Differences in the incorporation of [3H]-glucosamine into nascent polypeptide chains on free polysomes and two fractions of membrane-bound polysomes in mouse myeloma cells

The incorporation of [³H]-glucosamine into polypeptides of three fractions of polysomes in MPC-11 cells was studied. After short term incubation greatest incorporation was observed in a fraction of membrane-bound polysomes, which after nitrogen cavitation of cells, remained bound to the endoplasmic reticulum (ER) associated with the nucleus (fraction 2). Polypeptide chains on membrane-bound polysomes in the microsomal fraction (fraction 1) and free polysomes contained much less radioactivity. Since nascent polypeptide chains contained within membrane-bound polysomes of fraction 2 are glycosylated at an earlier stage than those in fraction 1 it is likely that this represents a difference in type of proteins synthesized in the respective fractions of ER.     

Labelling kinetics of RNA containing poly(a) in liver subcellular fractions

Kinetics of incorporation of (3H) uridine into cytoplasmic RNA fractions of rat liver is investigated. The fractions include free and membrane bound polysomes, rough membranes sedimenting with mitochondria and free cytoplasmic RNA particles. (1) Poly(A) containing RNA, isolated by oligo-dT cellulose, amounts to 0.4% of the total RNA in the homogenate, 0.5% in bound polysomes, 3.4% in free polysomes and 16% in free cytoplasmic RNA particles. (2) The rate of (3H) uridine incorporation into RNA lacking poly(A) proceeds uniformly in all subcellular fractions except for free cytoplasmic RNA particles, which accumulate negligible amounts of radioactivity. (3) The initial labelling of RNA containing poly(A) is most active in free cytoplasmic RNA particles supporting their identity as mRNA en route to polysomes. The initial specific radioactivities decrease in the following order: homogenate, bound polysomes, rough membranes sedimenting with mitochondria, free polysomes. The data suggest that mRNA is supplied to free and membrane-bound polysomes via different routes. The kinetic analysis indicates that free cytoplasmic RNA particles may be a precursor of mRNA of free polysomes rather than that of bound polysomes. (4) The kinetic differences of free and membrane bound polysomes are also demonstrated by comparing the radioactivity of RNA containing poly(A) to the total radioactivity at various incorporation times. In bound polysomes this decreases from 31% at 1 h to 10% at 25 h, whereas in free polysomes the corresponding ratio increases from 10 to 13%. RNA containing poly(A) of free cytoplasmic RNA particles represents 64% of the total radioactivity throughout the experiment.     

Morphological changes in Krebs II ascites tumour cells induced by insulin are associated with differences in protein composition and altered amounts of free,cytoskeletal-bound and membrane-bound polysomes

A three-step sequential detergent/salt extraction procedure was used in order to isolate three distinct subcellular fractions containing free (FP), cytoskeletal-bound (CBP) and membrane-bound polysomes (MBP), respectively, from Krebs II ascites cells (Vedeler et al., Mol Cell Biochem 100: 183–193,1991). The purpose was to study changes in the distribution of polysomes in these three fractions during long-term incubation with insulin under either stationary conditions or in roller suspension culture- Insulin caused a redistribution of polysomes between FP, CBP and MBP fractions. The hormone appeared to promote an entry of ribosomes into polysomes both in CBP and MBP populations. When cells were grown in stationary culture in the presence of insulin and thus promoted to attach to the substratum and undergo morphological changes, a diversion of ribosomes from CBP into MBP was observed. The level of protein synthesis was apparently very high in this latter fraction since more then 70% of ribosomes were in polysomes. Morphological changes observed following insulin treatment were accompanied by a shift of certain proteins among subcellular fractions (for example actin and p35). The fibronectin content was about 20% higher in attached compared to non-attached cells. The results suggest that morphological changes induced by stimulation with insulin are associated with an increased activity of MBP, presumably reflecting a requirement for an increased synthesis of membrane proteins. (Mol Cell Biochem 118: 131–140, 1992)     

A simple and rapid procedure for high-yield isolation of essentially undegraded free and membrane-bound polysomes from rat liver

A procedure is described for the preparation of free and membrane-bound polysomes from rat liver. The procedure involves: differential centrifugation of liver homogenate to separate free and membrane-bound polysomes; treatment of the membrane-bound polysome fraction with a detergent to release bound polysomes from membranes; and magnesium precipitation of both classes of polysomes. Free and bound polysomes prepared in this manner were essentially undegraded and highly active in cell-free protein synthesis. The recovery of polysomes was nearly quantitative and the distribution between the free and membrane-bound state was 41 and 59%, respectively. Polypeptides synthesized in vitro by the free and membrane-bound polysomes were quite different. The majority (81-84%) of mRNA activities of two secretory proteins (albumin and transferrin) were recovered in the membrane-bound polysomes, whereas the majority (81-85%) of mRNA activities of two cytosolic [aldolase B, EC 4.1.2.13, and argininosuccinate synthetase, EC 6.3.4.5], one mitochondrial [ornithine carbamoyltransferase, EC 2.1.3.3] and one peroxisomal [catalase, EC 1.11.1.6] proteins were recovered in the free polysomes. A polysome class synthesizing ornithine carbamoyltransferase was purified 42-fold from the free polysomes by immunoprecipitation. The procedure is rapid (4-5 h) and reproducible, and provides a nearly quantitative means of separating the two classes of polysomes.     

Metabolism of Poly(A) in Plant Cells: Discrete Classes Associated with Free and Membrane-bound Polysomes

In the subapical region of dark-grown pea epicotyls about 40% of the total polysomes are associated with membranes. The presence of poly(A) in polysomal mRNA was detected by hybridization of unlabeled RNA with (3)H-poly(U). Both free mRNA and messenger ribonucleoprotein particles in polysomes hybridize with (3)H-poly(U) quantitatively. The binding of (3)H-poly(U) to polysomes is increased by treatment with the detergent sodium dodecyl sulfate. Since detergent influenced the (3)H-poly(U) binding more in membrane-bound polysomes than in free, there may be more protein(s) associated with the poly(A) portion of the mRNA in membrane-bound polysomes. Analysis of the poly(A) segments isolated from the mRNA of these two classes of polysomes indicates that there are discrete classes of poly(A) and they appear to be differentially associated with free and membrane-bound polysomes. Mean size distribution of poly(A) in free polysomes is larger than in membrane-bound polysomes.Following treatment (2 days) with the plant growth hormone indoleacetic acid, there is a gradual decrease in the mean length of total poly(A), which appears to correspond to a decrease in the size of the polysomes and their associated mRNA.     

Intracellular site of prolactin synthesis in rat pituitary cells in culture

Free and membrane-bound polyribosomes were isolated from control and thyrotropin releasing hormone-treated GH₃ cells. The two polysome fractions were used to direct {³H}leucine incorporation into prolactin in both heterologous and homologous cell-free protein-synthesizing systems. Prolactin was measured by immunoprecipitation and SDS-disc gel electrophoresis of the reaction products. Only membrane-bound polysomes directed incorporation of {³H}leucine into labeled prolactin. In additon, intact cells were pulselabeled with {³H}leucine, free and membrane-bound polysomes were isolated, and newly synthesized prolactin associated with each polysome fraction was measured. In control cells, {³H}prolactin represented about 0.4 and 4.2% of total acid-insoluble radioactivity in free and membrane-bound polysomes, respectively; whereas, in thyrotropin releasing hormone-treated cells, these values were about 1 and 20%, respectively. Added {³H}prolactin did not associate nonspecifically with membrane-bound polysomes. We conclude that prolactin is synthesized predominantly on membrane-bound polysomes in GH₃ cells.     

Comparison of the Effect of Intravenous Administration of d-Lysergic Acid Diethylamide on Free and Membrane-Bound Polysomes in the Rabbit Brain

Abstract: The intravenous administration of LSD to young adult rabbits resulted in the disaggregation of both free and membrane-bound classes of brain polysomes. Based on the analysis of LSD dosage and the time course of the LSD-induced brain polysome shift, it was found that free polysomes were more sensitive to the drug than the membrane-bound polysome fraction. LSD-induced hyperthermia may be involved in the disaggregation of free and membrane-bound polysomes, since a correlation was found between the extent of LSD-induced hyperthermia and the degree of brain polysome shift. Prevention of LSD-induced hyperthermia by maintaining the animal at 4°C blocked the disaggregation of both polysome classes. Induction of hyperthermia by elevation of ambient temperature also resulted in a shift in free and membrane-bound polysomes. In all cases the disaggregation of polysomes to monosomes was not caused by RNase activation. During polysome disaggregation, polyadenylated mRNA associated with both free and membrane-bound polysomes was not degraded but was relocalized from polysomes to monosomes.     

Synthesis of exported proteins by membrane-bound polysomes from Escherichia coli.

A membrane-bound fraction of polysomes of Escherichia coli has been isolated after lysis of cells without the use of lysozyme. Protein-synthesis studies in vitro show that membrane-bound and free polysomes are different in the following respects. 1. Membrane-bound polysomes synthesize proteins which are exported from the cell. The products include proteins of the outer membrane and a secreted periplasmic protein, the maltose-binding protein. 2. The major product synthesized by free polysomes is elongation factor Tu, a soluble cytoplasmic protein. 3. The activity of membrane-bound polysomes in vitro is more resistant to puromycin than is the activity of free polysomes. In addition, the mRNA associated with membrane-bound polysomes is more stable than the bulk of cellular mRNA as revealed by studies with rifampicin.     

Polysomes in various phases of the cell cycle in synchronized plasmacytoma cells

The fraction of membrane-bound and free polysomes during different phases of the cell cycle was determined in suspension cultures of mouse plasmacytoma cells, synchronized by growth in isoleucine-deficient medium. The membrane-bound polysomes reached a maximum value (about 28 % of total polysomes) during the G 1 phase. In the S phase and G 2 phase only 18 to 20 % of the total polysomes were found to be membrane-bound. A high percentage of membrane-bound polysomes in the G 1 phase of the cell cycle agrees with the earlier finding that maximum synthesis of immunoglobulin light chain takes place on polysomes bound to the membrane in the G 1 phase of the cell cycle. The presence of a significant fraction of membrane-bound polysomes in the S and G 2 phases of the cell cycle would suggest that membrane-bound polysomes are also involved in the synthesis of proteins other than immunoglobulins.The ultrastructure of the cells during the various phases of the cell cycle was also studied. During the G 1 phase the surface of the majority of cells was distinguished by the presence of ruffles and slender villus-like cytoplasmic projections. In the S phase the surface contour tended to become smooth and even. These differences in the surface morphology may reflect the change in function which occurs during the transition from the G 1 to the S phase.     

The effect of insulin on proteins associated with free,cytoskeletal-bound and membrane-bound polysome populations.

It is known that insulin treatment increases the rate of protein synthesis in many cells and tissues and that it causes changes in the distribution of ribosomes between free (FP), cytoskeletal-bound (CBP) and membrane-bound polysome (MBP) populations. This paper concerns an analysis of the pattern of proteins in high-salt extracts of FP, CBP and MBP isolated from Krebs II ascites and MPC-11 cells. A combined detergent/salt extraction procedure was used to isolate the three fractions of polysomes from control cells and from cells following short-term stimulation with insulin. There were differences in the protein patterns in the individual fractions and changes occurred after insulin stimulation.     

Membrane-bound and free polysomes in transformed and untransformed fibroblast cells

A rapid and quantitative fractionation procedure has been used to measure the amounts of free and membrane-bound polysomes in growing and stationary Py3T3 and 3T3 (mouse) cells. A comparison of growing 3T3 and Py3T3 cells does not reveal any significant differences with regard to the ratio of the two polysome fractions. The amount of free and membrane-bound polysomes decreases in both 3T3 and Py3T3 cells as they approach the stationary state, an effect which is much more pronounced for free polysomes. At greatly reduced growth rates or in stationary cells, however, the amount of membrane-bound polysomes doubles in Py3T3 cells while it decreases even further in 3T3 cells. By contrast, the amount of free polysomes remains at a reduced level in both 3T3 and Py3T3 cells when cell multiplication is inhibited.Based on the hypothesis that membrane proteins are selectively synthesized on membrane-bound polysomes, an attempt is made to relate the results to accumulated data in the literature and discuss its possible significance with respect to the loss of growth control in Py3T3 cells.     

The formation and translational activity of polysomes from developing lupin seeds

The formation and in vitro translational activity of total, free and membrane-bound polysomes from various stages of developing cotyledons of yellow lupin seeds (Lupinus luteus L. cv. Iryd) has been investigated. The early stages of seed formation were characterized by a low level of polysomes that progressively increased. The main features of the cotyledons at the middle phase of development were full expansion growth and the highest amount of polysomes observed in all three poly so me fractions. In The final stages of emhryogenesis. the seed dehydration was accompanied by-gradual loss of all types of polysomes, at which the membrane-attached formations were degraded earlier than the free ones. By means of a wheat germ-derived cell-free system for protein synthesis, a correlation was demonstrated between cotyledon growth, polysome formation and their capacity for protein synthesis in vitro. As compared to the free polysomes, both the total and membrane-bound formations were more active in protein synthesis in vitro. Analysis of the translational products by means of immunoprecipitation and gel electrophoresis followed by fluorography showed that only membrane-bound polysomes produced polypeptides of higher molecular weight, including subunits of a legumin-like protein.     

In vitro amino acid incorporation into myosin by free polysomes of rat skeletal muscle

Rat skeletal muscle polysomes were separated into free and membrane-bound fractions by centrifugation through 2M sucrose. About 80% of total ribosomes extracted were recovered as free polysomes. Sucrose gradient experiments showed similar size distribution patterns for both free and bound polysomes. Chromatographic and electrophoretic analyses of proteins in the cell free amino acid incorporation system indicated that free polysomes are capable of synthesizing myosin.     

Polyribosomes from Peas: V. An Attempt to Characterize the Total Free and Membrane-bound Polysomal Population

Attempts were made to isolate and characterize the total population of free and membrane-bound polysomes from the elongating region of dark-grown pea stems (Pisum sativum L.). Partial separation of free from membrane-bound polysomes was achieved by relatively low speed centrifugation of the homogenate. Complete separation was not achieved. Based on analysis of the rRNA content of various subcellular fractions, fractionated tissue yielded greater than 95% of the rRNA found in whole tissue. Approximately 45% of the ribosomal material was membrane-bound (released by detergent) and was found in the “wall” (13%), the “nuclear” pellet (2%), and the “mitochondrial” pellet (29%). The remaining 55%, consisting primarily of free polysomes, could be recovered free from membranous material by sedimentation through a dense (700 mg/ml) sucrose pad for 90 hours. The advantages and disadvantages of using sucrose pads for the separation of free and membrane-bound polysomes are discussed.     

Structure and function of rat liver polysome populations. II. Characterization of polyadenylate-containing mRNA associated with subpopulations of membrane-bound particles

Poly(A)+RNA fractions prepared from free and loosely and tightly membrane-bound polysome populations (poly(A)+RNAfree, poly(A)+RNAloose, and poly(A)+RNAtight) were used to drive cDNA in homologous and heterologous hybridization reactions. A large fraction by mass of sequences was shared among the three poly(A)+RNA populations, but shared sequences exhibited distinct frequency distributions within the different populations. 13-15 in vitro translation products of poly(A)+RNAfree and poly(A)+RNAloose detected by gel electrophoresis were shared. Most of these were produced in different relative quantities by the two RNA populations. Five or six higher mol wt polypeptides were produced by poly(A)+RNAloose that were not detected as products of either poly(A)+free or poly(A)+RNAtight. We suggest that loosely bound polysomes may not be artifactually derived as reflected in their quantitatively distinct poly(A)+RNA population. Two tightly membrane-bound RNP fractions were prepared from rat liver on the basis of their release from or retention on purified rough microsomes or a crude membrane fraction after in vitro disaggregation of polysomes with high-salt and puromycin. Homologous and heterologous hybridizations involving their poly(A)+RNA fractions revealed that a large portion by mass of sequences was shared but that these sequences exhibited distinct frequency distributions in the two fractions. The RNA fractions produced exhibited distinct frequency distributions in the two fractions. The RNA fractions produced an identical set of in vitro translation products but individual polypeptides were produced in different relative quantities. This indicates that the two RNP fractions do not arise by any random artifactual process and suggests that they may represent functionally distinct populations.     

A method for visualizing and quantifying the total complement of free and membrane-bound ribosomes in rat liver.

A method is described for both visualization and quantification of the total complement of rat liver free and membrane-bound ribosomes, undegraded by nucleolysis and unaggregated by pelleting. The method involves: (a) differential centrifugation of liver homogenate which separates free and membrane-bound ribosomes; (b) treatment of the fractions with detergents to solubilize membranes and remove nuclei; (c) centrifugation of a portion of each fraction to remove all the ribosomes; (d) sedimentation of the samples and blanks on sucrose gradients; and (e) difference photometric scanning of the gradients, sample minus ribosome-free blank, to detect the ribosomes free of interference from nonribosomal materials. The use of the SW 56 rotor in the initial centrifugation and of a high Mg²⁺ concentration (20 mm) in the medium used to suspend the bound fraction prior to detergent treatment were found to be essential in obtaining bound polysomes of large size (～19-somes). The difference scanning technique is shown to be a sensitive, accurate, and reproducible means of eliminating interference from nonribosomal materials, principally detergents and protein, and of quantifying ribosomes in both fractions. The method is rapid (3.5 h), simple to perform, and well suited for the analysis of multiple liver samples. It can be used to assess the concentration, distribution, organization, and average size of the total complement of rat liver free and membrane-bound ribosomes in a single experiment.     

In vitro exchange of ribosomal subunits between free and membrane-bound ribosomes

Studies on the distribution of isotopieally labeled ribosomal subunits between free and membrane-bound ribosomes from rat liver showed that, upon release of nascent polypeptides in vitro, the small subunits of membrane-bound ribosomes could exchange with small subunits derived from free polysomes. However, under the same conditions, the large subunits of membrane-bound ribosomes did not exchange efficiently with large subunits derived either from free or bound polysomes; instead, the addition of large subunits caused a transfer of microsomal small subunits into a newly formed pool of free monomers.The small subunit exchange required a macromolecular fraction of the cell sap, was stimulated by ATP or GTP, and occurred at low concentrations of magnesium ions.Sodium dodecyl sulfate, polyacrylamide gel electrophoresis revealed close similarities between the protein complement of subunits from free and membrane-bound ribosomes, with the exception of one protein band which was more intense in free large subunits.     

Translation and identification of the viral mRNA species isolated from subcellular fractions of vesicular stomatitis virus-infected cells.

The cytoplasm of vesicular stomatitis virus (VSV)-infected BHK cells has been separated into a fraction containing the membrane-bound polysomes and the remaining supernatant fraction. Total poly(A)-containing RNA was isolated from each fraction and purified. A 17S class of VSV mRNA was found associated almost exclusively with the membrane-bound polysomes, whereas 14,5S and 12S RNAs were found mostly in the postmembrane cytoplasmic supernatant. Poly(A)-containing VSV RNA synthesized in vitro by purified virus was resolved into the same size classes. The individual RNA fractions isolated from VSV-infected cells or synthesized in vitro were translated in cell-free extracts of wheat germ, and their polypeptide products were compared by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. The corresponding in vivo and in vitro RNA fractions qualitatively direct the synthesis of the same viral polypeptides and therefore appear to contain the same mRNA species. By tryptic peptide analysis of their translation products, the in vivo VSV mRNA species have been identified. The 17S RNA, which is compartmentalized on membrane-bound polysomes, codes for a protein of molecular weight 63,000 (P-63) which is most probably a nonglycosylated form of the viral glycoprotein, G. Of the viral RNA species present in the remaining cytoplasmic supernatant, the 14.5S RNA codes almost exclusively for the N protein, whereas the 12S RNA codes predominantly for both the NS and M proteins of the virion.     

Interleukin 1: the patterns of translation and intracellular distribution support alternative secretory mechanisms.

Interleukin-1 (IL-1) is synthesized as a 31 kDa precursor protein, whose multiple extracellular activities are attributed to receptor binding of a processed, carboxy-terminal 17 kDa peptide. Unlike other secreted proteins, the IL-1 precursor lacks a hydrophobic leader sequence and is not found in organelles composing the classical secretory pathway. In order to further clarify the intracellular processing of IL-1, we studied its site of synthesis in human monocytes. Secreted and integral membrane proteins are translated on membrane-bound polyribosomes, while intracellular proteins are translated on free polyribosomes. Free and membrane-bound polysomes were isolated from Lipid A-stimulated monocyte lysates and immunoblotted using antibodies specific to the N-terminal regions of the IL-1 alpha and beta precursors. Free polysome fractions showed multiple small bands consistent with nascent peptide chains; membrane-bound polysomes yielded no detectable IL-1. Polysome fractions were then analyzed by immunoelectron microscopy; nascent IL-1 alpha and beta peptide chains were readily seen emerging from cytoskeletal-associated free polyribosomes, but not membrane-bound polyribosomes. Electron microscopic in situ hybridization revealed IL-1 mRNA chains attached to cytoskeletal-associated free, but not membrane-bound polyribosomes. The intracellular distribution of the fully synthesized IL-1 beta precursor was studied in human mesangial cells (HMC), whose cytoskeletal organization is more readily evaluated than that of monocytes. Dual immunofluorescence microscopy of these cells revealed a complex intracellular distribution of the fully synthesized 31 kDa IL-1 precursors. IL-1 was asymmetrically distributed between cytosolic, microtubule, and nuclear compartments, without association with actin or intermediate filaments. This demonstration of the sites of IL-1 synthesis and patterns of intracellular distribution provide further evidence for an extracellular release mechanism which is clearly distinct from the classical secretory pathway.     

Tubulin Synthesis in Rat Forebrain: Studies with Free and Membrane-Bound Polysomes

Abstract: Free and membrane-bound polysomes were prepared from rat forebrain and added to a cell-free system containing rabbit reticulocyte factors and L-[³⁵S]methionine. The translation products were analyzed by two-dimensional gel electrophoresis followed by autoradiography. The free polysomes synthesized actin and at least four major tubulin subunits (α¹, α², β¹, and α²) that are found in rat forebrain cytoplasm. The membrane-bound polysomes synthesized predominantly one protein (MB) in the tubulin region of the two-dimensional gel. MB has a molecular weight and isoelectric point similar to α-tubulin. Only trace amounts of α- and β-tubulin and actin were synthesized by the membrane-bound polysomes. MB co-purified with cytoplasmic tubulin after two cycles of aggregation and disaggregation. MB synthesized in vitro (from membrane-bound polysomes) and α- and β-tubulin and actin subunits (synthesized from free polysomes) were digested with Staphylococcus aureus V8 protease, and the resulting peptides were separated by slab gel electrophoresis followed by autoradiography. The peptide pattern of MB was similar but not identical to the peptide patterns of α- and β-tubulin; MB yielded peptides not found in tubulin. We conclude that membrane-bound polysomes from rat forebrain do not synthesize significant amounts of the predominant tubulin subunits synthesized by free polysomes. A major protein (MB) is synthesized by membrane-bound polysomes and is similar, but not identical, to α-tubulin synthesized by free polysomes on the basis of molecular weight, isoelectric point, and peptide analysis.