Aspects of the role of the endoplasmic reticulum in protein synthesis

The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.

Interaction between mRNA, ribosomes and the cytoskeleton

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The characterization of free, cytoskeletal and membrane-bound polysomes in Krebs II ascites and 3T3 cells

Polysomes from Krebs II ascites and 3T3 cells were separated into three populations by using a sequential extraction method. Free polysomes were released by using a combination of low salt (25 mM KCl) and NP-40 detergent in the lysis buffer. The cytoskeletal bound polysomes were subsequently released by raising the salt concentration to 130 mM and finally, polysomes bound to the membranes of the endoplasmic reticulum were extracted by the combined treatment with Triton X-100 and deoxycholate. The results presented here illustrate that the three polysome-containing fractions differ in many parameters such as polysome profiles, cytoskeletal components and phospholipid content. When polyA-containing mRNA was isolated from the three polysome fractions and translated in an in vitro system, some differences were observed in the patterns of proteins being synthesized.

Effects of colchicine on the intraneuronal transport of secretory material prior to the axon: a morphofunctional study in hypothalamic neurosecretory neurons of the rat

The effects of colchicine on neurosecretory neurons of the rat hypothalamus were studied by immunocytochemistry, high-resolution radioautography, and conventional electron microscopy. In control rats, intraneuronal immunocytochemical labeling of vasopressin, oxytocin and somatostatin occurred essentially in the Golgi apparatus, the neurosecretory granules and to a lesser extent, the endoplasmic reticulum. These immunostaining patterns were dramatically modified 24 h after the administration of colchicine: immunoreactive peptides were located in granular or tubular structures accumulated at the periphery of the perikarya, but the Golgi stacks were not immunostained. Two h after the administration of tritiated leucine, quantitative analysis of radioautographic labeling of supraoptic perikarya revealed large amounts of radioactive protein in the Golgi saccules of neurosecretory neurons in control rats, but in the neurons of colchicine-treated rats, radioautographic labeling was mainly located in granular structures accumulated at the periphery of the perikarya, with no significant labeling on the Golgi stacks. Lastly, 3 noteworthy effects of colchicine on the ultrastructural morphological features of these neurosecretory neurons consisted in: (1) a dramatic disorganization of the Golgi complexes, (2) an accumulation of electron-dense proteic material within the lumen of cisternae of both the rough and smooth endoplasmic reticulum and, (3) a marked depolymerization of perikaryal microtubules, specifically those associated with the Golgi stacks. Taken together, these data do not fit the prevailing concept that the colchicine-induced accumulation of secretory material within the perikarya of neurosecretory neurons essentially results from the blockade of axoplasmic transport mechanisms. Instead, they support the idea that the effects of colchicine are related to the inhibition of the intraneuronal transport of newly synthesized secretory material from the endoplasmic reticulum to the Golgi apparatus, suggesting that the microtubules associated with the Golgi stacks are possible sites of colchicine action.

Evidence that insulin increases the proportion of polysomes that are bound to the cytoskeleton in 3T3 fibroblasts

The association of polysome redistribution with changes in protein synthesis was investigated in insulin-stimulated fibroblasts. Free polysomes were released by Nonidet-P40 and 25 mM KCl, cytoskeletal-bound polysomes were retained at 25 mM KCl but released at 130 mM, while membrane-bound polysomes were released by deoxycholate. Insulin increased the proportion of polysomes which were retained at 25 mM KCl but had no effect when extraction was carried out at 130 mM KCl, suggesting that more polysomes were associated with the cytoskeleton. Insulin also reduced the amount of actin released from the detergent-insoluble cytoskeleton indicating that the hormone affects microfilament organization.

The migration of labeled phosphatidylcholine from the nuclear-associated endoplasmic reticulum to plasma membranes in L-929 cells

The nuclear-associated endoplasmic reticulum of L-929 cells was found to contain the highest amount of labeled phosphatidylcholine after a 60 min incubation with 14C-choline. Radioactivity was otherwise distributed relatively evenly among other membrane-containing organelles (nuclei, mitochondria, plasma membranes and endoplasmic reticulum membranes). During a 120 min chase following removal of isotope and addition of cold choline chloride, there was a considerable reduction in labeled phosphatidylcholine in the NER and nuclei. The decrease in radioactivity in these fractions was matched by an almost identical increase in the fraction containing mitochondria and plasma membranes. Separation of mitochondria and plasma membranes by centrifugation on discontinuous gradients showed that 14C-choline labeled phosphatidylcholine appeared most rapidly in the plasma membranes. The results indicate that phospholipid molecules migrate within a short period of time from their site of synthesis in the NER to plasma membranes.

Time-dependent alteration in endoplasmic reticulum membrane profiles during in vitro incubation of Krebs II ascites cells

Krebs II ascites cells were harvested from the mouse peritoneum 6-8 days after inoculation and incubated in vitro in roller suspension culture for up to 22 hr. Within 2 hr of incubation a large portion of the cells entered S phase as judged by the incorporation of 3H-thymidine into DNA. The incorporation of radioactive precursors into phospholipid, protein and RNA increased rapidly during in vitro incubation indicating a high degree of macromolecular synthesis. The rates of incorporation were maximal within 4 hr of incubation. When cells labeled with 3H-choline were disrupted by nitrogen cavitation and endoplasmic reticulum (ER) membranes analyzed for subfractions on discontinuous sucrose gradients, it was observed that only small amounts of radioactivity could be detected in the HR region after 1/2 hr incubation while 63.2% of total radioactivity in ER membranes appeared in the LR fraction. Between 8-18 hr there was a considerable increase in the amount of HR membranes. Of the total radioactivity in ER membranes that in the HR fraction increased from 16.9% to 53.9% during this period. There were only small changes in the amounts of radioactivity in LR and S membranes between 8 and 18 hr. The results suggest a time-dependent appearance of ER membrane subfractions during a 22 hr period of in vitro incubation of Krebs II ascites cells.

The phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate and stimulation of 3H-choline incorporation into endoplasmic reticulum membranes and other subcellular fractions of Krebs II ascites cells during in vitro incubation

After transfer of Krebs II ascites cells from the mouse peritoneum to suspension culture addition of the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA) causes an early stimulation of 3H-choline incorporation into phosphatidylcholine (PC). Choline transport into the treated cells, however, was unaffected. Within 30 min of TPA treatment 3H-choline incorporation was almost 300% above the control level. During a 5 hr period of suspension culture the overall patterns of 3H-choline incorporation were similar in TPA-treated and control cultures though the rate was greatly accentuated by the presence of the phorbol ester. Incubation of cells with cycloheximide prior to incubation with TPA did not result in an inhibition of the TPA-directed 3H-choline incorporation. After 3 hr incubation with TPA there were large increases in radioactivity in all subcellular fractions. At 20 hr, however, the values were not far from those of the control. During the first 3 hr of incubation with TPA the incorporation of 3H-choline into light rough (LR) and smooth (S) membranes was stimulated to levels of 400% and 320% respectively above control values. At later times the profiles of radioactivity in membrane subfractions in TPA-treated and control cultures were similar. The results illustrate an early effect of TPA on PC biosynthesis in Krebs II ascites cells while at later times of incubation the stimulatory effect was virtually abolished.

Ultrastructure and polysome content of the microsomal subfractions of mouse plasmacytoma cells

The endoplasmic reticulum (ER) of MPC-11 cells released as vesicles upon cell disruption by nitrogen cavitation was separated from the bulk of mitochondria, lysosomes and plasma membranes by a low speed centrifugation. The ER membranes were fractionated on discontinuous sucrose gradients into heavy rough (HR), light rough (LR) and smooth (S) membranes. The morphology of subcellular fractions was studied by electron microscopy and the ER membranes were shown to be virtually free of contaminating organelles. The S fraction was easily distinguishable because of the lack or ribosomes but there were no apparent morphological differences between the HR and LR fractions. Of total activity in the microsomal subfractions, 70% of the UDPase and 67% of the 5'-nucleotidase activity was associated with the S fraction. Polysomes were present in the HR, LR and nuclear-associated ER fractions but not in the S fraction. The HR and LR fractions did not appear to be contaminated to any great extent with free polysomes. RNA/protein and RNA/phospholipid ratios of the HR fraction were higher than those of the LR fraction, indicating a greater density of ribosomes in the former fraction. These ratios were much lower in the S fraction reflecting the low ribosome content.

Variation in microsomal subfractions obtained from normal animal tissues and transformed cells following cell disruption by nitrogen cavitation

Microsomal membranes were obtained from MPC-11 cells, L-cells, Krebs II ascites cells and various normal animal tissues following cell disruption by nitrogen cavitation. Membrane preparations were applied to discontinuous sucrose gradients designed to separate three fractions--heavy rough (HR), light rough (LR) and smooth (S) microsomes. In each of the transformed cell lines all three fractions were found whilst in the normal tissues tested the HR fraction was absent. Of the normal tissues liver and pancreas were rich in both LR and S microsomes, the presence of large amounts of LR indicating a rich protein synthesizing activity on membrane-bound polysomes. Kidney also contained appreciable LR but much less than both liver and pancreas. Both heart and lung contained virtually only S microsomal material--a reflection of low protein synthetic activity on membrane-bound polysomes. Attempts to promote the appearance of the HR fraction in liver, kidney and pancreas by incubation in tissue culture medium, or, in the case of pancreas, by cholecystokinin/pancreozymin/secretin stimulation both in vivo and in vitro were unsuccessful.