The prosome: an ubiquitous morphologically distinct RNP particle associated with repressed mRNPs and containing specific ScRNA and a characteristic set of proteins.

Abstract

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.

Cloning and sequencing of the gene encoding the large (α‐) subunit of the proteasome from Thermoplasma acidophilum

“…The SCLI-I mutauon of the sell + gene is a dominant suppressor of the cycloheximide-resistant, temperature-sensitive lethal mutaton, cri3 [25]. Th~s mutat]on-ehc~ted suppressor function of one of the S. cerevtstae proteasomal proteins sheds new light on the possible involvement of the parucle m the regulation of gene expression [27], repression or modulation of mRNA translation [28,29].…”

The protein of Mr 21 000 constituting the prosome‐like particle of duck erythroblasts is homologous to apoferritin

“…Various other Factors intervening in pre-translational regulation have been described, including small RNA species (Heywood et al, 1975;Dionne et al. 1982), proteins (Maundrell et al, 1979;Vincent et al, 1981;Richter and Smith, 1984), ribonucleoprotein particles (Schmid et al, 1984;Wolin, 1985).…”

“…Treatment of repressed mRNP with EDTA or 0.5 M KCI allows some of these components to be split off the mRNA, leaving behind a mRNP core which contains variable sets of proteins and is still unstranslatable in vitro (Civelli et al, 1980). Among the 'split factors' are the prosomes, a specific type of RNP of M , 720000, composed of variable sets of small proteins and RNA species that was discovered and characterized over the last years (Schmid et al, 1984;review in Scherrer, 1990). These particles, which have multi-catalytic proteinase activity (Rivett, 1989;Orlowski, 1990), might be involved in mRNA transport and differential pre-translational controls, being split off the mRNA in vivo prior to translation (Grainger and Winkler, 1987) and inhibit initiation of translation in vitro (Kleinschmidt and Buhl, 1987;Horsch et al, 1989).…”

Molecular cloning of an essential yeast gene encoding a proteasomal subunit

“…They are composed of multiple subunits (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) which are different in size and charge and which are organised into cylindrical 20 S particles [3][4][5][6]. The proteasome catalyses the cleavage of peptide bonds on the carboxyl side of basic, neutral and acidic residues [7][8][9] but its exact function in the cell is still unknown.…”

“…It has been implicated in playing a role in the degradation of proteins via the ATP-dependent ubiquitin-mediated proteolysis [10,11] and it was reported recently that prel-I yeast cells (mutations in the PREI gene encoding a proteasomal subunit affecting the proteolytic activity of the proteasome) exhibit decreased protein degradation and accumulate ubiquitin-protein conjugates [28]. The 20 S particles have been proposed to be also involved in tRNA processing and mRNA translation [12,13]. The expression of proteasomal subunits has been shown to be elevated in malignant cells [14] and tissue-specific expression has been reported in D. melanogaster [15].…”