The role of ribosomal ribonucleic acid in the structure and function of mammalian brain ribosomes

General features of certain RNA populations from gametes and cumulus cells

Results described herein provide insight regarding certain features of gamete RNA and how they compare to cumulus cell RNA. In particular, 28S/18S rRNA ratio and size distribution of RNA molecules differed in total RNA from oocytes versus surrounding cumulus cells. Specifically, oocyte total RNA had a lower rRNA ratio and an increased abundance of smaller RNA sizes compared to RNA from surrounding cumulus. Extensive efforts demonstrated that observed differences were repeatable whether oocyte maturation occurred in vitro or in vivo, and were similar between the nuclear stages examined. Features of oocyte RNA were conserved across six mammalian species, yet differed from surrounding cumulus. Profiles of sperm RNA were also examined but had no discernible ribosomal RNA peaks and were conserved across four mammalian species. Because the oocyte and spermatozoon are highly specialized cells representing unique molecular entities required for proper embryo development, dissimilarities described herein likely represent real gamete versus cumulus RNA differences.

Angiogenin abolishes cell-free protein synthesis by specific ribonucleolytic inactivation of ribosomes

Angiogenin is a potent inhibitor of cell-free protein synthesis. When incubated with rabbit reticulocyte lysate at a concentration of 40-60 nM, it completely abolishes the capacity of the lysate to support protein synthesis. The inhibition appears to be due to its ribonucleolytic activity since it (i) generates limited cleavage products from reticulocyte RNA and (ii) is prevented from both cleaving RNA and inhibiting protein synthesis by placental RNase inhibitor. The ribonucleolytic activity of angiogenin toward the reticulocyte RNA system is highly specific. Thus, under conditions where angiogenin totally abolishes protein synthesis, an equivalent concentration of pancreatic RNase A inhibits it only partially. In contrast, RNase A is a much more effective enzyme than angiogenin using isolated RNA as substrate. Angiogenin inhibits protein synthesis by cleaving rRNA, thereby inactivating the protein synthesis machinery. Addition of isolated reticulocyte ribosomes to an angiogenin-treated lysate restores the capacity for protein synthesis, whereas addition of tRNA or mRNA does not. This potent effect on protein synthesis suggests a possible physiological function of angiogenin whose overall relevance and implications should become evident as the mechanisms of neovascularization are deciphered. The use of angiogenin may also further elucidate ribosome structure and its role in protein synthesis.

Interaction between membrane functions and protein synthesis in reticulocytes. Isolation of RNase M, a membrane component inhibiting protein synthesis through specific endonucleolytic activity

An inhibitor of protein synthesis has been isolated from reticulocyte membranes by solubilization with Triton X-100; it has been purified using heat treatment, filtration on Amicon filters, DEAE-cellulose ion-exchange chromatography and Sephadex G-75 gel chromatography. A final purification of 120-fold was achieved. The purified inhibitor was found to be 95% homogenous when run on a dodecylsulfate/polyacrylamide gel system. Three independent methods were used to estimate the molecular weight of the purified inhibitor: Sephadex G-75 gel chromatography, dodecylsulfate/polyacrylamide gel electrophoresis and sucrose gradient all confirmed that the purified inhibitor was a small molecule with a sedimentation coefficient of 0.7 S and a molecular weight ranging between 5000 and 8000. The purified inhibitor was shown to possess a specific endonucleolytic activity, degrading the 28-S species of ribosomal RNA to species sedimenting between 10 and 14 S. Due to its membrane localisation the name RNase M is proposed. The purified inhibitor's endonucleolytic activity was characterized with regard to its kinetics, concentration dependence, pH optimum and its requirements for divalent cations. Kinetics showed that RNase M retained its specificity after 60 min of incubation with the RNA substrate. Specificity was also demonstrated by incubating the polysomal RNA with high concentrations of purified enzyme. The pH optimum was found to be between pH 6 and pH 7, and the enzyme did not require divalent cations for its activity. Pancreatic RNase B used at a similar protein synthesis inhibitory concentration as the RNase M caused a complete breakdown of ribosomal RNA to oligonucleotides and mononucleotides. The possible biological significance of the purified inhibitor in regulating protein synthesis in the maturing reticulocyte is discussed.

Mobility of ribosomes bound to microsomal membranes. A freeze-etch and thin-section electron microscope study of the structure and fluidity of the rough endoplasmic reticulum

The lateral mobility of ribosomes bound to rough endoplasmic reticulum (RER) membranes was demonstrated under experimental conditions. High-salt-washed rough microsomes were treated with pancreatic ribonuclease (RNase) to cleave the mRNA of bound polyribosomes and allow the movement of individual bound ribosomesmfreeze-etch and thin-section electron microscopy demonstrated that, when rough microsomes were treated with RNase at 4 degrees C and then maintained at this temperature until fixation, the bound ribosomes retained their homogeneous distribution on the microsomal surface. However, when RNase-treated rough microsomes were brought to 24 degrees C, a temperature above the thermotropic phase transition of the microsomal phospholipids, bound ribosomes were no longer distributed homogeneously but, instead, formed large, tightly packed aggregates on the microsomal surface. Bound polyribosomes could also be aggregated by treating rough microsomes with antibodies raised against large ribosomal subunit proteins. In these experiments, extensive cross-linking of ribosomes from adjacent microsomes also occurred, and large ribosome-free membrane areas were produced. Sedimentation analysis in sucrose density gradients demonstrated that the RNase treatment did not release bound ribosomes from the membranes; however, the aggregated ribosomes remain capable of peptide bond synthesis and were released by puromycin. It is proposed that the formation of ribosomal aggregates on the microsomal surface results from the lateral displacement of ribosomes along with their attached binding sites, nascent polypeptide chains, and other associated membrane proteins; The inhibition of ribosome mobility after maintaining rough microsomes at 4 degrees C after RNase, or antibody, treatment suggests that the ribosome binding sites are integral membrane proteins and that their mobility is controlled by the fluidity of the RER membrane. Examination of the hydrophobic interior of microsomal membranes by the freeze-fracture technique revealed the presence of homogeneously distributed 105-A intramembrane particles in control rough microsomes. However, aggregation of ribosomes by RNase, or their removal by treatment with puromycin, led to a redistribution of the particles into large aggregates on the cytoplasmic fracture face, leaving large particle-free regions.

Radioactive labelling of ribosomal proteins with reductive alkylation and its use in studying ribosome-cytosol interactions

Mouse brain ribosomes were radioactively labelled by a cell-free reductive alkylation reaction with NaBH4 and [14C]formaldehyde. The radioactivity was largely associated with ribosomal proteins, but little, if any, of the rRNA was radioactive after the alkylation procedure. Both ribosomal structural proteins and loosely associated components were successfully labelled by this procedure. The sedimentation properties of the ribosomes were unaltered and their ability to carry out poly(U)-directed protein synthesis, although decreased, was largely retained. Incubation of 14C-labelled ribosomes with brain cytosol resulted in a 17% loss of radioactivity, although treatment of the ribosomes with 1.0 m-KCl to remove the loosely associated factors rendered the ribonucleoprotein particles resistant to cytosol effects. The ribosome-cytosol interactions did not appear to be related to an exchange process, since the released radioactivity was largely degraded to acid-soluble material. In addition, the incubation of native ribosomes with brain cytosol resulted in an almost complete loss in the ability of the ribosomes to participate in cell-free protein synthesis.