissociation by 2 M Cs 2 SO 4 and 1% SDS; the 21‐kDa protein is not attacked by proteinase K unless the particle is extracted with phenol prior to treatment with the protease. The small RNA moiety of the particle hybridizes to the poly(A)‐rich mRNA derived from the free mRNPs, as well as to polyribosomal mRNA. These data indicate that prosomes may serve to regulate mRNA translation; they show furthermore that prosome‐like particles (about 600 kDa mass) may be built of up to 25 molecules of a single specific protein, rather than of the entire set of about 20 prosomal proteins previously indentified.