Translation of immunoglobulin mRNAs in a wheat germ cell-free system

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Cell-free synthesis of the myelin basic proteins in a wheat germ system programmed with brain messenger RNA

Poly A(+) messenger RNA (mRNA) was isolated from the brains of 3-week-old mice and translated in a cell-free system derived from wheat germ. Maximal stimulation of the system by brain mRNA was observed at a relatively low K+ concentration (45 mM) and low mRNA concentration (1-10 microgram/ml). The translational system was dependent on an energy-generating system and stimulated by the addition of spermidine and transfer RNA. Under optimal conditions, incorporation was linear for almost 45 min, but the overall stimulation with brain mRNA was relatively low (about twofold). In spite of the low stimulation, analysis of the translation products indicated that in the presence of brain mRNA polypeptides which co-chromatographed and co-electrophoresed with the two mouse myelin basic proteins could be detected. In control experiments with liver poly A(+) mRNA, which stimulated the translational system to a greater extent than brain mRNA, no such polypeptides could be detected. In this system the ratio of synthesis of small myelin basic protein to large myelin basic protein was found to be about 4.0, which correlates well with that found in vivo.

Induction of rat liver metallothionein mRNA and its distribution between free and membrane-bound polyribosomes

Total, membrane-bound and free polyribosomes were purified from livers of Zn2+-treated and control rats. Polyadenylated RNA was separated from the polyribosomal RNA extracts by oligo(dT)--cellulose chromatography and translated in a wheat-germ cell-free translation system. Newly synthesized 35S-labelled metallothionein was isolated from the other [35S]methionine-labelled translation products by activated-thiol--Sepharose 4B chromatography. The purity of the 35S-labelled metallothionein product was substantiated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Zinc administration resulted in an elevation of metallothionein mRNA activity to 11% of the total polyribosomal mRNA activity. The vast majority of biologically active metallothionein mRNA was localized in the free polyribosomal pool, at least 94% and 97% in control and zinc-treated rats respectively. The increase in the percentage of polyribosomal mRNA coding for metallothionein after zinc administration was 3-fold, whether measured directly in total polyribosomal mRNA or as a combination derived from membrane-bound and free polyribosomal mRNA. These data indicate that the induction of metallothionein mRNA by zinc involves only free polyribosomes and suggest that the function of metallothionein is limited to intracellular processes.

Synthesis of albumin and malic enzyme in wheat-germ lysates and Xenopus laevis oocytes programmed with chicken-liver messenger RNA

Undegraded, biologically-active, polyadenylated RNA was isolated from chicken liver by a rapid, simple procedure. Liver cells were dispersed mechanically and then broken gently by controlled Dounce homogenization in the absence of detergent or ribonuclease inhibitors. After removing lysosomes and mitochondria by centrifugation, RNA was precipitated at pH 5.2. Polyadenylated mRNA was isolated directly from the detergent-solubilized precipitate by oligo(dT)-cellulose chromatography. The resulting RNA was translated into liver-specific peptides in both the wheat germ lysate and Xenopus laevis oocytes. Translatable albumin mRNA was detected in the liver cytoplasm of both fed 3-week-old chicks and unfed, day-old chicks. Translatable malic enzyme mRNA was only detected in the livers from the fed chicks.

Messenger RNA for the insect storage protein calliphorin: in vitro translation and chromosomal hybridization analyses of a 20 S poly(A)-RNA fraction

A major poly(A)-containing RNA fraction of the approximate size expected of a monocistronic mRNA for the storage protein calliphorin has been isolated from the larval fat bodies of Calliphora vicina during early instar 3. This 20 S RNA fraction programs the synthesis by cell-free wheat embryo extracts of polypeptides of 86,000 daltons identified by tryptic peptide fingerprinting as precursors of the authentic calliphorin subunits of 83,000 daltons. Complementary DNA synthesized by AMV reverse transcriptase using the same 20 S RNA as template hybridized in situ to a single segment of one or two bands in the salivary polytene chromosomes of C. vicina.

Production and properties of vitamin-D-induced mRNA for chick calcium-binding protein

1. Calcium-binding protein synthesis on chick intestinal polysomes is induced within 2 h of injecting vitamin-D-deficient birds with 1,25-dihydroxycholecalciferol. The induction is short-lived: the maximum output of the binding protein is reached by 13 h after hormone injection, and declines rapidly thereafter. 2. This induction of calcium-binding protein synthesis occurs by the production of active mRNA for the protein. The sedimentation coefficient of this mRNA in denaturing conditions is 18 S, equivalent to a molecular weight of approximately 700000, and the molecule contains a tract of polyadenylate. 3. Both polysomal and poly(A)-containing RNA extracted from intestinal polysomes stimulate the synthesis of a range of proteins (up to 70000 molecular weight) by the wheat germ cell-free system. Immunoprecipitable calcium-binding protein is translated from RNA obtained from 1,25-dihydroxycholecalciferol-dosed birds but not from control birds. This calcium-binding protein is the same size (27000 molecular weight) as authentic chick calcium-binding protein; No other proteins are specifically precipitated by the antiserum. Thus in chickens 1,25-dihydroxycholecalciferol-induced calcium-binding protein is not synthesised via any precursor molecule. The implications of this result are discussed.

Modified nucleosides and 5'-end groups in purified mouse immunoglobulin light chain mRNA and rabbit globin mRNA detected by borohydride labelling

The borohydride reaction has been used to investigate modified nucleosides and end groups in purified immunoglobulin light chain mRNA and rabbit globin mRNA. 1. The light chain mRNA was isolated from the microsomal fraction of MOPC 41A mouse myelomas, which secrete kappa chains, by two cycles of oligo(dT) cellulose chromatography and glycerol gradient centrifugation. The 12 S mRNA was active in a Krebs II ascites cell-free system and appeared to be homogenous as judged by gradient centrifugation, polyacrylamide gel electrophoresis in 98% formamide and fingerprint analysis of 125I-labelled mRNA. 2. End group labelling of the light chain and globin mRNAs by oxidation with periodate and reduction with boro[3H]hydride showed that the RNAs have a 5'-terminal 7-methyl guanosine in 5'-pyrophosphate linkage with the next nucleoside. 3. To detect any modified residues in the interior of chains, nucleosides in complete digests of the mRNAs were converted by the borohydride reaction to 3H-labelled nucleoside trialcohols, which were fractionated by two dimensional chromatography (the Randerath technique). The light chain mRNA was found to contain N6-methyl adenosine (1 mole) but the rabbit globin mRNAs lacked this nucleoside. Deficiencies in this technique for analysis of minor constituents in large RNAs were noted.

Isolation and characterization of purified rat casein messenger ribonucleic acids

Purification of casein messenger ribonucleic acids (mRNAs) from lactating rat mammary gland RNA has been accomplished by a combination of sizing techniques, including Sepharose 4B chromatography and preparative agarose-urea gel electrophoresis, and affinity chromatography of poly(adenylic acid)-containing mRNA on oligo(dT)-cellulose. The separation of the individual casein mRNAs into discrete molecular species free of apparent ribosomal RNA contaminants was facilitated by the use of denaturing conditions either prior to or during each of the fractionation procedures. Two casein mRNA fractions were isolated: (1) a 15S mRNA doublet which directed the synthesis of the two largest rat caseins in the wheat-germ, cell-free, translation assay, and (2) a 12S mRNA which migrated as a single species during agarose-urea gel electrophoresis and directed the in vitro synthesis of the smallest of three rat caseins. These mRNAs had apparent molecular weights of 450 000 +/- 30 000 and 320 000 +/- 25 000 and contained poly(adenylic acid) sequences at their 3' termini ranging from 15 to 150 residues with number average lengths of 42 and 38 adenosines, respectively. The purity of the isolated casein mRNA'S was determined both by agarose-urea gel electrophoresis and by a careful comparison of the total products synthesized in the wheat-germ translation assay with those recognized by a specific casein antibody using an indirect immunoprecipitation technique. The specificity of the indirect immunoassay procedure was demonstrated by the selective displacement by purified rat casein of greater than 95% of the radioactive product synthesized in the cell-free system. Under optimal translation conditions for casein mRNA, at least 90% of the released protein synthesized in response to the 15S casein mRNA was specifically immunoprecipitable, representing a 178-fold purification compared with the initial RNA extract. Using these techniques a comparable purification was also obtained for a 15S mouse casein mRNA fraction. Finally, an analysis by fluorography on 5-20% (w/v) polyacrylamide gradient slab gels of the total proteins synthesized in response to both the 15S and 12S casein mRNAs revealed a close correspondence with those proteins which were specifically immunoprecipitated.

Light-induced enzyme synthesis in cell suspension cultures of Petroselinum hortense. Demonstration in a heterologous cell-free system of rapid changes in the rate of phenylalanine ammonia-lyase synthesis

The conditions for protein synthesis in vitro with polyribosomes from cell suspension cultures of parsel (Petroselinum hortense) and a wheat-germ extract were investigated. Two different criteria were used as estimated of the translational activity: (a) the total rate of incorporation of [35S]methionine into acid-insoluble material; (b) the ratio of large (molecular weight greater than 25000) to small (molecular weight less than 25000) peptide products. Depending on which of the criteria was employed, the pH optimum and the optimal concentrations for Tris=acetate, magnesium acetate, KCL, methionine and the wheat-germ extract differed considerably. The translational activity of the polyribosomes (both criteria) was effciently protected by 0.1 M Mg2+ against degradation during the isolation procedure. The rate of synthesis of phenylalanine ammonia-lyase in vitro with the polyribosomes was determined by measuring the incorporation rate of L-[35S]methionine into protein which was precipitable by a rabbit antiserum prepared for the purified enzyme. The immunoprecipitate was analyzed by disc gel electrophoresis in the presence of dodecylsulfate and was shown to contain small amounts of the complete enzyme subunits and relatively large amounts of shorter peptides which were also characteristic for the enzyme. The time course of light-induced changes in the rate of phenylalanine ammonia-lyase synthesis in vitro were investigated during a period of 15 h under two different conditions of induction: the cell cultures were irradiated with ultraviolet light eith (A) continuously or (B) for 2.5 h and then returned to darkness. Although the highest rate of enzyme synthesis was observed somewhat later inexperiment A than in experiment B, the periods of time during which the rate of synthesis increased rapidly were limited in both cases to only a few hours. The results obtained in vitro were identical within the limits of the experimental error with theoretical calculations of the changes in the rate constant of phenylalanine ammonia-lyase synthesis in vivo. These changes were calculated from the corresponding curves for the changes in the enzyme activity under the conditions of induction. The results are in agreement with previous observations suggesting that the induction of phenylalanine ammonia-lyase by light in the parsley cells was a short-term effect whose efficiency was greatly reduced within the 15 h of experimentation, even under continuous irradiation.

Partial amino-acid sequence of the precursor of an immunoglobulin light chain containing NH2-terminal pyroglutamic acid

Analyses of amino-acid sequences of the total cell-free products programmed by the mRNA of MOPC-104E gamma light (L)-chain show that over 95% of the products have sequences of a distinct protein that correspond to the L-chain precursor. In this precursor an extra piece is coupled to the NH2-terminus of the mature L-chain. Analyses of products labeled with [3H]alanine, [3H]leucine, and [3H]proline demonstrate that the extra piece is composed of at least 18 residues. Analyses of [35S]methione-labeled product indicate that the extra piece may contain an additional NH2-terminal methionine, which is detected in about 10% of the molecules. Partial recovery of the NJ2-terminal methionine (alanine, leucine, and proline are recovered in yields close to theoretical, greater than 95%) suggests that it is the initiator methionine, which is known to be short lived in eukaryotes due to rapid hydrolysis. Thus, the extra piece seems to be 19 residues in length, and it contains one methionine at the NH2-terminus, three alanines at positions 2, 12, and 17, and five leucines at positions 6, 8, 10, 11, and 13. The close gathering of leucine residues, as well as their abundance (26%), suggest that the extra piece would be quite hydrophobic. Hydrophobicity seems to be a general property of the extra piece, since similar clusters of leucine were found in the precursors of 3 KL-chains (Burstein, Y. & Schechter, I. (1976) Biochem. J. 157, 145-151). The NH2-terminus of the mature MOPC-104E gamma L-chain is blocked by pyroglutamic acid. The fact that in the precursor a peptide segment precedes this NH2-terminus establishes that pyroglutamic acid is not the initiator residue for synthesis of the L-chain. Apparently, the pyroglutamic acid is formed by cyclization of glutamic acid or glutamine during cleavage of the extra piece to yield the mature L-chain.

Synthesis of the brain-specific S-100 protein in a cell-free system from wheat embryo programmed with poly(A)-containing RNA from rabbit brain

Polyadenylated polysomal RNA was prepared from rabbit cerebral hemispheres using phenol extraction and chromatography on oligo(dT)-cellulose. This RNA directed the synthesis of the brain-specific S-100 protein in cell-free extracts from wheat embryo. S-100 protein was absent from the products of endogenous incorporation and from a reaction programmed with kidney mRNA. These results suggest that S-100 protein mRNA contains a poly(adenylic acid) sequence and rule out the necessity of a brain-specific factor for translation of -S100 protein mRNA.

Amino acid-sequence variability at the N-terminal extra piece of mouse immunoglobulin light-chain precursors of the same and different subgroups

The proteins programmed in the wheat-germ cell-free system by the mRNA coding for the MOPC-63 mouse myeloma L (light) chain were labelled with six radioactive amino acids: [35S]methionine, [4,5-3H]leucine, [3,4-3H]proline, [3-3H]serine, [4,5-3H]isoleucine or [2,3-3H]alanine. Amino acid-sequence analyses showed that over 90% of the total cell-free product was one homogeneous protein, which corresponds to the MOPC-63 L-chain precursor. In this precursor an extra piece, 20 amino acid residues in length, precedes the N-terminus of the mature L chain. The extra piece contains one methionine residue at the N-terminus, six leucine residues, which are clustered in two triplets at positions 6, 7, 8 and 11, 12, 13, one proline residue at position 16, and one serine residue at position 18. The closely gathered leucine residues, as well as their abundance (30%), suggest that the extra-piece moiety is hydrophobic. In the precursors, the extra piece is coupled to the variable region of the L chain. Partial sequences of precursors of L chains of the same and different subgroups that were labelled with the above six radioactive amino acids indicate that the extra piece is part of the variable region. Thus the precursors of MOPC-63 and MOPC-321 L chains, which are of the same subgroup, have extra pieces of identical size (20 residues), and so far their partial sequences are also identical (see above). On the other hand, in the precursor of MOPC-41 L chain, which is of a different subgroup, the extra piece is 22 residues in length. Further, the sequence of the MOPC-41 extra piece differs in at least ten positions from sequences of the extra pieces of the precursors of MOPC-63 and MOPC-321 L chains.

Identification of N-terminal methionine in the precursor of immunoglobulin light chain. Initiation of translation of messenger ribonucleic acid in plants and animals

The proteins programmed in the wheat-germ cell-free system by the mRNA coding for the MOPC-321 mouse myeloma L (light) chain were labelled with [35S]methionine, [4,5-3H]leucine or [3-3H]serine, and were subjected to amino acid-sequence analyses. Over 95% of the total cell-free product was sequenced as one homogeneous protein, which corresponds to the precursor of the L-chain protein. In the precursor, 20 amino acid residues precede the N-terminus of the mature protein. This extra piece contains one methionine residue at the N-terminus, one serine residue at position 18, and six leucine residues, which are clustered in two triplets at positions 6, 7, 8 and 11, 12, 13. The identification of methionine at the N-terminus of the precursor is in agreement with the evidence showing that unblocked methionine is the initiator residue for protein synthesis in eukaryotes. The absence of methionine at position 20, which precedes the N-terminal residue of the mature protein, suggests that myeloma cells synthesize the precursor. However, within the cell the precursor should be rapidly processed to the mature L chain, since precursor molecules have not yet been found in the intact animal. The abundance (30%) of leucine residues indicates that the extra-piece moiety is quite hydrophobic. The extra piece of the MOPC-321 L-chain precursor synthesized with the aid of the Krebs II ascites cell-free system is of identical size and it has the same leucine sequence [Schechter et al. (1975) Science 188, 160-162]. This indicates that cell-free systems derived from the plant and animal kingdom initiate mRNA translation from the same point. It is shown that the amino acid sequence of minute amounts of a highly labelled protein (0.1 pmol) can be faithfully determined in the presence of a large excess (over 2000 000-fold) of unrelated non-radioactive proteins.

Transfer of proteins across membranes. I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma

Fractionation of MOPC 41 DL-1 tumors revealed that the mRNA for the light chain of immunoglobulin is localized exclusively in membrane-bound ribosomes. It was shown that the translation product of isolated light chain mRNA in a heterologous protein-synthesizing system in vitro is larger than the authentic secreted light chain; this confirms similar results from several laboratories. The synthesis in vitro of a precursor protein of the light chain is not an artifact of translation in a heterologous system, because it was shown that detached polysomes, isolated from detergent-treated rough microsomes, not only contain nascent light chains which have already been proteolytically processed in vivo but also contain unprocessed nascent light chains. In vitro completion of these nascent light chains thus resulted in the synthesis of some chains having the same mol wt as the authentic secreted light chains, because of completion of in vivo proteolytically processed chains and of other chains which, due to the completion of unprocessed chains, have the same mol wt as the precursor of the light chain. In contrast, completion of the nascent light chains contained in rough microsomes resulted in the synthesis of only processed light chains. Taken together, these results indicate that the processing activity is present in isolated rough microsomes, that it is localized in the membrane moiety of rough microsomes, and, therefore, that it was most likely solubilized during detergent treatment used for the isolation of detached polysomes. Furthermore, these results established that processing in vivo takes place before completion of the nascent chain. The data also indicate that in vitro processing of nascent chains by rough microsomes is dependent on ribosome binding to the membrane. If the latter process is interfered with by aurintricarboxylic acid, rough microsomes also synthesize some unprocessed chains. The data presented in this paper have been interpreted in the light of a recently proposed hypothesis. This hypothesis, referred to as the signal hypothesis, is described in greater detail in the Discussion section.

Cell-free translation of immunoglobulin messenger RNA from MOPC-315 plasmacytoma and MOPC-315 NR, a variant synthesizing only light chain

Total poly(A)-containing mRNA was isolated from the MOPC-315 and MOPC-315 NR plasmacytomas. The RNA was further fractionated on sodium dodecyl sulfate-sucrose gradients. The MOPC-315 mRNA fractions directed the synthesis of both the heavy chain and light chain precursor of the MOPC-315 IgA protein in a cell-free extract of Ehrlich ascites tumor cells. None of the MOPC-315 NR mRNA fractions tested programmed the synthesis of the heavy chain in this system. Analysis of cell-free products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by immunoprecipitation demonstrated that no translatable heavy chain mRNA could be extracted from the MOPC-315 NR variant plasmacytoma.