mRNA-dependent synthesis of authentic precursor to human placental lactogen: conversion to its mature hormone form in ascites cell-free extracts

The chemistry and enzymology of the type I signal peptidases

The discovery that proteins exported from the cytoplasm are typically synthesized as larger precursors with cleavable signal peptides has focused interest on the peptidases that remove the signal peptides. Here, we review the membrane-bound peptidases dedicated to the processing of protein precursors that are found in the plasma membrane of prokaryotes and the endoplasmic reticulum, the mitochondrial inner membrane, and the chloroplast thylakoidal membrane of eukaryotes. These peptidases are termed type I signal (or leader) peptidases. They share the unusual feature of being resistant to the general inhibitors of the four well-characterized peptidase classes. The eukaryotic and prokaryotic signal peptidases appear to belong to a single peptidase family. This review emphasizes the evolutionary concepts, current knowledge of the catalytic mechanism, and substrate specificity requirements of the signal peptidases.

Intracellular traffic of newly synthesized proteins. Current understanding and future prospects

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Expression of different-sized placental alkaline phosphatase mRNAs in placenta and choriocarcinoma cells

The expression of human placental-type alkaline phosphatase (ALPase) in the placenta and in three choriocarcinoma cell lines was examined by translation in vitro and RNA blot analysis using a cDNA for placental ALPase. Placental RNA directed the synthesis of two polypeptides that could be immunoprecipitated with antiserum to placental ALPase. Translation of RNA from the choriocarcinoma cell lines, with or without sodium butyrate treatment, yielded a single immunoprecipitable product of molecular weight intermediate between those of the products from the placenta mRNA. Two cDNA clones for placental ALPase were isolated by antibody screening of a placental cDNA library constructed in lambda gt11. The overlapping cDNAs include 462 nucleotides of coding sequence. RNA blot analysis has confirmed that induction of placental-type ALPase levels during placental development is accompanied by an increase in steady-state placental ALPase mRNA concentrations. Examination of the mRNAs revealed a placental ALPase mRNA of 3.0 kilobases (kb) and a distinct choriocarcinoma placental-type ALPase mRNA of 2.6 kb, implying that transformation of normal to malignant trophoblast is associated with the expression of a distinct placental-type ALPase gene transcript and its protein product.

Translation of rat intestinal RNA yields two alkaline phosphatases

After translation of total rat intestinal RNA, immunoprecipitation using monospecific antiserum against rat intestinal alkaline phosphatase yielded two polypeptides in the adult duodenum and jejunum (molecular masses 62 and 65 kDa). Immunoprecipitation of both bands was blocked by a single purified alkaline phosphatase. In the adult ileum and in the entire small intestine of suckling pups, only the 62 kDa translation product was found. After fat feeding, translated alkaline phosphatase increased by an amount proportionate to the increase in enzyme activity previously seen in the serum. A small fraction of nascent alkaline phosphatase was translocated into microsomal vesicles, producing peptides of 65 and 69 kDa. Tunicamycin-treated membranes demonstrated a different signal peptide for each translation product. N-Terminal sequencing of the translation products showed leucine residues at similar positions, but overlap with the mature protein sequence was not demonstrated. On the basis of these data, we propose the presence of two mRNAs encoding alkaline phosphatase in the rat intestine.

Proteolytic processing and compartmentalization of the primary translation products of mammalian apolipoprotein mRNAs

The steps involved in the initial assembly of apolipoproteins and lipids into supramolecular arrays (nascent lipoprotein particles) are largely unknown. Examination of the proteolytic processing and compartmentalization of the primary translation products of apolipoprotein mRNAs represents one approach to deciphering the molecular details of lipoprotein assembly. The structures of the primary translation products of seven mammalian apolipoprotein mRNAs has been determined in the past several years. The organization of apolipoprotein signal peptides is typical of eukaryotic prepeptides, although an unusual degree of sequence conservation is present among the signal segments of apo AI, AIV, and E. For those apolipoprotein sequences studied in detail, SRP-dependent cotranslational translocation and proteolytic processing appears to be highly efficient and results in sequestration of the processed protein within the lumen of the endoplasmic reticulum (ER). However the mechanism by which these lipid-binding proteins avoid arrest during their translocation through the lipid bilayer of the ER membrane remains obscure. The two principal human HDL apolipoproteins undergo novel extracellular post-translational proteolytic processing, which results in removal of nonhomologous propeptides. The proteases responsible for proapo AI and AII processing appear to be different. The processing of these proapolipoproteins provides a potential series of steps for regulating the ordered assembly of HDL constituents.

Protein translocation across and integration into membranes

This review concentrates mainly on the translocation of proteins across the endoplasmic reticulum membrane and cytoplasmic membrane in bacteria. It will start with a short historical review and will pinpoint the crucial questions in the field. Special emphasis will be given to the present knowledge on the molecular details of the first steps, i.e., on the function of the signal recognition particle and its receptor. The knowledge on the signal peptidase and the ribosome receptor(s) will also be summarized. The various models for the translocation of proteins across and the integration of proteins into membranes will be critically discussed. In particular, the function of signal, stop-transfer, and insertion sequences will be dealt with and molecular differences discussed. The cotranslational mode of membrane transfer will be compared with the post-translational transport found for mitochondria and chloroplasts. This review will conclude with open questions and an outlook.

Partial purification of microsomal signal peptidase from hen oviduct

Signal peptidase has been purified approximately 600-fold from hen oviduct microsomes. Treatment of microsomes with ice-cold sodium carbonate at pH 11.5 removes soluble and extrinsic membrane proteins prior to solubilization of signal peptidase with Nonidet P-40. After dialysis to pH 8.2, the solubilized enzyme is chromatographed on diethylaminoethyl cellulose at pH 8.2. More than 90% of contaminating proteins bind to the column while signal peptidase and endogenous phospholipid are eluted in the column void volume. Enzyme activity subsequently binds to carboxymethyl cellulose at pH 5.8 and is eluted by approximately 100 to 200 mM NaCl during a NaCl gradient. Polypeptides present in partially purified hen oviduct signal peptidase have relative molecular masses ranging from 54 kD to less than 11 kD with major bands at 29, 23, 22, 19, 18 and 13 kD. The purified peptidase requires phospholipid for activity and is maximally active in the presence of 2 mg/ml phosphatidylcholine.

Structural requirements for sulfation of asparagine-linked oligosaccharides of lutropin

Human and bovine pituitary glycoprotein hormones (lutropin, follitropin, and thyrotropin) contain varying amounts of N-acetylgalactosamine and sulfate. The sulfate on asparagine-linked oligosaccharides of bovine lutropin (bLH) is present exclusively on GalNAc in the sequence GalNAc(beta 1-4)GlcNAc(beta 1-2)Man alpha. We have examined the structural requirements for sulfation of bLH oligosaccharides by using a reconstituted cell-free system. After cleavage from the protein, oligosaccharides containing the sequence GalNAc(beta 1-4)Glc-NAc(beta 1-2)Man alpha were sulfated by enzymes in pituitary membranes. Addition of one or two sulfates was observed, depending upon the number of GalNAc acceptor sites on the oligosaccharide. Neither GalNAc alone nor oligosaccharides devoid of GalNAc were sulfated. Membranes from placenta or liver did not sulfate oligosaccharides released from bLH, indicating that the sulfating activity is pituitary-specific. The lack of peptide dependence for sulfation, in conjunction with the oligosaccharide specificity, suggests that the sequence GalNAc(beta 1-4)GlcNAc(beta 1-2)Man alpha contains the recognition signal for the sulfotransferase(s).

Segregation of the polypeptide translocation apparatus to regions of the endoplasmic reticulum containing ribophorins and ribosomes. I. Functional tests on rat liver microsomal subfractions

A preparation of rat liver microsomes containing 70% of the total cellular endoplasmic reticulum (ER) membranes was subfractionated by isopycnic density centrifugation. Twelve subfractions of different ribosome content ranging in density from 1.06 to 1.29 were obtained and analyzed with respect to marker enzymes, RNA, and protein content, as well as the capacity of these membranes to bind 80S ribosomes in vitro. After removal of native polysomes from these microsomal subfractions by puromycin in a buffer of high ionic strength their capacity to rebind 80S ribosomes approached levels found in the corresponding native membranes before ribosome stripping. This indicates that in vitro rebinding of ribosomes occurs to the same sites occupied in the cell by membrane-bound polysomes. Microsomes in the microsomal subfractions were also tested for their capacity to effect the translocation of nascent secretory proteins into the microsomal lumen utilizing a rabbit reticulocyte translation system programmed with mRNA coding for the precursor of human placental lactogen. Membranes from microsomes with the higher isopycnic density and a high ribosome content showed the highest translocation activity, whereas membranes derived from smooth microsomes had only a very low translocation activity. These results indicate the membranes of the rough and smooth portions of the endoplasmic reticulum are functionally differentiated so that sites for ribosome binding and the translocation of nascent polypeptides are segregated to the rough domain of the organelle.

DNA-binding proteins of human placenta: purification and characterization of an endonuclease

DNA binding proteins present in the cytoplasm and nuclei of term placenta were isolated by DNA-cellulose chromatography and analysed by electrophoresis in high resolution polyacrylamide gradient gels. A denatured DNA specific protein of approximate molecular weight 34 000 daltons was the predominant DNA binding protein of the cytoplasm; this protein consisted of over 65% of the total DNA binding proteins of the 0.15 M NaCl eluate of the cytoplasm. The cytoplasmic extracts contained two additional DNA binding proteins of molecular weight 24 000 and 18 000 daltons and these proteins bound preferentially to ds DNA. All the three DNA binding proteins were also present in the nuclei and electrophoresis of histones in adjacent lanes indicated that they are not histones. The 34 000-dalton DNA binding protein has been purified by ammonium sulphate fractionation followed by phosphocellulose (PC) chromatography. The DBP eluted from the PC column between 0.125-0.15 M potassium phosphate. PC fractions containing electrophoretically pure 34 KD DBP showed an endonuclease activity capable of converting plasmid pBR 322 DNA to the linear form. Maximum endonucleolytic activity was observed in the presence of 3-5 mM Mg2+ and the enzyme activity was completely inhibited by 3 mM ethylenediamine tetraacetate.

Design and synthesis of a consensus signal sequence that inhibits protein translocation into rough microsomal vesicles

Most signal sequences are found to vary considerably in length and primary sequence, but possess some common structural features. Analysis of known signal sequences has led to the design of a 19-residue sequence that, although not a naturally occurring signal, possesses the structural features that commonly occur in pre-proteins. This peptide has been synthesized by solid-phase methods, and has been shown to inhibit, in a concentration-dependent manner, the processing in vitro of nascent pre-prolactin, pre-forms of pancreatic digestive enzymes, and pre-placental lactogen. The peptide acts at the cytoplasmic surface of microsomal vesicles added to the protein translation system, preventing translocation of the nascent chains to the lumenal space of vesicles where signal peptidase normally cleaves to remove the signal from nascent pre-proteins.

Sulfation of lutropin oligosaccharides with a cell-free system

Sulfate is covalently linked to the oligosaccharides on the alpha and beta subunits of bovine lutropin (luteinizing hormone; LH) but not to those on human chorionic gonadotropin (hCG). Since the amino acid sequences of the pituitary and placental alpha subunits are homologous, comparison of their asparagine-linked sugars can provide information regarding tissue specificity of oligosaccharide maturation. To characterize this post-translational modification, we have developed a reconstituted cell-free sulfation system. Sulfate is incorporated into exogenously added glycoproteins by sulfotransferases from Triton X-100-lysed Golgi membranes in the presence of 3'-phosphoadenosine 5'-phospho[35S]sulfate, which is generated from [35S]sulfate by a ribosome-free supernate from Krebs ascites tumor cells. LH is sulfated by pituitary and liver membranes but not by those from placenta. Desialylated hCG (AshCG) is sulfated by membranes from placenta and pituitary, but not liver, while hCG is not sulfated by any of these membranes. Endoglycosidase F releases all the incorporated sulfate from LH in the form of a heterogeneous mixture of mono- and disulfated oligosaccharides. In contrast, the sulfate added to AshCG is apparently attached to peptide rather than oligosaccharide. As found with the cell-free system, sulfate metabolically incorporated into LH by pituitary cells is present on a heterogeneous population of mono- and disulfated oligosaccharides. Thus the cell-free sulfation system accurately duplicates the in vivo process.

Independent regulation by sodium butyrate of gonadotropin alpha gene expression and cell cycle progression in HeLa cells

Sodium butyrate alters the growth and gene expression of a variety of differentiating and neoplastic cell types. For example, addition of 5 mM butyrate to HeLa cells is reported to both induce gonadotropin alpha subunit biosynthesis and block cell cycling in G1. We have studied these two actions of butyrate on HeLa cells and found that they are regulated in distinct ways. The induction of alpha subunit synthesis was due to an increase in the rate of transcription of the alpha gene. Using synchronized populations of HeLa cells, we determined that butyrate stimulates alpha transcription throughout the cell cycle. In contrast, treated cells arrest in G1 only if exposed to butyrate for a discrete period during the previous S phase. We conclude that butyrate inhibits DNA synthesis through a cell cycle-specific action that is independent from its direct action to stimulate transcription of the gonadotropin alpha gene.

Independent regulation by sodium butyrate of gonadotropin alpha gene expression and cell cycle progression in HeLa cells

Sodium butyrate alters the growth and gene expression of a variety of differentiating and neoplastic cell types. For example, addition of 5 mM butyrate to HeLa cells is reported to both induce gonadotropin alpha subunit biosynthesis and block cell cycling in G1. We have studied these two actions of butyrate on HeLa cells and found that they are regulated in distinct ways. The induction of alpha subunit synthesis was due to an increase in the rate of transcription of the alpha gene. Using synchronized populations of HeLa cells, we determined that butyrate stimulates alpha transcription throughout the cell cycle. In contrast, treated cells arrest in G1 only if exposed to butyrate for a discrete period during the previous S phase. We conclude that butyrate inhibits DNA synthesis through a cell cycle-specific action that is independent from its direct action to stimulate transcription of the gonadotropin alpha gene.

Biosynthesis and compartmentalization of rat-intestinal vitamin-D-dependent calcium-binding protein

We have purified the primary translation product of rat intestinal vitamin-D-dependent calcium-binding protein mRNA from wheat germ and ascites cell-free systems. We show that calcium-binding protein is neither synthesized as a larger percursor nor likely to be exported from the intestinal epithelium. Our conclusions are based on the following observations. (1) The primary translation product, NH2-terminally labeled with formyl[35S]methionine, comigrates with the mature cytoplasmic protein during electrophoresis through denaturing gels. (2) It does not possess a cleavable signal peptide sequence or internal signal equivalent as judged by co- and post-translational cleavage assays in vitro. (3) The NH2 terminus of the cell-free product is acetylated. (4) Comparison of the NH2-terminal amino acid sequences of the primary translation product and cyanogen bromide peptides obtained from the blocked, purified cytoplasmic protein. The kinetics of calcium-binding protein mRNA accumulation and decay in rachitic intestinal epithelium after primary and secondary stimulation with 1,25-dihydroxycholecalciferol (calcitriol) were studied using the cell-free translation system. The results are reminiscent of other steroid-hormone-inducible systems. Both the rate of mRNA accumulation and the peak response were greater after secondary stimulation.

Synthetic leader peptide modulates secretion of proteins from microinjected Xenopus oocytes

To investigate the role of the leader peptide in modulating secretion from living cells, we injected a synthetic peptide into Xenopus oocytes. The peptide consisted of the NH2-terminal leader sequence of mouse immunoglobulin light chain precursor. We found that the leader peptide has two different roles in regulating secretion from the oocytes. First, it competitively inhibits the synthesis of secretory and membrane proteins but not of cytoplasmic proteins. The inhibition occurs both with oocyte proteins and with proteins directed by coinjected myeloma mRNA. The inhibition reaches a maximum 2 hr after injection and decays within 3 hr. It appears to be mediated through the cell membrane, because 125I-labeled leader peptide segregates into the membrane fraction of microinjected oocytes simultaneously with the interference with methionine incorporation. A second role of the microinjected leader peptide is to induce a rapid acceleration in the rate of export of secretory proteins from the oocyte. The maximal enhancement effect is obtained upon injection of 50 ng of leader peptide per oocyte. It is not merely due to the small size, negative charge, or hydrophobicity of the peptide, because enhanced secretion does not occur when glucagon, poly-L-glutamic acid, or Triton X-100 is injected. Furthermore, immunoreaction of the peptide with specific antibodies prior to microinjection prevents the accelerated export. Our observations indicate that in Xenopus oocytes, the leader peptide is involved in both translocation and later step(s) in the secretory pathway.

Cleavage of human placental lactogen precursor by an enzyme from microbial membranes

When membranes from Escherichia coli MRE 600 are added to an in vitro translation system, they are able to cleave correctly prehuman placental lactogen to yield the mature hormone. The protein was identified by SDS/polyacrylamide-gel electrophoresis and by determining its amino acid sequence. These studies were aided by the development of a new method for the separation of human placental lactogen from its precursor.

Cloning and sequence of several alpha 2u-globulin cDNAs

We describe a simple cloning procedure for alpha 2u-globulin that requires neither enrichment of mRNA for cloning nor purification of a specific probe for screening recombinant colonies. Total adult male liver poly(A)+RNA was used as template for cloning, and the subsequent recombinant colonies were screened by comparing hybridization to radioactive cDNA probes prepared from hepatic male and female mRNA, respectively. Almost all of the selected "male-specific" clones were later shown to contain alpha 2u-globulin sequences. This cloned alpha 2u-globulin cDNA has been shown to specifically hybridize to male rat liver RNA, which, when isolated and translated in vitro, codes for a 21,000-dalton protein (pro-alpha 2u-globulin) immunologically identical to alpha 2u-globulin. When translation occurs in the presence of pancreatic microsomes this in vitro synthesized pro-alpha 2u-globulin is processed to the 19,000-dalton mature form of alpha 2u-globulin. The nucleotide sequence of the alpha 2u-globulin cDNA has been determined, thus elucidating the complete amino acid sequence of alpha 2u-globulin and most of the hydrophobic "leader" sequence of pro-alpha 2u-globulin. The amino acid sequence deduced from the cDNA is in agreement with the partial sequence that we previously determined by sequential Edman degradation of the purified protein. alpha 2u-Globulin cDNA clones contain within the 3'-untranslated region one or both of the two putative polyadenylylation/transcription termination sites (A-A-T-A-A-A and A-A-T-T-A-A-A). Either of these can be used, generating alpha 2u-globulin mRNA species of two lengths. A codon usage analysis of the cDNA showed that, although all six leucine codons are used for the 14 leucine residues in mature alpha 2u-globulin, the seven leucines in the partial leader sequence reported are all encoded by the same codon, CTG. The primary amino acid sequence contains a unique Asn-Gly-Ser sequence, likely to be in beta-turn conformation, as the probable site of glycosylation for this glycoprotein.

Purification of a membrane-associated protein complex required for protein translocation across the endoplasmic reticulum

The capacity of microsomal membranes to translocate nascent presecretory proteins across their lipid bilayer can be largely abolished by extracting them with high ionic strength buffers. It can be reconstituted by adding the salt extract back to the depleted membranes [Warren, G. & Doberstein, B. (1978) Nature (London) 273, 569-571]. Utilizing hydrophobic chromatography, we purified to homogeneity a protein component of the salt extract that reconstitutes the translocation activity of the extracted membranes. This component behaves as a homogeneous species upon gel filtration, ion-exchange chromatography, adsorption chromatography, and sucrose-gradient centrifugation. When examined by polyacrylamide gel electrophoresis in NaDodSO4, six polypeptides with apparent Mr of 72,000, 68,000, 54,000, 19,000, 14,000, and 9000 are observed in about equal and constant stoichiometry, suggesting that they are subunits of a complex. The sedimentation coefficient of 11S is in good agreement with the sum of the Mr of the subunits. The Mr 68,000 and 9000 subunits label intensely with N-[3H]ethylmaleimide. Thus, the reported sulfhydryl group requirement of the translocation activity in the unfractionated extract [Jackson, R. C., Walter, P. & Blobel, G. (1980) Nature (London), 286, 174-176] may be localized to either or both the Mr 68,000 and 9000 subunits of the purified complex.

A membrane component essential for vectorial translocation of nascent proteins across the endoplasmic reticulum: requirements for its extraction and reassociation with the membrane

Previous reports have shown that rough microsomes treated with high salt (Warren and Dobberstein, 1978, Nature, 273:569-571) or proteases (Walter et al., 1979, Proc. Natl. Acad. Sci, U. S. A., 76:1,795) are unable to vectorially translocate nascent proteins. Readdition of the high salt or protease extracts restored activity to such inactive rough microsomes. A detailed study was carried out to determine how this factor interacts with the rough microsomal membrane. Proteolytic cleavage was found to be necessary but not sufficient to remove this factor from the membrane. A subsequent treatment with high salt had to be carried out. Endogenous (pancreatic) protease could effect the required cleavage, but low levels of trypsin, clostripain, or elastase were far more efficient. Several proteases were not effective. The minimum level of salt (after proteolysis) required to solubilize the active factor was approximately 200 mM KCl. Salt extracts prepared by treatment with one of the effective proteases were capable of restoring activity to inactive microsomes produced by treatment with one of the others.

Isolation of mRNA from bovine pituitary. The cell-free synthesis of the alpha and beta subunits of luteinizing hormone

RNA derived from bovine steer pituitary was translated in wheat germ cell-free extracts containing [35S]methionine. Antisera generated against purified denatured alpha and beta subunits of lutropin were used to demonstrate the synthesis of both proteins in vitro. The immunoprecipitated products of the cell-free system were resolved on sodium dodecyl sulfate/polyacrylamide gels and it was observed that the molecular weight of the immunoprecipitated alpha subunit protein was approximately 14,000, while that of the beta protein was estimated to be 16,000. Since the molecular weights of authentic alpha and beta subunits are 10,600 and 14,000 respectively, the cell-free products presumably represented their pre-protein forms. The ratio of the immunoprecipitated subunit pre-proteins was dependent on the magnesium concentration in the translation mixtures; at 2.1 mM, translation of lutropin alpha and beta mRNAs was comparable. RNA isolated from cow pituitary tissue directed the synthesis of fivefold less of the alpha and beta immunoprecipitated proteins than did steer RNA. Since the blood levels of gonadal steroids are higher in the cow, the results supported the hypothesis that lutropin alpha and beta mRNA biosynthesis is repressed by these steroids. The data also suggest that synthesis of lutropin alpha and beta subunits is coordinately expressed in certain physiological situations.