A novel ribonucleoprotein (RNP) particle showing a highly compact and characteristic structure in the electron microscope was found associated with globin and other repressed mRNA in the cytoplasm of duck, mouse and HeLa cells. This 19S complex is of extraordinary stability: dissociated by 0.5 M KCl or EDTA from the (still repressed) core globin mRNP, it can be purified on gradients containing 1% Sarkosyl, and resists (unfixed) caesium sulphate‐dimethylsulphoxide density centrifugation. Its density of 1.31 g/cm3 indicates an RNP complex with a 15% RNA component. In mouse and duck it contains approximately 10 proteins in the 20 000‐30 000 mol. wt. range, a few components of 50 000‐70 000 mol. wt., and two specific small cytoplasmic RNAs (ScRNA) of 70‐90 nucleotides. Both of these RNAs have identical 3′‐terminal oligonucleotides. We propose the name ‘prosome’ for this ScRNP particle which somehow participates in negative control of mRNA translation, and we believe will prove to be ubiquitous to animal species.

To investigate the role of the 73-kDa poly(A)-binding protein in protein synthesis, the effect of the addition of homo-polyribonucleotides on the translation of polyadenylated and non-adenylated mRNA was studied in the rabbit reticulocyte lysate. Poly(A) was found to be the most effective polynucleotide in inhibiting duck-globin mRNA translation, whereas it had no effect on the translation of polyribosomal duck-globin mRNP, or on the endogenous synthesis of the rabbit reticulocyte lysate. The translation of poly(A)-free mRNA was not affected by the addition of poly(A).Furthermore, we found that the inhibiting effect of poly(A) can be reversed by addition of purified poly(A)-binding protein. It is thus likely that the 73-kDa poly(A)-binding protein is an essential factor necessary for poly(A)-rich mRNA translation.The presence of a stretch of adenylate residues at the 3' end of eukaryotic mRNA has been known for over a decade, but its precise role still remains undefined [l -41. Only a few classes of somatic-cell mRNA do not possess a poly(A) tail long enough to be retained on an oligo(dT) column, as those coding for the histones and some viral mRNAs such as reovirus, TMV and TYMV [5 -71. Investigation of the cytoplasmic role of poly(A) has concentrated in the past on mRNA stability and efficiency of translation.A role of the poly(A) sequence in maintaining message stability was suggested by microinjection experiments showing, in the case of rabbit-globin mRNA and human-histone mRNA, a stringent relationship between the presence of a poly(A) tail and mRNA stability in the frog oocyte and HeLa cells [6,[8][9][10]. This relationship is however apparently not true for all types of mRNA. For instance interferon [ l l ] and mengovirus mRNA [12], whether in their natural polyadenylated or enzymatically deadenylated form, have the same half-life in oocytes. Furthermore, all ten reovirus mRNAs have been shown to be stable for several days following microinjection [7]. A study of the endogenous Xenopus leavis oocyte mRNA coding for the histones has shown that as much as 25-50% is non-adenylated and yet is clearly stable during the long period of oogenesis [13, 141. As another possibility, a function of the poIy(A) tail (and the protein associating with it) in mRNA translation was suggested [15, 161. The efficiency of translation of enzymatically deadenylated mRNA has been compared to that of natural poly(A)-rich mRNA in a variety of in vitro proteinsynthesizing systems. A significant difference between the two forms of mRNA, suggesting a facilitating effect of the poly(A) tail on translation, was only observed when systems with a high rate of re-initiation, such as the reticulocyte lysate, were employed. On the other hand, in the wheat germ or Krebs In eukaryotic cells, the mRNA recovered from polyribosomes dissociated with EDTA or puromycin in the form of mRNP is found tightly associated with a major protein of about 73 kDa, as well as with several other minor proteins [18, 191. This 73-kDa protein appears to be ubiqui...

A cytoplasmic particle displaying properties in common with a structure present in duck erythroblasts, termed the prosome, has been isolated from eggs and embryos of two species of sea urchin. This particle was partially purified by sedimentation in sucrose gradients containing 0.5 M KCI, and some of its physical properties and its behavior during early development were determined. The prosome sediments between 16 and 19 S and has a buoyant density of 1.30 g/cm3 in Cs2SO4 gradients. Biochemically, the particle is characterized as 20-25 polypeptides of molecular size 24-35 kDa with about 10 small RNAs. A monoclonal antibody directed against the 27-kDa protein of duck erythroblast prosome recognizes a 27-kDa protein of the sea urchin prosome. We have used this protein, as representative of the prosome, to immunologically determine the level and the subcellular localization of the particle during development. Immunoblotting and cellular fractionation studies show that the 27-kDa prosome polypeptide is present almost entirely in the postribosomal supernatant of unfertilized egg lysates. After fertilization and during early development, the total amount of 27-kDa protein per embryo remains constant, but the amount in the postribosomal supernatant decreases; there is a concomitant increase in the level of the 27-kDa protein in a rapidly sedimenting, particulate fraction containing nuclei. Immunofluorescence studies further show that the 27-kDa protein is located mainly in the cytoplasm of eggs and two-cell embryos. The subcellular location of the prosome, therefore, appears to change during development. In vivo labeling experiments have failed to detect the synthesis of either the prosome proteins or RNAs in eggs and embryos up to 48 hr of development, suggesting that this cytoplasmic particle is not synthesized de novo in early embryogenesis and thus is metabolically stable. The prosome is thus a normal cellular constituent of the sea urchin and is most likely synthesized during oogenesis and stored in the unfertilized egg.