Control of haemoglobin synthesis: rate of translation of the messenger RNA for the alpha and beta chains

Translational control of protein synthesis: the early years

For the past fifty-five years, much of my research has focused on the function and biogenesis of red blood cells, including the cloning and study of many membrane proteins such as glucose and anion transporters and the erythropoietin receptor. We have also elucidated the mechanisms of membrane insertion, folding, and maturation of many plasma membrane and secreted proteins. Despite all of this work and more, I remain extremely proud of our very early work on the regulation of mRNA translation: work on bacteriophage f2 RNA in the 1960s and on translation of α- and β-globin mRNAs in the early 1970s. Using techniques hopelessly antiquated by today's standards, we correctly elucidated many important aspects of translational control, and I thought readers would be interested in learning how we did these experiments.

Kinetic signatures of microRNA modes of action

MicroRNAs (miRNAs) are key regulators of all important biological processes, including development, differentiation, and cancer. Although remarkable progress has been made in deciphering the mechanisms used by miRNAs to regulate translation, many contradictory findings have been published that stimulate active debate in this field. Here we contribute to this discussion in three ways. First, based on a comprehensive analysis of the existing literature, we hypothesize a model in which all proposed mechanisms of microRNA action coexist, and where the apparent mechanism that is detected in a given experiment is determined by the relative values of the intrinsic characteristics of the target mRNAs and associated biological processes. Among several coexisting miRNA mechanisms, the one that will effectively be measurable is that which acts on or changes the sensitive parameters of the translation process. Second, we have created a mathematical model that combines nine known mechanisms of miRNA action and estimated the model parameters from the literature. Third, based on the mathematical modeling, we have developed a computational tool for discriminating among different possible individual mechanisms of miRNA action based on translation kinetics data that can be experimentally measured (kinetic signatures). To confirm the discriminatory power of these kinetic signatures and to test our hypothesis, we have performed several computational experiments with the model in which we simulated the coexistence of several miRNA action mechanisms in the context of variable parameter values of the translation.

The role of alpha-hemoglobin stabilizing protein in redox chemistry, denaturation, and hemoglobin assembly

Hemoglobin biosynthesis in erythrocyte precursors involves several steps. The correct ratios and concentrations of normal alpha (alpha) and beta (beta) globin proteins must be expressed; apoproteins must be folded correctly; heme must be synthesized and incorporated into these globins rapidly; and the individual alpha and beta subunits must be rapidly and correctly assembled into heterotetramers. These events occur on a large scale in vivo, and dysregulation causes serious clinical disorders such as thalassemia syndromes. Recent work has implicated a conserved erythroid protein known as Alpha-Hemoglobin Stabilizing Protein (AHSP) as a participant in these events. Current evidence suggests that AHSP enhances alpha subunit stability and diminishes its participation in harmful redox chemistry. There is also evidence that AHSP facilitates one or more early-stage post-translational hemoglobin biosynthetic events. In this review, recent experimental results are discussed in light of several current models describing globin subunit folding, heme uptake, assembly, and denaturation during hemoglobin synthesis. Particular attention is devoted to molecular interactions with AHSP that relate to alpha chain oxidation and the ability of alpha chains to associate with partner beta chains.

Modulation of poliovirus replicative fitness in HeLa cells by deoptimization of synonymous codon usage in the capsid region

We replaced degenerate codons for nine amino acids within the capsid region of the Sabin type 2 oral poliovirus vaccine strain with corresponding nonpreferred synonymous codons. Codon replacements were introduced into four contiguous intervals spanning 97% of the capsid region. In the capsid region of the most highly modified virus construct, the effective number of codons used (N(C)) fell from 56.2 to 29.8, the number of CG dinucleotides rose from 97 to 302, and the G+C content increased from 48.4% to 56.4%. Replicative fitness in HeLa cells, measured by plaque areas and virus yields in single-step growth experiments, decreased in proportion to the number of replacement codons. Plaque areas decreased over an approximately 10-fold range, and virus yields decreased over an approximately 65-fold range. Perhaps unexpectedly, the synthesis and processing of viral proteins appeared to be largely unaltered by the restriction in codon usage. In contrast, total yields of viral RNA in infected cells were reduced approximately 3-fold and specific infectivities of purified virions (measured by particle/PFU ratios) decreased approximately 18-fold in the most highly modified virus. The replicative fitness of both codon replacement viruses and unmodified viruses increased with the passage number in HeLa cells. After 25 serial passages (approximately 50 replication cycles), most codon replacements were retained, and the relative fitness of the modified viruses remained well below that of the unmodified virus. The increased replicative fitness of high-passage modified virus was associated with the elimination of several CG dinucleotides. Potential applications for the systematic modulation of poliovirus replicative fitness by deoptimization of codon usage are discussed.

Synaptic regulation of protein synthesis and the fragile X protein

Protein synthesis occurs in neuronal dendrites, often near synapses. Polyribosomal aggregates often appear in dendritic spines, particularly during development. Polyribosomal aggregates in spines increase during experience-dependent synaptogenesis, e.g., in rats in a complex environment. Some protein synthesis appears to be regulated directly by synaptic activity. We use "synaptoneurosomes," a preparation highly enriched in pinched-off, resealed presynaptic processes attached to resealed postsynaptic processes that retain normal functions of neurotransmitter release, receptor activation, and various postsynaptic responses including signaling pathways and protein synthesis. We have found that, when synaptoneurosomes are stimulated with glutamate or group I metabotropic glutamate receptor agonists such as dihydroxyphenylglycine, mRNA is rapidly taken up into polyribosomal aggregates, and labeled methionine is incorporated into protein. One of the proteins synthesized is FMRP, the protein that is reduced or absent in fragile X mental retardation syndrome. FMRP has three RNA-binding domains and reportedly binds to a significant number of mRNAs. We have found that dihydroxyphenylglycine-activated protein synthesis in synaptoneurosomes is dramatically reduced in a knockout mouse model of fragile X syndrome, which cannot produce full-length FMRP, suggesting that FMRP is involved in or required for this process. Studies of autopsy samples from patients with fragile X syndrome have indicated that dendritic spines may fail to assume a normal mature size and shape and that there are more spines per unit dendrite length in the patient samples. Similar findings on spine size and shape have come from studies of the knockout mouse. Study of the development of the somatosensory cortical region containing the barrel-like cell arrangements that process whisker information suggests that normal dendritic regression is impaired in the knockout mouse. This finding suggests that FMRP may be required for the normal processes of maturation and elimination to occur in cerebral cortical development.

High accumulation of components of the RNA polymerase II transcription machinery in rodent spermatids

Levels of mRNA and protein encoded by the TATA-binding protein (tbp) gene are shown to increase dramatically during late spermatogenesis in rodents, culminating in a highly testis-enriched expression pattern. Whereas adult spleen and liver contained roughly 0.7 and 2.3 molecules of TBP mRNA per haploid genome-equivalent, respectively, adult testis contained 80–200 molecules of TBP mRNA per haploid genome-equivalent. Comparison of nuclear and cytoplasmic levels of TBP mRNA in liver and testis suggested that nuclear events (transcription or processing) contribute roughly 12-fold, and cytoplasmic events (mRNA stability) roughly 6-fold, to testis-specific overaccumulation. Levels of nuclear TBP protein in testis cells were, on average, 8- and 11-fold higher than those in liver and spleen cells, respectively. Overexpression of TBP mRNA in testis began about 20 days after birth and reached a plateau around day 40, corresponding to the developmental emergence of haploid cells. Besides TBP, two other components of the general RNA polymerase II machinery, TFIIB and RNA polymerase II, were also overexpressed in testis. By immunostaining, it was found that TBP and RNA polymerase II were particularly rich in round spermatid nuclei. Our results suggest a molecular explanation for how early spermatids are able to accumulate all of the mRNA necessary for the final week of spermiogenesis.

Translational control during heat shock

Regulation of translation during heat shock of Drosophila and mammalian cells is reviewed. Protein synthesis is severely inhibited by elevated temperatures but synthesis of heat shock proteins (HSPs) is resistant to this inhibition. The primary site of regulation is polypeptide chain initiation. The activities of two initiation factors, eIF-2 and eIF-4F, are modulated during heat shock. A protein kinase which modulates eIF-2 activity appears to be associated with heat shock proteins (HSPs). Evidence is emerging that HSP70 acts as a heat sensor by detecting the presence of accumulating denatured proteins. In the rabbit reticulocyte lysate denatured proteins bind HSP70 releasing an eIF-2 kinase to shut down protein synthesis. It appears highly likely that a similar mechanism is acting in heat shocked cells. Cell-free protein synthesizing systems prepared from heat shocked cells are deficient in eIF-4F. Modulation of eIF-4F can explain in part the apparent preferential translation of HSP mRNAs.

Effect of age and restricted feeding on polypeptide chain assembly kinetics in liver protein synthesis in vivo

Polypeptide assembly rates during in vivo hepatic protein synthesis were studied as a function of age and restricted feeding in male rats. With ageing the time to assemble the average peptide in the liver of fully-fed rats significantly increased. In young rats maintained on a restricted feeding regime known to retard ageing, the time to assemble the average polypeptide was increased 2.5 times. With ageing the rate of peptide elongation increased so that at 2 years of age the underfed animals assembled peptides at a significantly faster rate than their age-matched controls. The rate of elongation of peptides during hepatic protein synthesis was shown to be directly dependent upon circulating T3 levels rather than the dietary status of the animal. On refeeding young diet restricted rats, polypeptide assembly kinetics did not immediately return to control values although the rate of protein synthesis was significantly increased. Total liver RNA content increased significantly in refed animals allowing for a greater rate of chain initiation to offset the slow rate of chain elongation. A period of 28 days of ad libitum feeding was required before assembly kinetics returned to control values and is probably indicative of a persistent impaired monodeiodination of T4 to T3.

Expression of epithelial Na channels in Xenopus oocytes

Epithelial Na channel activity was expressed in oocytes from Xenopus laevis after injection of mRNA from A6 cells, derived from Xenopus kidney. Poly A(+) RNA was extracted from confluent cell monolayers grown on either plastic or permeable supports. 1-50 ng RNA was injected into stage 5-6 oocytes. Na channel activity was assayed as amiloride-sensitive current (INa) under voltage-clamp conditions 1-3 d after injection. INa was not detectable in noninjected or water-injected oocytes. This amiloride-sensitive pathway induced by the mRNA had a number of characteristics in common with that in epithelial cells, including (a) high selectivity for Na over K, (b) high sensitivity to amiloride with an apparent K1 of approximately 100 nM, (c) saturation with respect to external Na with an apparent Km of approximately 10 mM, and (d) a time-dependent activation of current with hyperpolarization of the oocyte membrane. Expression of channel activity was temperature dependent, being slow at 19 degrees C but much more rapid at 25 degrees C. Fractionation of mRNA on a sucrose density gradient revealed that the species of RNA inducing channel activity had a sedimentation coefficient of approximately 17 S. Treatment of filter-grown cells with 300 nM aldosterone for 24 h increased Na transport in the A6 cells by up to fivefold but did not increase the ability of mRNA isolated from those cells to induce channel activity in oocytes. The apparent abundance of mRNA coding for channel activity was 10-fold less in cells grown on plastic than in those grown on filters, but was increased two- to threefold by aldosterone.

A Koelliker hemoglobin in chick erythrocytes

1. Adult chicken hemoglobins were analysed by ion exchange chromatography and isoelectric focusing and a minor hemoglobin fraction (HbK) was isolated. 2. Analysis of the constituent chains shows that HbK differs from the two major hemoglobins HbA and HbD in the alpha globin. 3. The amino acid composition, the tryptic peptide maps, the results of carboxypeptidase digestion and the functional properties show that the HbK alpha globin is quite similar to that of HbA except that the C-terminal amino acid Arg 141 is lacking. 4. HbK must then be considered a Koelliker-type hemoglobin.

Heat shock-induced translational control of HSP70 and globin synthesis in chicken reticulocytes

Incubation of chicken reticulocytes at elevated temperatures (43 to 45 degrees C) resulted in a rapid change in the pattern of protein synthesis, characterized by the decreased synthesis of normal proteins, e.g., alpha and beta globin, and the preferential and increased synthesis of only one heat shock protein, HSP70. The repression of globin synthesis was not due to modifications of globin mRNA because the level of globin mRNA and its ability to be translated in vitro were unaffected. The HSP70 gene in reticulocytes was transcribed in non-heat-shocked cells, yet HSP70 was not efficiently translated until the cells had been heat shocked. In non-heat-shocked reticulocytes, HSP70 mRNA was a moderately abundant mRNA present at 1 to 2% of the level of globin mRNA. The rapid 20-fold increase in the synthesis of HSP70 after heat shock was not accompanied by a corresponding increase in the rate of transcription of the HSP70 gene or accumulation of HSP70 mRNA. These results suggest that the elevated synthesis of HSP70 is due to the preferential utilization of HSP70 mRNA in the heat-shocked reticulocyte. The heat shock-induced alterations in the reticulocyte protein-synthetic apparatus were not reversible. Upon return to control temperatures (37 degrees C), heat-shocked reticulocytes continued to synthesize HSP70 at elevated levels whereas globin synthesis continued to be repressed. Despite the presence of HSP70 mRNA in non-heat-shocked reticulocytes, we found that continued transcription was necessary for the preferential translation of HSP70 in heat-shocked cells. Preincubation of reticulocytes with the transcription inhibitor actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole blocked the heat shock-induced synthesis of HSP70. Because the level of HSP70 mRNA was only slightly diminished in cells treated with actinomycin D, we suggest two possible mechanisms for the preferential translation of HSP70 mRNA: the translation of only newly transcribed HSP70 mRNA or the requirement of a newly transcribed RNA-containing factor.

Heat shock-induced translational control of HSP70 and globin synthesis in chicken reticulocytes

Incubation of chicken reticulocytes at elevated temperatures (43 to 45 degrees C) resulted in a rapid change in the pattern of protein synthesis, characterized by the decreased synthesis of normal proteins, e.g., alpha and beta globin, and the preferential and increased synthesis of only one heat shock protein, HSP70. The repression of globin synthesis was not due to modifications of globin mRNA because the level of globin mRNA and its ability to be translated in vitro were unaffected. The HSP70 gene in reticulocytes was transcribed in non-heat-shocked cells, yet HSP70 was not efficiently translated until the cells had been heat shocked. In non-heat-shocked reticulocytes, HSP70 mRNA was a moderately abundant mRNA present at 1 to 2% of the level of globin mRNA. The rapid 20-fold increase in the synthesis of HSP70 after heat shock was not accompanied by a corresponding increase in the rate of transcription of the HSP70 gene or accumulation of HSP70 mRNA. These results suggest that the elevated synthesis of HSP70 is due to the preferential utilization of HSP70 mRNA in the heat-shocked reticulocyte. The heat shock-induced alterations in the reticulocyte protein-synthetic apparatus were not reversible. Upon return to control temperatures (37 degrees C), heat-shocked reticulocytes continued to synthesize HSP70 at elevated levels whereas globin synthesis continued to be repressed. Despite the presence of HSP70 mRNA in non-heat-shocked reticulocytes, we found that continued transcription was necessary for the preferential translation of HSP70 in heat-shocked cells. Preincubation of reticulocytes with the transcription inhibitor actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole blocked the heat shock-induced synthesis of HSP70. Because the level of HSP70 mRNA was only slightly diminished in cells treated with actinomycin D, we suggest two possible mechanisms for the preferential translation of HSP70 mRNA: the translation of only newly transcribed HSP70 mRNA or the requirement of a newly transcribed RNA-containing factor.

Formation of the triple helix of type I procollagen in cellulo. Temperature-dependent kinetics support a model based on cis in equilibrium trans isomerization of peptide bonds

The kinetics of triple-helix formation in type I procollagen at 37 degrees C in cellulo have been found to agree with predictions from the following model: triple-helix formation is initiated after completion of the synthesis of the procollagen polypeptide chains and after the chains associate to form interchain disulfide bonds within the C-propeptide; triple-helix formation propagates from this single nucleation site toward the N terminus of the molecule, interrupted by the random occurrence of peptide bonds in the cis configuration; cis-trans isomerization controls the rate of triple-helix formation. This model predicts that the activation energy of the rate-limiting process should be strongly positive. However, studies of triple-helix formation in vitro using thermally denatured material have shown only a low, or even negative, dependence of the rate on temperature in the physiological range. Here we report the temperature dependence of the rate of triple-helix formation in cellulo and a novel procedure for analyzing the resulting data to give an estimate of the Arrhenius activation energy of the rate-controlling process. It was found that this rate showed a strong, positive dependence on temperature, as expected, and that the activation energy was in satisfactory agreement with independent direct determinations of this parameter for cis-trans isomerizations. These findings lend further support to the model of triple-helix formation described above.

Biosynthesis of platelet-activating factor (PAF-acether). V. Enhancement of acetyltransferase activity in murine peritoneal cells by calcium ionophore A23187

The activity of the acetyltransferase capable of transferring the acetyl moiety of acetyl-CoA onto 2-lyso PAF-acether (1-alkyl-sn-glycero-3-phosphocholine) to form PAF-acether was compared in ionophore A23187-stimulated and in non-stimulated rat peritoneal cells. Stimulation resulted in a doubling of the acetyltransferase activity within 30 s. This effect was abolished in the presence of EDTA (1 mM) or EGTA (1 mM) and restored by addition of Ca2+ (10 mM). The specificity of acetyltransferase measured in ionophore-stimulated as well as in untreated cells is the same. In both situations we observed the same Km values for acetyl-CoA, whereas the Vmax values were different. The wide similarities of the two enzyme preparations lead us to conclude that stimulation by the ionophore involves an increase in the number of enzyme molecules rather than a change in the kinetic parameters of the acetyltransferase.

The control of protein synthesis during heat shock in Drosophila cells involves altered polypeptide elongation rates

When Drosophila tissue culture cells are shifted from 25 to 36 degrees C (heat shocked) the pre-existing mRNAs (25 degrees C mRNAs) remain in the cytoplasm but their translation products are underrepresented relative to the induced heat shock proteins. Many of these undertranslated 25 degrees C mRNAs are found in association with polysomes of similar size in heat-shocked and control cells. Furthermore, the messages encoding alpha-tubulin, beta-tubulin, and actin are found associated with one-third to one-half as many total ribosomes in heat-shocked cells as in cells incubated at 25 degrees C. Increased temperature should lead to increased output of protein per ribosome. However, the 25 degrees C proteins are actually synthesized at less than 10% of 25 degrees C levels in heat-shocked cells. Thus, the rates of both elongation and initiation of translation are significantly (15- to 30-fold) slower on 25 degrees C mRNAs than they are on heat shock mRNAs in heat-shocked cells.

Is the major Drosophila heat shock protein present in cells that have not been heat shocked?

When eukaryotic cells are exposed to elevated temperatures they respond by vigorously synthesizing a small group of proteins called the heat shock proteins. An essential element in defining the role of these proteins is determining whether they are unique to a stressed state or are also found in healthy, rapidly growing cells at normal temperatures. To date, there have been conflicting reports concerning the major heat-induced protein of Drosophila cells, HSP 70. We report the development of monoclonal antibodies specific for this protein. These antibodies were used to assay HSP 70 in cells incubated under different culture conditions. The protein was detectable in cells maintained at normal temperatures, but only when immunological techniques were pushed to the limits of their sensitivity. To test for the possibility that these cells contain a reservoir of protein in a cryptic antigenic state (i.e., waiting posttranslational modification for use at high temperature), we treated cells with cycloheximide or actinomycin D immediately before heat shock. HSP 70 was not detected in these cells. Finally, we tested for the presence of a reservoir of inactive messages by using a high stringency hybridization of 32P-labeled cloned gene sequences to electrophoretically separated RNAs. Although HSP 70 mRNA was detectable in rapidly growing cells, it was present at less than 1/1,000th the level achieved after induction.

Direct measurement of histone peptide elongation rate in cleaving sea urchin embryos

Regulation of protein synthesis can be exercised at a number of levels. One of the more experimentally difficult levels to approach has been the measurement of peptide elongation rate. This paper presents a new application of the cyanogen bromide (CNBr) cleavage of proteins in a direct measurement of histone peptide elongation rate in cleaving sea urchin embryos (Strongylocentrotus purpuratus). The data indicate an elongation rate (at 15 degrees C) for histones H2B and H1 alpha of 0.69 and 0.80 codons per s, respectively. These values fall within the range of previously published values of average peptide elongation rate for total protein in these cells. This method should be generally applicable to many systems for which the measurement of peptide elongation rate may provide a key to the understanding of the regulation of protein synthesis.

Mononucleotide and dinucleotide frequencies, and codon usage in poliovirion RNA

The polio type 1 (Mahoney) RNA sequence (1) has been analyzed in terms of the distribution of its mononucleotides, dinucleotides and trinucleotides (codons). The distribution of adenosine in the sequence is nonuniform, being lower at the 5' end and higher at the 3' end. The dinucleotide CG is relatively rare and the dinucleotides UG and CA are relatively more common than expected. Codon usage is decidedly nonrandom. Codons containing CG are avoided and those ending in adenosine are favored. The asymmetric use of mononucleotides, dinucleotides and codons in polio RNA is unexplained at the present time although the lowered CG frequency may be the result of a DNA origin for polio RNA.

Translational elongation rate changes in encephalomyocarditis virus-infected and interferon-treated cells

Infection of mouse L cells with encephalomyocarditis virus results in a rapid inhibition of host protein synthesis before the synthesis of viral proteins. Although no alterations in initiation factor activities have been demonstrated in encephalomyocarditis virus-infected mouse cells, a defect in polypeptide chain elongation has been shown to occur in infected cell extracts. We investigated the significance of this elongation defect in the host shutoff phenomenon in vivo. Average polypeptide chain elongation rates were measured at various times after infection. Interferon was used as a reagent to separate temporarily the virus-induced alterations. Encephalomyocarditis virus infection of L cells was shown to lead to a progressive reduction in the elongation rate. Whereas interferon pretreatment delayed the decrease in elongation rate in a dose-dependent manner, it failed to alter the kinetics of host shutoff, suggesting that slowing of elongation steps played no significant role in this phenomenon. In addition, interferon pretreatment of either mock-infected or virus-infected cells led to no elongation defect that could be attributed to interferon action.

Evidence for control of protein synthesis in HeLa cells via the elongation rate

The effects of fresh medium and serum on protein synthesis in suspension-cultured HeLa cells after growth to high cell density (> 5 x 10(5) cells/ml) were studied. Cells which were resuspended in fresh medium plus serum and grown for 24 hours (control) were compared with cells grown for 2 hours after resuspension (stimulated). The spectrum of proteins being synthesized by control and stimulated cells does not appear to be grossly different; that is, the weight and number average molecular weights of newly synthesized whole-cell protein are about the same in both cultures. Also, no significant differences were observed in the number of ribosomes per polysome or in the fraction of total ribosomes in polysomes. However, the transit times (combined elongation and termination times) were found to differ significantly; the average transit time for control cells was 2.24 minutes, while the average transit time for stimulated cells was 1.26 minutes. (An appendex evaluating the methodology involved in measuring the transit time is included.) In aggreement with the difference in transit time, the absolute rate of protein synthesis in stimulated cells was approximately 1.8 times the rate measured in control cells. These data are taken as evidence that under certain conditions, the rate of elongation and/or termination of polypeptide chains limits the overall rate of translation, and that cells can respond to growth conditions by changing the elongation and/or termination rate of protein synthesis.

Early events in the cellular formation of proparathyroid hormone

Early events in the cellular synthesis and subsequent transfer into membrane-limited compartments of pre-proparathyroid hormone (pre-proPTH) and proparathyroid hormone (proPTH) were investigated by electrophoretic analyses of newly synthesized proteins in subcellular fractions of parthyroid gland slices pulse-labeled for 0.5-5 min with [(35)S] methionine. During these short times of incubation, both pre-proPTH and proPTH were confined to the microsomal fraction. Labeled pre-proPTH and proPTH were detected in a 30-s interval between 0.5 and 1.0 min of incubation. The radioactivity in proPTH became relatively constant between 3 and 5 min, whereas the radioactivity in ProPTH increased markedly over this period. When corrected for the known content of methionine in the prohormone and the prohormone, we found four times as much radiolabeled prohormone as prehormone between 0.5 and 1.0 min of synthesis. Sequestration of labeled prohomrone into endoplasmic reticulum compartments was shown by treatment of the microsomal fraction with chymotrypsin and trypsin, which resulted in the degradation of the prehormone but not of the prohormones. Approximately 50 percent of pre-prohormone and 25 percent of prohormone were released from the microsomes by their extraction with 1.0 M KCl, whereas 80-90 percent of both was released by treatment with Triton X-100. These results in intact cells support the signal hypothesis proposed by Blobel and his co-workers in studies utilizing cell-free systems, inasmuch as the results indicate transfer of prohormone into the cisternal space of the rough endoplasmic reticulum concomitant with the growth of the nascent polypeptide chain. Appearance of membrane-sequestered proPTH takes place without entry of pre-proPTH into the cisternal space, suggesting that proteolytic removal of the leader peptide occurs during transfer of the polypeptide through the lipid bilayer. Further evidence in support of this process is that pre-proPTH is only partly extracted from the microsomes by treatment with 1.0 M KCl, suggesting that a substantial fraction of the nascent pre-proPTH is integrally inserted into the membranes before it is cleaved to form proPTH.

The initiation and elongation steps in protein synthesis: relative rates in Chinese hamster ovary cells during and after hyperthermic and hypothermic shocks

The relative rates of the initiation and elongation phases of protein synthesis have been determined in heat- and cold-shocked CHO cells from measurements of the incorporation of 35S-methionine into N-terminal and internal positions of growing peptides by a modified Edman degradation. When the cells are shifted from 37 degrees C to temperatures between 10 degrees C and 34 degrees C, the rate of initiation is at first reduced more extensively than that of elongation. After 20 to 30 minutes at the lower temperature, however, the cells undergo a metabolic adjustment which includes increasing the rate of initiation until it corresponds to the rate of elongation at that temperature. Calculated apparent energies of activation for initiation and elongation are in reasonable agreement with those determined in other mammalian cells. When the cooled cells are returned to 37 degrees C, the rates of initiation and elongation recover immediately but do not exceed the control values. Exposure to elevated temperatures (43 degrees C) causes an immediate cessation of initiation and thus a delayed inhibition of elongation; upon return to 37 degrees C, the rate of initiation is transiently elevated above the control rate, and the rate of elongation returns to the control rate after a 2- to 3-minute delay. Hence, a factor which leads to supranormal rates of initiation may accumulate at high but not at low temperatures.

Alterations in the protein synthetic apparatus of Friend erythroleukemia cells during their erythroid differentiation

The changes in rate of protein synthesis and cell division and the distribution of polyribosomes and globin mRNA on the polyribosomes of Friend erythroleukemia (FL) cells exposed to 2% DMSO and maintained at low cell density, were examined at different times after exposure to DMSO. The rate of protein synthesis and the capacity of cells to divide declined in concert to 50% of the level found in untreated cell cultures at 24 hours after exposure. Thereafter these rates recovered to 70% of the rate found in untreated control cultures until 96 hours post-exposure and then irreversibly declined as the cells lost the capacity to divide. The proportion of ribosomes present as polyribosomes in cells exposed to DMSO paralleled the capacity of these cells to synthesize protein. The distribution of polyribosomes analyzed by sedimentation in sucrose gradients demonstrated that a discrete, abundant class of polyribosomes composed of pentamers to heptamers appeared as early as 48 hours after exposure to DMSO. The appearance of an abundant class of polyribosomes was correlated with globin synthesis by demonstrating that a discrete class of polyribosomes arises in cells treated with the inducers hexamethylene bisacetamide and hemin.

Resolution of simian virus 40 proteins in whole cell extracts by two-dimensional electrophoresis: heterogeneity of the major capsid protein

The major capsid protein (VP1) of simian virus 40 (SV40) has been analyzed by two-dimensional electrophoresis. This system separates protein according to isoelectric point by isoelectric-focusing, and according to molecular weight by sodium dodecylsulphate electrophoresis (O'Farrell, 1975). VP1 synthesis in infected CV-1 cells can be monitored directly by analysis of unfractionated whole cell extracts; the resolution of VP1 from cellular proteins allows its detection as early as 13 hr after infection. The two-dimensional separation of VP1 reveals that it is heterogeneous, consisting of one major protein (molecular weight 47,000 daltons and isoelectric point of approximately pH 6.8) and five minor protein components. The minor forms of VP1 are 10% of the total VP1 and differ from the major form of VP1 both in molecular weight (by approximately 500 daltons) and isoelectric point (ranging from approximately pH 6.7 to pH 6.9). Evidence is presented to show that two of the minor forms are phosphorylated derivatives of VP1, and it is further suggested that all the different forms of VP1 are the result of modifications of the primary product of translation. A temperature-sensitive mutant of the BC complementation group (BC11) of SV40 results in the synthesis of VP1 with an altered electrophoretic mobility; both the major form of VP1 and the minor forms are shifted in their isoelectric points. In addition to the specific case of SV40, two aspects of these studies should be generally significant to investigators studying eucaryotic gene expression by two-dimensional gel electrophoresis: first, the genetic origin of a protein can be determined by a temperature-sensitive mutation which causes a charge change in the resultant protein; and second, two or more protein spots on a two-dimensional separation may be the products of a single gene.

On the mechanism of delayed inhibition of protein synthesis in heme-defecient rabbit reticulocyte lysates

In the absence of added hemin, protein synthesis in a rabbit reticulocyte lysate declines abruptly (shuts off) after about 5 min at 30 degrees. In these studies we have examined the basis for the lag period preceding shut-off. The initiation factor that binds Met-tRNAf, previously shown to be rate-limiting in inhibited, heme-deficient lysates, is found to be used stoichiometrically in the presence of excess inhibitor. We suggest that a principal effect of the inhibitor is to impair the recycling of the Met-tRNAf-binding factor; the lag period is attributable largely to the presence of a pool of excess Met-tRNAf-binding factor, which, once used in initiation, cannot be recycled because of the action of the inhibitor.

Histone-gene reiteration in the genome of mouse

Histone messenger RNA labelled to high specific activity has been isolated and purified from the mouse tissue-culture cell line L929. It had an electrophoretic mobility equivalent to a mean molecular weight of 1.4 X 10(5). The synthesis of this RNA was suppressed, but apparently not completely so, by inhibition of cellular DNA synthesis to more than 95% by addition of cytosine arabinoside. The synthesis of other RNA species remained unaffected by this treatment. Poly(A)-containing mRNA, representing a 20% contaminant of the electrophoretically purified histone mRNA, was removed by poly(U)-Sepharose chromatography. Histone mRNA thus purified was hybridized to DNA fractions enriched for histone genes of the sea urchin Psammechinus miliaris by actinomycin/CsCl gradient centrifugation. The mRNA was eluted from the hybrids and challenged with mouse DNA in vast DNA excess conditions. A (cot1/2) of 360 mol s 1(-1) was obtained from the RNA trace curve, suggesting a 10-20-fold reiteration of the histone genes in the haploid genome. Thus histone genes in mouse are much less highly reiterated than in sea urchins, but nevertheless are present in considerable excess over the number theoretically necessary for histone synthesis during the S phase of the cell cycle.

Liver protein synthesis. Molecular weight distribution of pulse-labeled polypeptide chains in normal and thyroidectomized rats

Equations are presented for determination of elongation rate in vivo for a heterogenous population of polypeptide chain molecular weights. The distribution of pulse-labeled polypeptide chains in rat liver deoxycholate-soluble protein has been obtained by sodium dodecyl sulfate-gel electrophoresis and used to compute a theoretical curve for determination of synthesis time of a 50000 mol. wt. polypeptide chain (tc50). Values of tc50 for normal and thyro-parathyroidectomized Long-Evans male rats were 1.2 and 1.75 min, respectively, representing protein synthetic rates of about 7.5 and 5.1 mg protein/g liver/h. No difference in the molecular weight profile of liver polypeptide chains on the basis of labeling or Amido-black staining was observed between the two groups. The distributions of radioactivity before and after secretion of labeled plasma protein are compared. The role of protein-synthetic rate in the changing enzyme levels associated with thyroid hormone is discussed.

Globin synthesis during erythroid cell maturation in alpha thalassemia

Globin chain synthesis was examined in erythroid cells of increasing maturity, fractionated from bone marrow of two patients with hemoglobin H disease and in one alpha thalassemia 1 heterozygote. In contrast to beta thalassemia where a gradient of alpha/beta chain ratios increasing with erythroid cell maturation is observed, in alpha thalassemia the alpha/beta chain ratio remains constant throughout maturation. This finding suggests that in alpha thalassemia there is no modification of the imbalance in globin chain synthesis either by increased alpha chain production or decreased beta chain synthesis in erythroid precursors. Furthermore, the constant alpha/beta ratio reflects a limited degree of beta chain destruction, indicating that the ability of the excess beta chains to associate into tetramers protects them from proteolytic digestion.

Regulation of translation in rabbit reticulocytes and mouse L-cells; comparison of the effects of temperature

Various parameters of protein synthesis were analyzed in rabbit reticulocytes exposed to various temperatures for up to five hours. Between 10 degrees C and 40 degrees C total protein synthesis exhibited two different apparent activation energies (36 kcal/mole, 10-24 degrees C; 22 kcal/mole, 24-40 degrees C), as did protein elongation and release (35 kcal/mole, 10-25 degrees C; 12 kcal/mole, 25-40 degrees C). However, the level of polysomes remained essentially unchanged between 0 degrees C and 42 degrees C which implies that the activation energy for polypeptide initiation is quite similar to that for elongation and is also biphasic. This situation is different from that in cultured mouse L-cells where the polysome level is dependent on temperatures. Nevertheless, reticulocytes and L-cells appear to be similar in their temperature dependence of initiation and in their rate of elongation (5-6 amino acids/second at 36 degrees C.

Globin mRNA translation on Artemia salina ribosomes with components from Friend leukemia cells

Globin mRNA can be translated with relatively high efficiency in a fractionated cell-free system containing ribosomes prepared from cytst of Artemia salina. These ribosomes have unusually low endogenous activity for peptide synthesis in the absence of added mRNA. The system requires components from the postribosomal supernatant and from the 0.5 M KCl ribosomal wash fraction. Both these fractions were derived from either rabbit reticulocytes or unstimulated Friend leukemia cells that produce little or no hemoglobin. The activity of mRNA and enzyme fractions from rabbit reticulocytes and Friend leukemia cells were tested in this system in vitro for their ability to direct the synthesis of the alpha and beta chains of globin. The alpha:beta chain ratio synthesized from mRNA in the rabbit reticulocyte salt wash fraction was 4:1. The corresponding value for the 9-S mRNA fraction from the salt-washed reticulocyte ribosomes was 1:4, thus these two fractions appear to provide sources enriched in either alpha or beta globin mRNA. Under all conditions tested, the ratio and amounts of peptides formed in vitro appear to reflect mRNA composition. Globin mRNA from dimethysulfoxide-stimulated Friend leukemia cells when translated in vitro produced alpha and beta chains in a ratio of 1:1. These peptides are formed in the same ratio in the intact cells.

The hemoglobins of the developing chicken embryos. Fractionation and globin composition of the individual component of total erythrocytes and of a single erythrocyte type

The hemoglobins of the chicken embryo at several stages of development have been isolated in pure form by column chromatography and their relative amounts and globin compositions determined. The analyses on separated primitive and definitive erythrocytes show that the first contain four hemoglobins different from the adult ones. The two major ones at four days, decrease gradually and are no longer detectable from 15 days on. The two minor ones increase up to 6-7 days, then decrease but are still present at hatching. The definitive embryonic erythrocytes contain two hemoglobins identical to the adult ones but their ratios change gradually during development and approach that of the adult hemoglobins at hatching.