Emetine resistance in chinese hamster ovary cells is associated with an altered ribosomal protein S14 mRNA

Targeting the Lnc-OPHN1-5/androgen receptor/hnRNPA1 complex increases Enzalutamide sensitivity to better suppress prostate cancer progression

Long non-coding RNAs (lncRNAs) have been found to play critical roles in regulating gene expression, but their function in translational control is poorly understood. We found lnc-OPHN1-5, which lies close to the androgen receptor (AR) gene on chromosome X, increased prostate cancer (PCa) Enzalutamide (Enz) sensitivity via decreasing AR protein expression and associated activity. Mechanism dissection revealed that lnc-OPHN1-5 interacted with AR-mRNA to minimize its interaction with the RNA binding protein (RBP) hnRNPA1. Suppressing lnc-OPHN1-5 expression promoted the interaction between AR-mRNA and hnRNPA1, followed by an increase of ribosome association with AR-mRNA and translation. This effect was reversed by increasing lnc-OPHN1-5 expression. Consistently, the in vivo mice model confirmed that knocking down lnc-OPHN1-5 expression in tumors significantly increased the tumor formation rate and AR protein expression compared with the control group. Furthermore, knocking down hnRNPA1 blocked/reversed shlnc-OPHN1-5-increased AR protein expression and re-sensitized cells to Enz treatment efficacy. Evidence from Enz-resistant cell lines, patient-derived xenograft (PDX) models, clinical samples, and a human PCa study accordantly suggested that patients with low expression of lnc-OPHN1-5 likely have unfavorable prognoses and probably are less sensitive to Enz treatment. In summary, targeting this newly identified lnc-OPHN1-5/AR/hnRNPA1 complex may help develop novel therapies to increase Enz treatment sensitivity for suppressing the PCa at an advanced stage.

Efficacy and Mechanism of Action of Low Dose Emetine against Human Cytomegalovirus

Infection with human cytomegalovirus (HCMV) is a threat for pregnant women and immunocompromised hosts. Although limited drugs are available, development of new agents against HCMV is desired. Through screening of the LOPAC library, we identified emetine as HCMV inhibitor. Additional studies confirmed its anti-HCMV activities in human foreskin fibroblasts: EC₅₀−40±1.72 nM, CC₅₀−8±0.56 μM, and selectivity index of 200. HCMV inhibition occurred after virus entry, but before DNA replication, and resulted in decreased expression of viral proteins. Synergistic virus inhibition was achieved when emetine was combined with ganciclovir. In a mouse CMV (MCMV) model, emetine was well-tolerated, displayed long half-life, preferential distribution to tissues over plasma, and effectively suppressed MCMV. Since the in vitro anti-HCMV activity of emetine decreased significantly in low-density cells, a mechanism involving cell cycle regulation was suspected. HCMV inhibition by emetine depended on ribosomal processing S14 (RPS14) binding to MDM2, leading to disruption of HCMV-induced MDM2-p53 and MDM2-IE2 interactions. Irrespective of cell density, emetine induced RPS14 translocation into the nucleus during infection. In infected high-density cells, MDM2 was available for interaction with RPS14, resulting in disruption of MDM2-p53 interaction. However, in low-density cells the pre-existing interaction of MDM2-p53 could not be disrupted, and RPS14 could not interact with MDM2. In high-density cells the interaction of MDM2-RPS14 resulted in ubiquitination and degradation of RPS14, which was not observed in low-density cells. In infected-only or in non-infected emetine-treated cells, RPS14 failed to translocate into the nucleus, hence could not interact with MDM2, and was not ubiquitinated. HCMV replicated similarly in RPS14 knockdown or control cells, but emetine did not inhibit virus replication in the former cell line. The interaction of MDM2-p53 was maintained in infected RPS14 knockdown cells despite emetine treatment, confirming a unique mechanism by which emetine exploits RPS14 to disrupt MDM2-p53 interaction. Summarized, emetine may represent a promising candidate for HCMV therapy alone or in combination with ganciclovir through a novel host-dependent mechanism.

Infection with human Cytomegalovirus (HCMV) is a growing and pressing problem, creating ongoing management and therapeutic challenges. Despite the availability of DNA polymerase inhibitors, development of new strategies for HCMV therapy is needed. We report for the first time on the efficacy of an old drug (emetine) against HCMV in vitro and mouse CMV in vivo, using exceedingly low drug doses. We also provide evidence for a specific host-dependent anti-CMV mechanism of emetine in vitro, thus uncovering a cellular function that can be further studied for drug development. Our work provides a novel direction for HCMV therapeutics through repurposing of an old agent, at substantially lower doses, and inhibiting HCMV indirectly through host activities critical for virus replication.

Duration of the unconditioned stimulus in appetitive conditioning of honeybees differentially impacts learning, long-term memory strength, and the underlying protein synthesis

This study examines the role of stimulus duration in learning and memory formation of honeybees (Apis mellifera). In classical appetitive conditioning honeybees learn the association between an initially neutral, conditioned stimulus (CS) and the occurrence of a meaningful stimulus, the unconditioned stimulus (US). Thereby the CS becomes a predictor for the US eliciting a conditioned response (CR). Here we study the role of US duration in classical conditioning by examining honeybees conditioned with different US durations. We quantify the CR during acquisition, memory retention, and extinction of the early long-term memory (eLTM), and examine the molecular mechanisms of eLTM by interfering with protein synthesis. We find that the US duration affects neither the probability nor the strength of the CR during acquisition, eLTM retention, and extinction 24 h after conditioning. However, we find that the resistance to extinction 24 h after conditioning is susceptible to protein synthesis inhibition depending on the US duration. We conclude that the US duration does not affect the predictability of the US but modulates the protein synthesis underlying the eLTM's strength. Thus, the US duration differentially impacts learning, eLTM strength, and its underlying protein synthesis.

Cryo-EM structure of the Plasmodium falciparum 80S ribosome bound to the anti-protozoan drug emetine

Malaria inflicts an enormous burden on global human health. The emergence of parasite resistance to front-line drugs has prompted a renewed focus on the repositioning of clinically approved drugs as potential anti-malarial therapies. Antibiotics that inhibit protein translation are promising candidates for repositioning. We have solved the cryo-EM structure of the cytoplasmic ribosome from the human malaria parasite, Plasmodium falciparum, in complex with emetine at 3.2 Å resolution. Emetine is an anti-protozoan drug used in the treatment of ameobiasis that also displays potent anti-malarial activity. Emetine interacts with the E-site of the ribosomal small subunit and shares a similar binding site with the antibiotic pactamycin, thereby delivering its therapeutic effect by blocking mRNA/tRNA translocation. As the first cryo-EM structure that visualizes an antibiotic bound to any ribosome at atomic resolution, this establishes cryo-EM as a powerful tool for screening and guiding the design of drugs that target parasite translation machinery.

DOI:http://dx.doi.org/10.7554/eLife.03080.001

Each year, malaria kills more than 600,000 people, mostly children younger than 5 years old. Humans who have been bitten by mosquitoes infected with malaria-causing parasites become ill as the parasites rapidly multiply in blood cells. Although there are several drugs that are currently used to treat malaria, the parasites are rapidly developing resistance to them, setting off an urgent hunt for new malaria drugs.

Developing new antimalarial medications from scratch is likely to take decades—too long to combat the current public health threat posed by emerging strains of drug-resistant parasites. To speed up the process, scientists are investigating whether drugs developed for other illnesses may also act as therapies for malaria, either when used alone or in combination with existing malaria drugs.

Certain antibiotics—including one called emetine—have already shown promise as antimalarial drugs. These antibiotics prevent the parasites from multiplying by interfering with the ribosome—the part of a cell that builds new proteins. However, humans become ill after taking emetine for long periods because it also blocks the production of human proteins.

Tweaking emetine so that it acts only against the production of parasite proteins would make it a safer malaria treatment. To do this, scientists must first map the precise interactions between the drug and the ribosomes in parasites. Wong et al. have now used a technique called cryo-electron microscopy to examine the ribosome of the most virulent form of malaria parasite. This technique uses very cold temperatures to rapidly freeze molecules, allowing scientists to look at molecular-level details without distorting the structure of the molecule—a problem sometimes encountered in other techniques.

The images of the parasitic ribosome taken by Wong, Bai, Brown et al. show that emetine binds to the end of the ribosome where the instructions for how to assemble amino acids into a protein are copied from strands of RNA. In addition, the images revealed features of the parasitic ribosome that are not found in the human form. Drug makers could exploit these features to improve emetine so that it more specifically targets the production of proteins by the parasite and is less toxic to humans.

DOI:http://dx.doi.org/10.7554/eLife.03080.002

Alteration of ribosomal protein maps in herpes simplex virus type 1 infection

At present, the effect of herpes simplex virus infection on the entire proteomes of infected cells is very poorly documented. Following several studies performed over the past few years, the modifications of a sub-cellular fraction induced by herpes simplex virus type 1 can be documented. These studies were performed in order to characterize the virally-induced modifications of a major component of the translational apparatus, the ribosomes. The very basic nature of most of the ribosomal proteins renders them very difficult to separate using isoelectric focusing (IEF). Therefore these studies were achieved using several different but related two-dimensional electrophoretic systems which allowed several two-dimensional ribosomal protein maps to be built. Comparison of the ribosomal protein maps built from non-infected cells with those built from infected cells demonstrated that infection by herpes simplex virus type 1 (HSV-1) induces important modifications of ribosomes: (i) non-reversible phosphorylation of ribosomal protein S6; (ii) unusual phosphorylation of several proteins of the small and the large subunits; and (iii) association of viral and cellular proteins to the ribosomal fraction. An overview of these published studies is presented in this review.

Ribosomal protein S14 of Saccharomyces cerevisiae regulates its expression by binding to RPS14B pre-mRNA and to 18S rRNA

Production of ribosomal protein S14 in Saccharomyces cerevisiae is coordinated with the rate of ribosome assembly by a feedback mechanism that represses expression of RPS14B. Three-hybrid assays in vivo and filter binding assays in vitro demonstrate that rpS14 directly binds to an RNA stem-loop structure in RPS14B pre-mRNA that is necessary for RPS14B regulation. Moreover, rpS14 binds to a conserved helix in 18S rRNA with approximately five- to sixfold-greater affinity. These results support the model that RPS14B regulation is mediated by direct binding of rpS14 either to its pre-mRNA or to rRNA. Investigation of these interactions with the three-hybrid system reveals two regions of rpS14 that are involved in RNA recognition. D52G and E55G mutations in rpS14 alter the specificity of rpS14 for RNA, as indicated by increased affinity for RPS14B RNA but reduced affinity for the rRNA target. Deletion of the C terminus of rpS14, where multiple antibiotic resistance mutations map, prevents binding of rpS14 to RNA and production of functional 40S subunits. The emetine-resistant protein, rpS14-EmRR, which contains two mutations near the C terminus of rpS14, does not bind either RNA target in the three-hybrid or in vitro assays. This is the first direct demonstration that an antibiotic resistance mutation alters binding of an r protein to rRNA and is consistent with the hypothesis that antibiotic resistance mutations can result from local alterations in rRNA structure.

Functional analysis of human RPS14 null alleles

Previously we described a large collection of cloned human DNAs that encode chemically defined missense mutations within the ribosomal protein S14 sequence. We determined that biologically inactive (i.e. null) alleles resulted primarily from point mutations targeted to two internal segments of the S14-coding sequence and designated these functionally critical regions as domains B and D. Further, we inferred that structural determinants within domains B and D are required for proper incorporation of the S14 protein into nascent 40 S ribosomal particles and/or for the normal function of mature cytoplasmic ribosomes. In this study we have used immunofluorescence to monitor the intracellular trafficking of epitopically labeled human S14 protein isoforms transiently expressed by cultured Chinese hamster cells. Data obtained distinguish null alleles of RPS14 which encode proteins that are not incorporated into pre-ribosomal subunit particles from null alleles whose products are compatible with normal ribosome assembly processes but result in functionally inactive cytoplasmic 40 S ribosomal subunits. Mutations assigned to the first allele class involve amino acid replacements located within S14 domains B and D; whereas mutations assigned to the second class are distributed throughout the S14 protein-coding sequence.

Regulation of human RPS14 transcription by intronic antisense RNAs and ribosomal protein S14

RNase protection studies reveal two stable RNAs (250 and 280 nucleotides) transcribed from the antisense strand of the human ribosomal protein gene RPS14's first intron. These transcripts, designated alpha-250 and alpha-280, map to overlapping segments of the intron's 5' sequence. Neither RNA encodes a polypeptide sequence, and both are expressed in all human cells and tissues examined. Although alpha-280 is detected among both the cells' nuclear and cytoplasmic RNAs, the great majority of alpha-250 is found in the cytoplasmic subcellular compartment. As judged by its resistance to high concentrations of alpha-amanitin, cell-free transcription of alpha-250 and alpha-280 appears to involve RNA polymerase I. Tissue culture transfection and cell-free transcription experiments demonstrate that alpha-250 and alpha-280 stimulate S14 mRNA transcription, whereas free ribosomal protein S14 inhibits it. Electrophoretic mobility shift experiments indicate specific binary molecular interactions between r-protein S14, its message and the antisense RNAs. In light of these data, we propose a model for fine regulation of human RPS14 transcription that involves RPS14 intron 1 antisense RNAs as positive effectors and S14 protein as a negative effector.

Deoxycytidine methylation and the origin of spontaneous transition mutations in mammalian cells

Previously we described a recurrent, site-specific G4784 --> A transition mutation affecting exon V of the Chinese hamster ovary cell RPS14 gene. Because the mutation is located within a CpG dinucleotide, we considered the possibility that deoxycytidine methylation might be responsible for the transition's unusually high frequency and site specificity. Therefore, we used a procedure based on the PCR amplification of bisulfite-modified genomic DNA to analyze the pattern of DNA cytosine methylation in exon V of the CHO cell RPS14 locus. Our data indicate that the CpG dinucleotide targeted by the transition mutation is stably methylated in CHO cell chromosomes. This finding supports the notion that deoxycytidine methylation promotes "spontaneous", site-specific transition mutations in mammalian cells.

Fine-structure map of the human ribosomal protein gene RPS14

We have used polymerase chain reaction-mediated chemical mutagenesis (J.-J. Diaz, D. D. Rhoads, and D. J. Roufa, BioTechniques 11:204-211, 1991) to analyze the genetic fine structure of a human ribosomal protein gene, RPS14. Eighty-three DNA clones containing 158 random single-base substitution mutations were isolated. Mutant RPS14 alleles were tested for biological activity by transfection into cultured Chinese hamster cells. The resulting data permitted us to construct a map of the S14-coding sequence that is comparable to available fine-structure genetic maps of many prokaryotic and lower eukaryotic gene loci. As predicted from the multiplicity of protein-protein and protein-RNA interactions required for ribosomal protein transport and assembly into functional ribosomal subunits, the distribution of null mutations indicated that S14 is composed of multiple, functionally distinct polypeptide domains. Two of the protein's internal domains, designated domains B and D, were essential for S14 biological activity. In contrast, mutations which altered or deleted S14's amino-terminal 20 amino acid residues (domain A) had no observable effect on the protein's assembly and function in mammalian ribosomes. Interestingly, S14 structural domains deduced by in vitro mutagenesis correlate well with the RPS14 gene's exon boundaries.

Fine-structure map of the human ribosomal protein gene RPS14

We have used polymerase chain reaction-mediated chemical mutagenesis (J.-J. Diaz, D. D. Rhoads, and D. J. Roufa, BioTechniques 11:204-211, 1991) to analyze the genetic fine structure of a human ribosomal protein gene, RPS14. Eighty-three DNA clones containing 158 random single-base substitution mutations were isolated. Mutant RPS14 alleles were tested for biological activity by transfection into cultured Chinese hamster cells. The resulting data permitted us to construct a map of the S14-coding sequence that is comparable to available fine-structure genetic maps of many prokaryotic and lower eukaryotic gene loci. As predicted from the multiplicity of protein-protein and protein-RNA interactions required for ribosomal protein transport and assembly into functional ribosomal subunits, the distribution of null mutations indicated that S14 is composed of multiple, functionally distinct polypeptide domains. Two of the protein's internal domains, designated domains B and D, were essential for S14 biological activity. In contrast, mutations which altered or deleted S14's amino-terminal 20 amino acid residues (domain A) had no observable effect on the protein's assembly and function in mammalian ribosomes. Interestingly, S14 structural domains deduced by in vitro mutagenesis correlate well with the RPS14 gene's exon boundaries.

Regulation of ribosomal protein S25 in HL60 cells isolated for resistance to adriamycin

The ribosomal protein S25 gene is highly overexpressed in HL60 cells isolated for resistance to adriamycin. In contrast there is no overexpression of 3 other ribosomal genes which code for proteins S14, S17 and S24. Studies with an antibody against a synthetic peptide of the S25 protein show that although the S25 gene is overexpressed in resistant cells there is no corresponding increase in the levels of S25 protein. These results suggest that the r-protein levels are highly regulated by translational controls or protein turnover.

Herpes simplex virus type-1 immediate-early gene expression and shut off of host protein synthesis are inhibited in neomycin-treated human epidermoid carcinoma 2 cells

Infection of human epidermoid carcinoma-2 (HEp-2) cells by Herpes simplex virus type 1 (HSV-1) leads to significant activation of inositol phospholipid turnover after 15 min. The effect of neomycin, an inhibitor of inositol phospholipid turnover, has been investigated for its effect on HSV-1 multiplication in HEp-2 cells. HSV-1 multiplication is inhibited by neomycin. This inhibition is not due to a block of virus adsorption or penetration. Neomycin inhibits the expression of virus immediate-early genes, as well as expression of early genes and viral DNA synthesis. In neomycin-treated cells, the usual virion-associated shut off of host protein synthesis does not occur. These results indicate that the inositol phospholipid pathway is involved in immediate-early gene expression and shut off of host protein synthesis in HEp-2 cells.

Herpes simplex virus type-1-induced stimulation of ribosomal protein S6 phosphorylation is inhibited in neomycin-treated human epidermoid carcinoma 2 cells and in ras-transformed cells

Neomycin, an inhibitor of inositol phospholipid turnover, prevents Herpes-simplex-virus-type-1 (HSV-1)-induced stimulation of ribosomal protein S6 phosphorylation, but does not impair the S6 phosphorylation induced by serum. Long-term treatment with phorbol 12-myristate 13-acetate, which down-regulates protein kinase C activity, does not inhibit virus-induced S6 phosphorylation. In ras-transformed cells, S6 phosphorylation is not stimulated after HSV-1 infection. These results suggest that activation of the inositol phospholipid pathway is involved in the HSV-1-induced stimulation of S6 phosphorylation. However, protein kinase C activation does not appear to be necessary for HSV-1-induced S6 phosphorylation.

Genetic analysis of a vital mammalian housekeeping locus using CHO cells that express a transfected mutant allele

We describe a novel approach for the isolation of null mutations in a vital Chinese hamster ovary (CHO) cell housekeeping gene. Our experimental strategy required introduction of an expressible DNA clone encoding a recessive emetine-resistance allele of ribosomal protein S14 into wild-type CHO cells. Transgene heterozygote (TGH) cell lines, which harbor multiple emetine-resistance S14 transgenes, survive mutations that inactivate the CHO RPS14 locus by virtue of the transgenes' biological function. Null mutations in RPS14 yield TGH clones that display the transgene's drug-resistance phenotype. A large collection of emetine-resistant clones was isolated from one TGH cell line and shown to consist of three types of S14 mutations: (1) nonsense null mutations in the RPS14 protein coding sequence; (2) missense null mutations that affect S14 amino acid residues that have been conserved stringently during eukaryotic evolution; and (3) a recurrent missense mutation that results in a new, functional RPS14 emetine-resistance allele.

A Drosophila ribosomal protein functions in mammalian cells

A cDNA expression vector encoding Drosophila ribosomal protein S14 was transfected into cultured Chinese hamster ovary (CHO) cells that harbor a recessive RPS14 emetine resistance mutation. Transformants synthesized the insect mRNA and polypeptide and consequently displayed an emetine-sensitive phenotype. These observations indicate that the insect protein was accurately expressed and correctly assembled into functional mammalian 40S ribosomal subunits.

A Drosophila ribosomal protein functions in mammalian cells

A cDNA expression vector encoding Drosophila ribosomal protein S14 was transfected into cultured Chinese hamster ovary (CHO) cells that harbor a recessive RPS14 emetine resistance mutation. Transformants synthesized the insect mRNA and polypeptide and consequently displayed an emetine-sensitive phenotype. These observations indicate that the insect protein was accurately expressed and correctly assembled into functional mammalian 40S ribosomal subunits.

Ribosome and protein synthesis modifications after infection of human epidermoid carcinoma cells with herpes simplex virus type 1

Modifications of ribosomes have been investigated in human epidermoid carcinoma-2 cells at different stages of herpes simplex virus type 1 infection. Very early in infection, there is an increase in ribosomal protein S6 phosphorylation even in the absence of serum. The same result is obtained in the presence of actinomycin D. At early infection time, ribosomal proteins S2, S3a and Sa are newly phosphorylated. At early and early-late times, three phosphorylated non-ribosomal proteins (v1, v2 and v3) are differently associated temporally to ribosomes. Analyses of proteins extracted from 40S subunits, 80S ribosomes and polysomes show that v1 and v2 are distributed differently among the different ribosomal populations. S6 phosphopeptides were found to be identical after serum stimulation and after viral infection. In every case phosphoserine and phosphothreonine were identified in S6. Only phosphoserine was found in other phosphorylated proteins. Our results indicate that herpes simplex virus type 1 is able to modify pre-existing ribosomes: (i) by stimulating a pre-existing kinase for S6 phosphorylation even in the absence of serum and of viral genome expression; (ii) by inducing new specific kinase activity(ies); and (iii) by association of new, phosphorylated proteins to ribosomes. These ribosomal modifications are correlated with changes in protein synthesis, as shown by two-dimensional electrophoretic analyses of newly synthesized 35S-labelled proteins.

A cloned human ribosomal protein gene functions in rodent cells

Cloned fragments of human ribosomal protein S14 DNA (RPS14) were transfected into cultured Chinese hamster (CHO) cells. Transient expression assays indicated that DNA with as little as 31 base pairs of upstream flanking sequence was transcribed into a polyadenylated, 650-base mRNA that is largely bound to the polyribosomes. In these respects the exogenous human S14 message appeared to function normally in CHO cells. Interestingly, transcription of human RPS14 did not require the TATA sequence located 26 base pairs upstream of exon 1. Stably transformed clones were selected from cultures of emetine-resistant CHO cells (Emr-2) after transfection with pSV2Neo-human RPS14 constructs. Human RPS14 complemented the mutationally based drug resistance of the Chinese hamster cells, demonstrating that the cloned human ribosomal protein gene is functional in rodent cells. Analysis of transformed cells with different amounts of integrated RPS14 indicated that human S14 mRNA levels are not tightly regulated by CHO cells. In contrast, the steady-state S14 level fluctuated only slightly, if at all, in transformed clones whose S14 message contents differed by more than 30-fold. These data support the conclusion that expression of human RPS14 is regulated, at least partially, posttranscriptionally.

A cloned human ribosomal protein gene functions in rodent cells

Cloned fragments of human ribosomal protein S14 DNA (RPS14) were transfected into cultured Chinese hamster (CHO) cells. Transient expression assays indicated that DNA with as little as 31 base pairs of upstream flanking sequence was transcribed into a polyadenylated, 650-base mRNA that is largely bound to the polyribosomes. In these respects the exogenous human S14 message appeared to function normally in CHO cells. Interestingly, transcription of human RPS14 did not require the TATA sequence located 26 base pairs upstream of exon 1. Stably transformed clones were selected from cultures of emetine-resistant CHO cells (Emr-2) after transfection with pSV2Neo-human RPS14 constructs. Human RPS14 complemented the mutationally based drug resistance of the Chinese hamster cells, demonstrating that the cloned human ribosomal protein gene is functional in rodent cells. Analysis of transformed cells with different amounts of integrated RPS14 indicated that human S14 mRNA levels are not tightly regulated by CHO cells. In contrast, the steady-state S14 level fluctuated only slightly, if at all, in transformed clones whose S14 message contents differed by more than 30-fold. These data support the conclusion that expression of human RPS14 is regulated, at least partially, posttranscriptionally.

Primary structure of human ribosomal protein S14 and the gene that encodes it

Chinese hamster ribosomal protein S14 cDNA was used to recognize homologous human cDNA and genomic clones. Human and Chinese hamster S14 protein sequences deduced from the cDNAs are identical. Two overlapping human genomic S14 DNA clones were isolated from a Charon 28 placental DNA library. A fragment of single-copy DNA derived from an intron region of one clone was mapped to the functional RPS14 locus on human chromosome 5q by using a panel of human X Chinese hamster hybrid cell DNAs. The human S14 gene consists of five exons and four introns spanning 5.9 kilobase pairs of DNA. Polyadenylated S14 transcripts purified from HeLa cell cytoplasma display heterogeneous 5' ends that map within noncoding RPS14 exon 1. This precludes assignment of a unique 5' boundary of RPS14 transcripts with respect to the cloned human genomic DNA. Apparently HeLa cells either initiate transcription at multiple sites within RPS14 exon 1, or capped 5' oligonucleotides are removed from most S14 mRNAs posttranscription. In contrast to the few murine ribosomal protein and several other mammalian housekeeping genes whose structures are known, human RPS14 contains a TATA sequence (TATACTT) upstream from exon 1. Three related short sequence motifs, also observed in murine and yeast ribosomal protein genes, occur in this region of the RPS14 gene. RPS14 introns 3 and 4 both contain Alu sequences. Interestingly, the Alu sequence in intron 3 is located slightly downstream from a chromosome 5 deletion breakpoint in one human X hamster hybrid clone analyzed.

Ribosomal protein phosphorylation in vivo and in vitro by vaccinia virus

Ribosomal protein phosphorylation was investigated in Ehrlich ascites tumor cells infected with vaccinia virus (Copenhagen strain). After 90 min of simultaneous infection and 32P-labelling, ribosomal proteins Sa, S2 and S13 appear specifically phosphorylated as well as Sb/La, P1 and S6, which are also phosphorylated in control cells. Sa is an acidic protein, whose phosphorylation has not been observed previously. A kinetic study showed that S2 is phosphorylated very rapidly within 10 min after the beginning of infection and it is complete 1 h later. The phosphorylation of S13 begins after a lag time of about 1 h and is completed after about 2.5 h of infection. Moreover only one phosphate is incorporated into S13 on a serine residue while up to four phosphates are incorporated into S2, the first on a serine and the three following on threonine residues. In vivo experiments, carried out in the presence of cycloheximide and cordycepin, suggest a viral origin for the kinase involved in the phosphorylation of S2 and S13. Moreover, in vitro experiments demonstrated that the kinase associated with the viral cores is capable of phosphorylating S2 on a serine residue only. In our cell/virus system, no significant difference in S6 phosphorylation was detected, when compared to uninfected cells. It is concluded that the specific and efficient phosphorylation of three ribosomal proteins from the 40S ribosomal subunit correlate well with possible translational mechanisms ensuring the efficient expression of early and late genes of vaccinia virus. In the light of these and previous results [Person, A. and Beaud, G. (1986) J. Biol. Chem. 261, 8283-8289], a mechanism is proposed for the shut-off of host protein synthesis and the selective translation of mRNAs of viral origin.

Primary structure of human ribosomal protein S14 and the gene that encodes it

Chinese hamster ribosomal protein S14 cDNA was used to recognize homologous human cDNA and genomic clones. Human and Chinese hamster S14 protein sequences deduced from the cDNAs are identical. Two overlapping human genomic S14 DNA clones were isolated from a Charon 28 placental DNA library. A fragment of single-copy DNA derived from an intron region of one clone was mapped to the functional RPS14 locus on human chromosome 5q by using a panel of human X Chinese hamster hybrid cell DNAs. The human S14 gene consists of five exons and four introns spanning 5.9 kilobase pairs of DNA. Polyadenylated S14 transcripts purified from HeLa cell cytoplasma display heterogeneous 5' ends that map within noncoding RPS14 exon 1. This precludes assignment of a unique 5' boundary of RPS14 transcripts with respect to the cloned human genomic DNA. Apparently HeLa cells either initiate transcription at multiple sites within RPS14 exon 1, or capped 5' oligonucleotides are removed from most S14 mRNAs posttranscription. In contrast to the few murine ribosomal protein and several other mammalian housekeeping genes whose structures are known, human RPS14 contains a TATA sequence (TATACTT) upstream from exon 1. Three related short sequence motifs, also observed in murine and yeast ribosomal protein genes, occur in this region of the RPS14 gene. RPS14 introns 3 and 4 both contain Alu sequences. Interestingly, the Alu sequence in intron 3 is located slightly downstream from a chromosome 5 deletion breakpoint in one human X hamster hybrid clone analyzed.

Emetine resistance of Chinese hamster cells: structures of wild-type and mutant ribosomal protein S14 mRNAs

The Chinese hamster ovary (CHO) cell 40S ribosomal subunit protein S14 provides a unique opportunity to investigate an important mammalian housekeeping gene and its mRNA and protein products. The S14 gene appears to be single copy, and CHO cell S14 mutants have been isolated as emetine-resistant (emtB) clones in tissue culture. Thus, S14 is the only mammalian ribosomal protein whose gene structure and function are amenable to straightforward genetic and biochemical analysis. Recently, we isolated a wild-type Chinese hamster lung cell cDNA clone, pCS14-1, including an almost complete copy of the ribosomal protein S14 message (N. Nakamichi, D. D. Rhoads, and D. J. Roufa, J. Biol. Chem. 258: 13236-13242, 1983). Here we describe comparable cDNAs from wild-type and emtB CHO cells. We report both mRNA and polypeptide sequences of the wild-type and mutant ribosomal protein transcripts. As a consequence of the genetic methods used to obtain our emetine-resistant mutants, the emtB S14 cDNAs differ from wild-type cDNA by single-base changes. Physical and chemical features of polypeptides encoded by the cDNAs are consistent with well-characterized S14 protein polymorphisms. The three emtB mutations analyzed affect two adjacent arginine codons within the very basic S14 carboxyl region, indicating a significant role for this portion of the protein in the function and architecture of the mammalian 40S ribosomal subunit.

Emetine resistance of Chinese hamster cells: structures of wild-type and mutant ribosomal protein S14 mRNAs

The Chinese hamster ovary (CHO) cell 40S ribosomal subunit protein S14 provides a unique opportunity to investigate an important mammalian housekeeping gene and its mRNA and protein products. The S14 gene appears to be single copy, and CHO cell S14 mutants have been isolated as emetine-resistant (emtB) clones in tissue culture. Thus, S14 is the only mammalian ribosomal protein whose gene structure and function are amenable to straightforward genetic and biochemical analysis. Recently, we isolated a wild-type Chinese hamster lung cell cDNA clone, pCS14-1, including an almost complete copy of the ribosomal protein S14 message (N. Nakamichi, D. D. Rhoads, and D. J. Roufa, J. Biol. Chem. 258: 13236-13242, 1983). Here we describe comparable cDNAs from wild-type and emtB CHO cells. We report both mRNA and polypeptide sequences of the wild-type and mutant ribosomal protein transcripts. As a consequence of the genetic methods used to obtain our emetine-resistant mutants, the emtB S14 cDNAs differ from wild-type cDNA by single-base changes. Physical and chemical features of polypeptides encoded by the cDNAs are consistent with well-characterized S14 protein polymorphisms. The three emtB mutations analyzed affect two adjacent arginine codons within the very basic S14 carboxyl region, indicating a significant role for this portion of the protein in the function and architecture of the mammalian 40S ribosomal subunit.

Accounting for the ATP-consuming processes in rabbit reticulocytes

The report deals with a detailed balance of ATP production and consumption of the rabbit reticulocyte. The sum-total of ATP produced amounts to 135 mmol . 1-1 . h-1. About 70% of the ATP consumption has been accounted for by specific processes. The main contributing processes are the globin synthesis with about 28%, the Na+, K+-ATPase with 23% and the proteolysis with more than 15%. 30% of ATP consumption has not been accounted for. Cycloheximide (20 microM) leads to a dissociation between synthesis and degradation of proteins, which argues against any obligatory connection between these processes. More than 90% of the lysine liberated from mitochondria by proteolysis were reutilized for the globin synthesis demonstrating the high nitrogen economy of reticulocytes. Each of the ATP-consuming processes studied appears to control ATP production in an independent manner without competition with each other.

Genetic and biochemical distinction among Chinese hamster cell emtA, emtB, and emtC mutants

Genetic and biochemical experiments have enabled us to more clearly distinguish three genetic loci, emtA, emtB, and emtC, all of which can be altered to give rise to resistance to the protein synthesis inhibitor, emetine, in cultured Chinese hamster cells. Genetic experiments have demonstrated that, unlike the emtB locus, neither the emtA locus nor the emtC locus is linked to chromosome 2 in Chinese hamster cells, clearly distinguishing the latter two genes from emtB. emtA mutants can also be distinguished, biochemically, from emtB and emtC mutants based upon different degrees of cross-resistance to another inhibitor of protein synthesis, cryptopleurine. Two-dimensional gel electrophoretic analysis of ribosomal proteins failed to detect any electrophoretic alterations in ribosomal proteins from emtA or emtC mutants that could be correlated with emetine resistance. However, a distinct electrophoretic alteration in ribosomal protein S14 was observed in an emtB mutant. In addition, the parental Chinese hamster peritoneal cell line of an emtC mutant, and the emtC mutant itself, are apparently heterozygous for an electrophoretic alteration in ribosomal protein L9.

Ribosomal protein S14 is altered by two-step emetine resistance mutations in Chinese hamster cells

Four two-dimensional polyacrylamide gel electrophoresis systems were used to identify 78 Chinese hamster cell ribosomal proteins by the uniform nomenclature based on rat liver ribosomal proteins. The 40S ribosomal subunit protein affected by Chinese hamster ovary (CHO) cell one-step emetine resistance mutations is designated S14 in the standard nomenclature. To seek unambiguous genetic evidence for a cause and effect relationship between CHO cell emetine resistance and mutations in the S14 gene, we mutagenized a one-step CHO cell mutant and isolated second-step mutant clones resistant to 10-fold-higher concentrations of emetine. All of the highly resistant, two-step CHO cell mutants obtained displayed additional alterations in ribosomal protein S14. Hybridization complementation tests revealed that the two-step CHO cell emetine resistance mutants were members of the same complementation group defined by one-step CHO cell mutants, EmtB. Two-step mutants obtained from a Chinese hamster lung cell emetine-resistant clone belong to the EmtA complementation group. The two-step and EmtB mutants elaborated 40S ribosomal subunits, which dissociated to 32S and 40S core particles in buffers containing 0.5 M KCl at 4 degrees C. In contrast, 40S ribosomal subunits purified from all EmtA, one-step EmtB EmtC mutants, and wild-type CHO and lung cells were stable at this temperature in buffers containing substantially higher concentrations of salt. Thus, two-step emtB mutations affect the structure of S14 protein directly and the stability of the 40S ribosomal subunit indirectly.

Biochemical evidence for multiple independent emetine resistance genes in Chinese hamster cells

Hybridization-complementation studies indicated that mutations in multiple genes can render Chinese hamster cells resistant to the alkaloid translation inhibitor emetine. Two of the genes, emtA and emtB, recognized in Chinese hamster lung and ovary cell lines, respectively, are known to affect the ribosomes of the cells directly. Although mutations in a third gene, emtC, affect the translation apparatus of Chinese hamster peritoneal cells in vitro (Wasmuth et al., Mol. Cell. Biol. 1:58-65, 1981), the molecular product of the emtC locus remains to be determined. To study the molecular basis for genetic complementation among emetine-resistant Chinese hamster cell mutants, we analyzed ribosomal proteins elaborated by complementing, emetine-sensitive hybrid clones (EmtB X EmtA and EmtB X EmtC) and by emetine-resistant clones that segregated from the hybrids. The electrophoretic forms of ribosomal protein S14 (the emtB gene product) elaborated by these clones indicated that the EmtA and EmtC phenotypes are independent of the emtB locus and that the emtA and emtC loci are not chromosomally linked to emtB.

Construction and characterization of Chinese hamster cell EmtA EmtB double mutants

Starting with hybrid cell lines between a Chinese hamster cell EmtA mutant and a Chinese hamster cell EmtB mutant, we have constructed cell lines that are homozygous for mutant alleles at both the emtA locus and the emtB locus, by using a two-step segregation protocol. The EmtA EmtB double mutants are approximately 10-fold more resistant to emetine inhibition than either of the parental mutants. Having both the EmtA mutation and the EmtB mutation expressed in the same cell also results in a level of resistance to cryptopleurine that is significantly higher than a simple additive effect of the two mutations alone. Analysis of ribosomal proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated that a parental hybrid and a first-step segregant, which has lost the wild-type emtA allele, synthesize both a normal and an altered form of ribosomal protein S14, whereas an EmtA EmtB double mutant synthesizes only the altered form of this ribosomal protein. This result confirms that the emtB locus is the structural gene for ribosomal protein S14. Our results also suggest that the products of the emtA and emtB loci interact directly, indicating that the emtA locus, like the emtB locus, encodes a component of the ribosome.

Construction and characterization of Chinese hamster cell EmtA EmtB double mutants

Starting with hybrid cell lines between a Chinese hamster cell EmtA mutant and a Chinese hamster cell EmtB mutant, we have constructed cell lines that are homozygous for mutant alleles at both the emtA locus and the emtB locus, by using a two-step segregation protocol. The EmtA EmtB double mutants are approximately 10-fold more resistant to emetine inhibition than either of the parental mutants. Having both the EmtA mutation and the EmtB mutation expressed in the same cell also results in a level of resistance to cryptopleurine that is significantly higher than a simple additive effect of the two mutations alone. Analysis of ribosomal proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated that a parental hybrid and a first-step segregant, which has lost the wild-type emtA allele, synthesize both a normal and an altered form of ribosomal protein S14, whereas an EmtA EmtB double mutant synthesizes only the altered form of this ribosomal protein. This result confirms that the emtB locus is the structural gene for ribosomal protein S14. Our results also suggest that the products of the emtA and emtB loci interact directly, indicating that the emtA locus, like the emtB locus, encodes a component of the ribosome.

Genetic and biochemical distinction among Chinese hamster cell emtA, emtB, and emtC mutants

Genetic and biochemical experiments have enabled us to more clearly distinguish three genetic loci, emtA, emtB, and emtC, all of which can be altered to give rise to resistance to the protein synthesis inhibitor, emetine, in cultured Chinese hamster cells. Genetic experiments have demonstrated that, unlike the emtB locus, neither the emtA locus nor the emtC locus is linked to chromosome 2 in Chinese hamster cells, clearly distinguishing the latter two genes from emtB. emtA mutants can also be distinguished, biochemically, from emtB and emtC mutants based upon different degrees of cross-resistance to another inhibitor of protein synthesis, cryptopleurine. Two-dimensional gel electrophoretic analysis of ribosomal proteins failed to detect any electrophoretic alterations in ribosomal proteins from emtA or emtC mutants that could be correlated with emetine resistance. However, a distinct electrophoretic alteration in ribosomal protein S14 was observed in an emtB mutant. In addition, the parental Chinese hamster peritoneal cell line of an emtC mutant, and the emtC mutant itself, are apparently heterozygous for an electrophoretic alteration in ribosomal protein L9.

Selective linkage disruption in human-Chinese hamster cell hybrids: deletion mapping of the leuS, hexB, emtB, and chr genes on human chromosome 5

Chinese hamster-human interspecific hybrid cells, which contain human chromosome 5 and express four genes linked on that chromosome, were subjected to selective conditions requiring them to retain one of the four linked genes, leuS (encoding leucyl-tRNA synthetase), but lose another, either emtB (encoding ribosomal protein S14) or chr. Cytogenetic and biochemical analyses of spontaneous segregants isolated by using these unique selective pressures have enabled us to determine the order and regional location of the leuS, hexB, emtB, and chr genes on human chromosome 5. These segregants arise primarily by terminal deletions of various portions of the long arm of chromosome 5. Our results indicate that the order of at least three of these genes is the same on human chromosome 5 and Chinese hamster chromosome 2. Thus, there appears to be extensive homology between Chinese hamster chromosome 2 and human chromosome 5, which represents an extreme example of the conservation of gene organization between very divergent mammalian species. In addition, these hybrids and selective conditions provide a very simple and quantitative means to assess the potency of various agents suspected of inducing gross chromosomal damage.

Biochemical evidence for multiple independent emetine resistance genes in Chinese hamster cells

Hybridization-complementation studies indicated that mutations in multiple genes can render Chinese hamster cells resistant to the alkaloid translation inhibitor emetine. Two of the genes, emtA and emtB, recognized in Chinese hamster lung and ovary cell lines, respectively, are known to affect the ribosomes of the cells directly. Although mutations in a third gene, emtC, affect the translation apparatus of Chinese hamster peritoneal cells in vitro (Wasmuth et al., Mol. Cell. Biol. 1:58-65, 1981), the molecular product of the emtC locus remains to be determined. To study the molecular basis for genetic complementation among emetine-resistant Chinese hamster cell mutants, we analyzed ribosomal proteins elaborated by complementing, emetine-sensitive hybrid clones (EmtB X EmtA and EmtB X EmtC) and by emetine-resistant clones that segregated from the hybrids. The electrophoretic forms of ribosomal protein S14 (the emtB gene product) elaborated by these clones indicated that the EmtA and EmtC phenotypes are independent of the emtB locus and that the emtA and emtC loci are not chromosomally linked to emtB.

Ribosomal protein S14 is altered by two-step emetine resistance mutations in Chinese hamster cells

Four two-dimensional polyacrylamide gel electrophoresis systems were used to identify 78 Chinese hamster cell ribosomal proteins by the uniform nomenclature based on rat liver ribosomal proteins. The 40S ribosomal subunit protein affected by Chinese hamster ovary (CHO) cell one-step emetine resistance mutations is designated S14 in the standard nomenclature. To seek unambiguous genetic evidence for a cause and effect relationship between CHO cell emetine resistance and mutations in the S14 gene, we mutagenized a one-step CHO cell mutant and isolated second-step mutant clones resistant to 10-fold-higher concentrations of emetine. All of the highly resistant, two-step CHO cell mutants obtained displayed additional alterations in ribosomal protein S14. Hybridization complementation tests revealed that the two-step CHO cell emetine resistance mutants were members of the same complementation group defined by one-step CHO cell mutants, EmtB. Two-step mutants obtained from a Chinese hamster lung cell emetine-resistant clone belong to the EmtA complementation group. The two-step and EmtB mutants elaborated 40S ribosomal subunits, which dissociated to 32S and 40S core particles in buffers containing 0.5 M KCl at 4 degrees C. In contrast, 40S ribosomal subunits purified from all EmtA, one-step EmtB EmtC mutants, and wild-type CHO and lung cells were stable at this temperature in buffers containing substantially higher concentrations of salt. Thus, two-step emtB mutations affect the structure of S14 protein directly and the stability of the 40S ribosomal subunit indirectly.

Selective linkage disruption in human-Chinese hamster cell hybrids: deletion mapping of the leuS, hexB, emtB, and chr genes on human chromosome 5

Chinese hamster-human interspecific hybrid cells, which contain human chromosome 5 and express four genes linked on that chromosome, were subjected to selective conditions requiring them to retain one of the four linked genes, leuS (encoding leucyl-tRNA synthetase), but lose another, either emtB (encoding ribosomal protein S14) or chr. Cytogenetic and biochemical analyses of spontaneous segregants isolated by using these unique selective pressures have enabled us to determine the order and regional location of the leuS, hexB, emtB, and chr genes on human chromosome 5. These segregants arise primarily by terminal deletions of various portions of the long arm of chromosome 5. Our results indicate that the order of at least three of these genes is the same on human chromosome 5 and Chinese hamster chromosome 2. Thus, there appears to be extensive homology between Chinese hamster chromosome 2 and human chromosome 5, which represents an extreme example of the conservation of gene organization between very divergent mammalian species. In addition, these hybrids and selective conditions provide a very simple and quantitative means to assess the potency of various agents suspected of inducing gross chromosomal damage.