Primary structural requirements for the enzymatic formation of the N-glycosidic bond in glycoproteins. Studies with alpha-lactalbumin

Role of Flippases in Protein Glycosylation in the Endoplasmic Reticulum

Glycosylation is essential to the synthesis, folding, and function of glycoproteins in eukaryotes. Proteins are co- and posttranslationally modified by a variety of glycans in the endoplasmic reticulum (ER); modifications include C- and O-mannosylation, N-glycosylation, and the addition of glycosylphosphatidylinositol membrane anchors. Protein glycosylation in the ER of eukaryotes involves enzymatic steps on both the cytosolic and lumenal surfaces of the ER membrane. The glycans are first assembled as precursor glycolipids, on the cytosolic surface of the ER, which are tethered to the membrane by attachment to a long-chain polyisoprenyl phosphate (dolichol) containing a reduced α-isoprene. The lipid-anchored building blocks then migrate transversely (flip) across the ER membrane to the lumenal surface, where final assembly of the glycan is completed. This strategy allows the cell to export high-energy biosynthetic intermediates as lipid-bound glycans, while constraining the glycosyl donors to the site of assembly on the membrane surface. This review focuses on the flippases that participate in protein glycosylation in the ER.

Complete genome sequence of a Newcastle disease virus strain isolated from broiler breeder flocks in China

In 2010 and 2011, several devastating Newcastle disease (ND) outbreaks occurred in China, affecting broilers, layers, and breeders. The CK-JSX1-201005 virus was isolated from broiler breeder flocks vaccinated with the classical ND virus (NDV) vaccine program, but laying rate decreased from 80% to 30 to 40% in the clinic. Here, we report the complete genome sequence and molecular characteristic of the CK-JSX1-201005 NDV. These findings provide additional insights into the genetic variation of NDV circulating in China and are useful for vaccine development for NDV.

Suppression of Rft1 expression does not impair the transbilayer movement of Man5GlcNAc2-P-P-dolichol in sealed microsomes from yeast

To further evaluate the role of Rft1 in the transbilayer movement of Man(5)GlcNAc(2)-P-P-dolichol (M5-DLO), a series of experiments was conducted with intact cells and sealed microsomal vesicles. First, an unexpectedly large accumulation (37-fold) of M5-DLO was observed in Rft1-depleted cells (YG1137) relative to Glc(3)Man(9)GlcNAc(2)-P-P-Dol in wild type (SS328) cells when glycolipid levels were compared by fluorophore-assisted carbohydrate electrophoresis analysis. When sealed microsomes from wild type cells and cells depleted of Rft1 were incubated with GDP-[(3)H]mannose or UDP-[(3)H]GlcNAc in the presence of unlabeled GDP-Man, no difference was observed in the rate of synthesis of [(3)H]Man(9)GlcNAc(2)-P-P-dolichol or Man(9)[(3)H]GlcNAc(2)-P-P-dolichol, respectively. In addition, no difference was seen in the level of M5-DLO flippase activity in sealed wild type and Rft1-depleted microsomal vesicles when the activity was assessed by the transport of GlcNAc(2)-P-P-Dol(15), a water-soluble analogue. The entry of the analogue into the lumenal compartment was confirmed by demonstrating that [(3)H]chitobiosyl units were transferred to endogenous peptide acceptors via the yeast oligosaccharyltransferase when sealed vesicles were incubated with [(3)H]GlcNAc(2)-P-P-Dol(15) in the presence of an exogenously supplied acceptor peptide. In addition, several enzymes involved in Dol-P and lipid intermediate biosynthesis were found to be up-regulated in Rft1-depleted cells. All of these results indicate that although Rft1 may play a critical role in vivo, depletion of this protein does not impair the transbilayer movement of M5-DLO in sealed microsomal fractions prepared from disrupted cells.

Oligosaccharyltransferase directly binds to ribosome at a location near the translocon-binding site

Oligosaccharyltransferase (OT) transfers high mannose-type glycans to the nascent polypeptides that are translated by the membrane-bound ribosome and translocated into the lumen of the endoplasmic reticulum through the Sec61 translocon complex. In this article, we show that purified ribosomes and OT can form a binary complex with a stoichiometry of approximately 1 to 1 in the presence of detergent. We present evidence that OT may bind to the large ribosomal subunit near the site where nascent polypeptides exit. We further show that OT and the Sec61 complex can simultaneously bind to ribosomes in vitro. Based on existing data and our findings, we propose that cotranslational translocation and N-glycosylation of nascent polypeptides are mediated by a ternary supramolecular complex consisting of OT, the Sec61 complex, and ribosomes.

Complete coding sequence of bovine aggrecan: comparative structural analysis

The previously available sequence for bovine aggrecan included only the KS domain, the C-terminal portion of the CS-2 domain, and the entire CS-3 and G3 domains. We have isolated cDNA clones for previously uncharacterized portions of the bovine aggrecan sequence, and, when we combined them with previously published incomplete sequences, have obtained a complete sequence for the entire core protein. The bovine aggrecan sequence, which is a composite of new sequence data and previously published incomplete sequences, is 2327 residues in length. Although there is significant conservation of G1, G2, and G3 globular domains between species, there are differences in the length of the interglobular domain, in the number of KS domain hexapeptide repeats and CS domain repeats, and in alternative splicing within the G3 domain. The bovine aggrecan KS domain contains 24 repeats of a hexapeptide motif. The largely uncharacterized CS-1 domain of bovine aggrecan was found to contain 27 variable repeats of a 21-residue consensus sequence. A notable feature of the bovine CS-1 domain is in the distribution of single Ser-Gly dipeptides, the majority of which are separated by 7 or 8 amino acids, compared to the human, where discrete pairs of Ser-Gly dipeptides are separated by 13 amino acids. The CS-2 domain contains a total of six "homology domains" with 4 complete and 2 partial approximately 100-residue repeats. Each "homology domain" contains a "nodal" region with few sites for CS chain addition that is highly conserved between species, suggesting a possible role in aggrecan biosynthesis or catabolism.

Isolation and characterization of a thiamin transport gene, THI10, from Saccharomyces cerevisiae

We isolated a thiamin transporter gene, THI10, from a genomic library of Saccharomyces cerevisiae by the complementation of a yeast mutant defective in thiamin transport activity. The THI10 gene contained an open reading frame of 1,794 base pairs encoding a 598-amino acid polypeptide with a calculated molecular weight of 66, 903. The nucleotide sequence of THI10 is completely identical to that of an anonymous DNA (open reading frame L8083.2) mapped to chromosome XII; two other genes (open reading frames YOR071c and YOR192c) in chromosome XV are extremely similar to THI10. Moreover, the THI10 gene product showed significant sequence homology with yeast allantoin and uracil transporters. Hydropathy profile suggested that THI10 product is highly hydrophobic and contains many transmembrane regions. Gene disruption of the THI10 locus completely abolished the thiamin transport activity and thiamin binding activity in yeast plasma membrane fraction. Both the transport and thiamin binding activities were restored in the disrupted cells when the THI10 open reading frame was expressed by yeast GAL1 promoter, suggesting that the THI10 gene encodes for the thiamin transport carrier protein. Northern blot analysis demonstrated that THI10 gene expression is regulated at the mRNA level by intracellular thiamin pyrophosphate and that it requires a positive regulatory factor encoded by THI3 gene.

Structures and functionalities of milk proteins

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.

Structural study of the glycosylated and unglycosylated forms of a genetic variant of human serum albumin (63 Asp-->Asn)

A genetic variant of human serum albumin (alloalbumin) exhibited atypical electrophoretic mobility and chromatographic behavior apparently because of the effect of a point substitution on the molecular conformation. Three forms of albumin were isolated by DEAE HPLC chromatography: normal albumin, and two variant forms V1 and V2. The point substitution (Asp-63-->Asn) generated a canonical tripeptide acceptor sequence for glycosylation with an N-linked oligosaccharide (Asn-Lys-Ser). Neuraminidase digestion followed by electrophoresis showed that the V2 variant form was glycosylated and the V1 form was not. Time-of-flight mass spectrometry yielded a molecular weight of about 2000 for the carbohydrate. Structural analysis of the carbohydrate was done by chromatographic comparison of the pyridylaminated derivatives with standards and was confirmed by proton NMR of the three pronase glycopeptides and of the pyridylaminated oligosaccharide. The oligosaccharide had a complex biantennary structure with two sialic acid residues. In normal albumin Asp-63 is exposed and is adjacent to the first disulfide bond, Cys-62-->Cys-53. The apparent effect on molecular conformation resulting in incomplete glycosylation and atypical electrophoretic mobility suggests that glycosylation may interfere with disulfide bond formation at this site.

Genetic and molecular characterization of a gene encoding a wide specificity purine permease of Aspergillus nidulans reveals a novel family of transporters conserved in prokaryotes and eukaryotes

In Aspergillus nidulans, loss-of-function mutations in the uapA and azgA genes, encoding the major uric acid-xanthine and hypoxanthine-adenine-guanine permeases, respectively, result in impaired utilization of these purines as sole nitrogen sources. The residual growth of the mutant strains is due to the activity of a broad specificity purine permease. We have identified uapC, the gene coding for this third permease through the isolation of both gain-of-function and loss-of-function mutations. Uptake studies with wild-type and mutant strains confirmed the genetic analysis and showed that the UapC protein contributes 30% and 8-10% to uric acid and hypoxanthine transport rates, respectively. The uapC gene was cloned, its expression studied, its sequence and transcript map established, and the sequence of its putative product analyzed. uapC message accumulation is: (i) weakly induced by 2-thiouric acid; (ii) repressed by ammonium; (iii) dependent on functional uaY and areA regulatory gene products (mediating uric acid induction and nitrogen metabolite repression, respectively); (iv) increased by uapC gain-of-function mutations which specifically, but partially, suppress a leucine to valine mutation in the zinc finger of the protein coded by the areA gene. The putative uapC gene product is a highly hydrophobic protein of 580 amino acids (M(r) = 61,251) including 12-14 putative transmembrane segments. The UapC protein is highly similar (58% identity) to the UapA permease and significantly similar (23-34% identity) to a number of bacterial transporters. Comparisons of the sequences and hydropathy profiles of members of this novel family of transporters yield insights into their structure, functionally important residues, and possible evolutionary relationships.

Intracellular folding of tissue-type plasminogen activator. Effects of disulfide bond formation on N-linked glycosylation and secretion

The addition of N-linked core oligosaccharides to membrane and secretory glycoproteins occurs co-translationally at asparagine residues in the tripeptide sequon Asn-Xaa-Ser/Thr soon after translocation of the nascent polypeptide into the lumen of the endoplasmic reticulum. However, the presence of the sequon does not automatically ensure core glycosylation, as many proteins contain sequons that remain either unglycosylated or glycosylated to a variable extent. To investigate whether intracellular protein folding can influence sequon utilization, we have expressed tissue-type plasminogen activator (t-PA) in cell culture in the presence of mild concentrations of the reducing agent dithiothreitol to prevent co-translational disulfide bond formation in the endoplasmic reticulum. We show that conditions that prevent disulfide bond formation lead to complete glycosylation of a sequon that otherwise undergoes variable glycosylation in untreated cells. This demonstrated that folding and disulfide bond formation of t-PA determines its extent of core N-linked glycosylation. When dithiothreitol was removed from the cells, the reduced and overglycosylated t-PA formed disulfide bonds, folded, and was secreted. We also show t-PA present within cells is more susceptible to reduction with low concentrations of dithiothreitol than secreted t-PA.

Efficiency of N-linked core glycosylation at asparagine-319 of rabies virus glycoprotein is altered by deletions C-terminal to the glycosylation sequon

In N-linked core glycosylation, the oligosaccharide Glc3Man9GlcNAc2 is transferred to the tripeptide sequon Asn-X-Ser/Thr. However, this process must be regulated by additional protein signals, since many sequons are either poorly glycosylated or not glycosylated at all. Since N-linked glycosylation can influence protein structure and function, understanding these signals is essential for the design and expression of recombinant glycoproteins. Core glycosylation usually occurs cotranslationally in the rough endoplasmic reticulum (RER) during translocation of nascent proteins. Since only regions of a protein immediately near to a sequon or N-terminal to it are thought to be in the RER when core glycosylation occurs, most models predict that regions C-terminal to the sequon do not influence this process. We tested whether regions C-terminal to a sequon can influence its core glycosylation. Full-length (505 amino acid) rabies virus glycoprotein (RGP) mutants, each containing only one of the three sequons normally present in RGP, were used for these studies. Using a cell-free system, the core glycosylation efficiency at each sequon was determined. Termination codons were then introduced into these mutants at defined sites to produce C-terminal truncations, and the effect of each of these truncations on the core glycosylation efficiency at each sequon was assessed. While deletion of the C-terminal transmembrane and cytoplasmic domains did not affect core glycosylation, more extensive C-terminal deletions did result in altered core glycosylation in a site-specific fashion. Specifically, C-terminal truncations resulting in proteins containing 386 or 344 amino acids decreased the efficiency of core glycosylation at Asn319. This demonstrates that core glycosylation efficiency can be influenced by the presence or absence of regions in a protein more than 68 amino acids C-terminal to a specific glycosylation site.

Human skin mast cell carboxypeptidase: functional characterization, cDNA cloning, and genealogy

We functionally characterized human skin mast cell carboxypeptidase A (MC-CPA), and explored its evolutionary relationship to other carboxypeptidases to understand further the structural basis for the substrate preferences of this enzyme. Purified human skin MC-CPA displayed more activity than did bovine pancreatic carboxypeptidase A (CPA) against carboxyl-terminal leucine residues, about equal activity with phenylalanine and tyrosine residues, and no activity with tryptophan or alanine. To correlate kinetic data with structure, we isolated and sequenced a cDNA encoding MC-CPA from human skin, and directly sequenced 30% of the purified protein. These sequences agreed with that of human lung MC-CPA, and further support the evidence for a single MC-CPA gene in humans. Four amino acid replacements, resulting in a net positive change in non-hydrogen atoms in the S1' subsite of MC-CPA, were associated with less alteration in substrate specificity, relative to bovine CPA, than might be expected from studies using rat CPA1 and CPA2. We noted two consensus N-linked glycosylation sites in human MC-CPA that are not found in rat and mouse MC-CPA, or in bovine CPA; that at least one of these sites is glycosylated in vivo was verified by N-glycosidase F treatment, lentil lectin binding, and Concanavalin A-Sepharose chromatography. Evolutionary trees constructed from the known carboxypeptidase sequences suggested that MC-CPA most likely evolved from a carboxypeptidase B-like enzyme, independent of the pancreatic CPA. Thus, in the carboxypeptidase gene family, MC-CPA displays a unique genealogy and several amino acid replacements in its S1' binding pocket that result in substrate specificity quite similar to bovine CPA.

Primary structure and regulation of a glucoamylase-encoding gene (STA2) in Saccharomyces diastaticus

We have determined the complete nucleotide (nt) sequence of a 5070-bp DNA fragment containing a glucoamylase-encoding gene (STA2) from Saccharomyces diastaticus. The 5' transcription start points for STA1, STA2 and STA3 were determined by primer extension of their respective mRNAs using reverse transcriptase. The sequence data show one major open reading frame (ORF) of 767 amino acids encoding GAII with a calculated Mr of 82,514. The 5' region in the ORF contains two ATG sequences within 30 nt of each other. The upstream region of STA2 was amplified by the polymerase chain reaction (PCR) and fused to the Escherichia coli lacZ gene. Some of the PCR products contained mutations in ATG1 and/or ATG2. Results indicated that both ATG1 and ATG2 encode functional translation start codons, but ATG2 was shown to encode the stronger initiator. The upstream region of STA2 contains a canonical sequence that is homologous to known sites of repression by the MATa/MAT alpha-encoded repressor. Also, consensus RAP1 (Repressor-Activator Protein 1)-binding sites are located in the 5' upstream region and within the coding region of STA2.

Molecular characterization of a new hemagglutinin, subtype H14, of influenza A virus

Two influenza A viruses whose hemagglutinin (HA) did not react with any of the reference antisera for the 13 recognized HA subtypes were isolated from mallard ducks in the USSR. Antigenic analysis by hemagglutination inhibition and double immunodiffusion tests showed that the HAs of these viruses are similar to each other but distinct from the HAs of other influenza A viruses. Nucleotide sequence analysis showed that these HA genes differ from each other by only 21 nucleotides. However, they differ from all other HA subtypes at the amino acid level by at least 31% in HAI. Thus, we propose that the HAs of these viruses [A/Mallard/Gurjev/263/82 (H14N5) and A/Mallard/Gurjev/244/82 (H14N6) belong to a previously unrecognized subtype, and are designated H14. Unlike any other HAs of influenza viruses, the H14 HAs contained lysine at the cleavage site between HA1 and HA2 instead of arginine. Experimental infection of domestic poultry and ferrets with A/Mallard/Gurjev/263/82 (H14N5) showed that the virus is avirulent for these animals. Based on comparative sequence analysis of different HA genes, it is suggested that differences of 30% or more at the amino acid level in HA1 constitute separate subtypes. Phylogenetic analysis of representatives of each HA subtype showed that H14 is one of the most recently diverged lineages while H8 and H12 branched off early during the evolution of the HA subtypes. These latter two subtypes (H8 and H12) have been isolated very infrequently in recent years, suggesting that these old subtypes may be disappearing from the influenza reservoirs in nature.

The hydrophilic and acidic N-terminus of the integral membrane enzyme phosphatidylserine synthase is required for efficient membrane insertion

The product of the yeast CHO 1 gene, phosphatidylserine synthase (PSS), is an integral membrane protein that catalyses a central step in cellular phospholipid biosynthesis. A 1.2 kb fragment containing the regulatory and structural components of the CHO 1 gene was sequenced. Transcription initiation in wild-type cells was found to occur between -1 and -15 relative to the first ATG of a large open reading frame capable of encoding a 30,804 molecular weight protein. This translation initiation site was active in vivo and in vitro in a heterologous system. In both cases it supported production of a protein of approximately 30,000 molecular weight. A second potential translation initiation site was detected 225 or 228 bases downstream from the first ATG. This second site was active in vitro where it supported production of a protein of 22,400 molecular weight. A subclone, lacking the 5' regulatory region and the sequence encoding the first 12 amino acids of the large open reading frame, allowed translation in vivo starting at the second ATG. The resulting protein was 22,000 molecular weight, lacked the 74 N-terminal amino acids and was capable of complementing the choline auxotrophy of a cho 1 null-mutant. In transformants carrying this construct, PSS activity and 22 kDa protein was found to be associated with membrane fractions corresponding to mitochondria and endoplasmic reticulum. However, most of the truncated PSS protein accumulated in the cytosol in an inactive form. A hybrid-protein containing the 63 N-terminal amino acids of PSS fused to mouse dihydrofolate reductase was found exclusively in the cytosol when expressed in wild-type yeast. Thus, the hydrophilic, highly acidic N-terminus of PSS is required for efficient membrane insertion but does not appear to contain sequences required for a targeting to the membrane compartment.

Vascular endothelial growth factor is a secreted angiogenic mitogen

Vascular endothelial growth factor (VEGF) was purified from media conditioned by bovine pituitary folliculostellate cells (FC). VEGF is a heparin-binding growth factor specific for vascular endothelial cells that is able to induce angiogenesis in vivo. Complementary DNA clones for bovine and human VEGF were isolated from cDNA libraries prepared from FC and HL60 leukemia cells, respectively. These cDNAs encode hydrophilic proteins with sequences related to those of the A and B chains of platelet-derived growth factor. DNA sequencing suggests the existence of several molecular species of VEGF. VEGFs are secreted proteins, in contrast to other endothelial cell mitogens such as acidic or basic fibroblast growth factors and platelet-derived endothelial cell growth factor. Human 293 cells transfected with an expression vector containing a bovine or human VEGF cDNA insert secrete an endothelial cell mitogen that behaves like native VEGF.

Intracellular targeting and structural conservation of a prohormone-processing endoprotease

The prohormone-processing endoprotease (KEX2 gene product) of the yeast Saccharomyces cerevisiae is a membrane-bound, 135,000-dalton glycoprotein, which contains both asparagine-linked and serine- and threonine-linked oligosaccharide and resides in a secretory compartment. Analysis of mutant kex2 genes truncated at their 3' end indicates that carboxyl terminal domains of the enzyme are required for its proper localization within the cell. A human gene product, "furin," shares 50% identity with the catalytic domain of Kex2 protease and is, therefore, a candidate for a human prohormone-processing enzyme.

The proline transport protein of Aspergillus nidulans is very similar to amino acid transporters of Saccharomyces cerevisiae

In Aspergillus nidulans, the gene prnB encoding the major proline transport system is one of a cluster of four genes necessary and sufficient for the utilization of proline as sole nitrogen and/or carbon source. The prn cluster has been cloned and the sequence and transcript map of the prnB gene are presented in this paper. The predicted translated sequence consists of 570 amino acids, resulting in a molecular weight of 63,028 Daltons. Its hydropathy profile shows 10 hydrophobic segments typical of integral membrane proteins. No N-terminal hydrophobic signal peptide is present, the N-terminal and C-terminal ends of the protein being hydrophilic. Similar results were previously found for the arginine and histidine transporters of Saccharomyces cerevisiae, with which the prnB transporter shares regions of highly conserved amino acid sequences. Using S1 mapping and Northern blot analyses, we confirm the presence of a unique inducible prnB transcript of 1.9 kb.

Structure and biosynthesis of prokaryotic glycoproteins

Glycoproteins as components of cell surfaces are not restricted to eukaryotes. The prokaryotic glycoprotein studied in greatest detail so far is the cell surface glycoprotein of the archaebacterium Halobacterium halobium. This bacterial glycoprotein contains 3 different types of glycoconjugates, and each type of glycoconjugate involves a different carbohydrate-protein linkage unit: 1) One glycosaminoglycan chain, constructed from a repeating sulfated pentasaccharide block, is linked to one protein molecule via the novel N-glycosyl linkage unit asparaginyl-N-acetylgalactosamine. 2) Ten sulfated oligosaccharides that contain glucose, glucuronic acid and iduronic acid are bound to the protein via the hitherto unknown N-glycosyl linkage unit asparaginylglucose. 3) About 15 disaccharides, glucosylgalactose, are O-glycosyl-linked to a cluster of threonine residues close to the C-terminus of the core protein. The overall structure of the cell surface glycoprotein of halobacteria is thus reminiscent of animal proteoglycans and a functional role of the glycosaminoglycan chain in maintaining the rod shape of halobacteria is discussed. Biosynthesis of the two N-glycosyl linkage units involves dolichol monophosphate and dolicholdiphosphate-linked saccharide precursors. Sulfation and epimerization of the glycoconjugates occur at the lipid-linked level and the mature saccharides are transferred to the protein core on the cell surface. The sulfated oligosaccharides that finally become bound to asparagine via glucose are transiently methylated at their lipid-linked stage and this transient chemical modification seems to be required for the biosynthesis of the corresponding N-glycosyl bond.

Glycosylation site binding protein, a component of oligosaccharyl transferase, is highly similar to three other 57 kd luminal proteins of the ER

A 57 kd component of oligosaccharyl transferase, termed glycosylation site binding protein, specifically recognizes a photoaffinity probe containing the N-glycosylation site sequence Asn-Lys-Thr. It is present in the lumen of the ER (endoplasmic reticulum) and its release from this compartment results in a loss of N-glycosylation. Antibodies against this protein were used to identify cDNA clones from a lambda gt11 expression library. Analysis of its cDNA sequence reveals high sequence similarity to three other 57 kd luminal endoplasmic reticulum proteins: protein disulfide isomerase, the beta-subunit of prolyl hydroxylase, and thyroid hormone binding protein. This finding suggests that the capacity to recognize multiple polypeptide domains may reside in a single luminal protein that participates in co- and/or posttranslational modifications of newly synthesized proteins.

Cloning and expression of cDNA for human vascular anticoagulant, a Ca2+-dependent phospholipid-binding protein

Based on sequence information from tryptic peptides an almost full-size cDNA coding for the human vascular anticoagulant was isolated from a placental cDNA library and sequenced. The coding region was cloned into an Escherichia coli expression vector and the protein expressed at high levels. The recombinant protein was purified and found to be indistinguishable from its natural counterpart in several biological assays.

Molecular cloning and characterization of the gene encoding cholinephosphate cytidylyltransferase in Saccharomyces cerevisiae

1. The structural gene for cholinephosphate cytidylyltransferase (CCT) was isolated from a Saccharomyces cerevisiae genomic library by means of complementation in a mutant of the yeast defective in the enzyme. The cloned DNA restored both the growth and cholinephosphate cytidylyltransferase activity of the mutant. Whereas the enzyme of the mutant was thermolabile, the enzyme produced by the transformant was indistinguishable in heat stability from that produced by the wild type. 2. Strains carrying a multicopy recombinant plasmid overproduced cholinephosphate cytidylyltransferase. The overproduction of the enzyme brought about an increase in the synthesis of CDPcholine in the transformant, but there was no increase in the overall rate of phosphatidylcholine synthesis. 3. The cloned DNA was subcloned into a 2.5-kb DNA fragment. The nucleotide sequence which contained CCT was determined by the dideoxy chain-termination method. The sequence contained an open reading frame capable of encoding a protein of 424 amino acid residues with a calculated relative molecular mass of 49,379.31. Northern blot analysis showed that this DNA segment is transcribed in yeast cells and the length of the transcript is consistent with the putative translation product. 4. Hydropathy analysis according to Kyte and Doolittle indicated that the primary translation product contains extended hydrophilic stretches in its N- and C-terminal regions. 5. The primary translation product contains a region showing local sequence homology with nucleotidyl-transfer enzymes such as DNA polymerase (Escherichia coli), CDPdiacylglycerol pyrophosphatase (E. coli), 3-deoxy-manno-octulosonate cytidylyltransferase (E. coli) and DNA ligase (T4 phage), suggesting that these five enzymes are evolutionarily related. Statistically significant sequence homology was also noted between the human c-fos gene product and the enzyme.

Nucleotide sequence of the glucoamylase gene GLU1 in the yeast Saccharomycopsis fibuligera

The complete nucleotide sequence of the glucoamylase gene GLU1 from the yeast Saccharomycopsis fibuligera has been determined. The GLU1 DNA hybridized to a polyadenylated RNA of 2.1 kilobases. A single open reading frame codes for a 519-amino-acid protein which contains four potential N-glycosylation sites. The putative precursor begins with a hydrophobic segment that presumably acts as a signal sequence for secretion. Glucoamylase was purified from a culture fluid of the yeast Saccharomyces cerevisiae which had been transformed with a plasmid carrying GLU1. The molecular weight of the protein was 57,000 by both gel filtration and acrylamide gel electrophoresis. The protein was glycosylated with asparagine-linked glycosides whose molecular weight was 2,000. The amino-terminal sequence of the protein began from the 28th amino acid residue from the first methionine of the putative precursor. The amino acid composition of the purified protein matched the predicted amino acid composition. These results confirmed that GLU1 encodes glucoamylase. A comparison of the amino acid sequence of glucoamylases from several fungi and yeast shows five highly conserved regions. One homology region is absent from the yeast enzyme and so may not be essential to glucoamylase function.

Nucleotide sequence and characterization of the yeast PSS gene encoding phosphatidylserine synthase

1. A yeast chromosomal DNA which contains the structural gene for phosphatidylserine synthase (PSS) was isolated by genetic complementation from a wild-type yeast genomic library. The PSS gene was subcloned into a 1.1-kb fragment of the yeast DNA on the YEp13 vector. 2. The PSS gene on the multicopy plasmid caused the fourfold over-production of the enzyme and fully restored the phosphatidylserine content of the transformant. The phospholipid composition of the transformant was similar to that of the wild type. 3. Sequence analysis showed that this DNA fragment contains an open reading frame capable of encoding 276 amino acid residues with a calculated relative molecular mass of 30,804. Northern blot analysis of poly(A)-rich RNA of the wild-type yeast indicated that this DNA segment is transcribed into a single mRNA species. 4. The DNA sequence contained two putative transcriptional initiation signals, each followed by the ATG initiator codon. Deletion experiments indicated that the 5'-proximal ATG codon is essential for the synthesis of the functional phosphatidylserine synthase.

Nucleotide sequence of the alpha-amylase gene (ALP1) in the yeast Saccharomycopsis fibuligera

The complete nucleotide sequence of the secretable alpha-amylase gene ALP1 from the yeast Saccharomycopsis fibuligera has been determined. The ALP1 DNA hybridized to a polyadenylated RNA of 2.0 kilobases. A single open reading frame encodes a 494-amino acid protein which is highly homologous with alpha-amylase (Taka-amylase) of a fungus Aspergillus oryzae.

cDNA and protein sequence of a major pea seed albumin (PA 2 : Mr≈26 000)

Pea albumin 2 (PA2:Mr≈26000) is a major component of the albumin fraction derived from aqueous salt extracts of pea seed. Sodium dodecylsulfate-polyacrylamide gel electrophoresis and chromatography on DEAE-Sephacel resolve PA2 into two closely related components (PA2a and PA2b). A cDNA clone coding for one of these components has been sequenced and the deduced amino acid sequence compared with partial, chemically-determined sequences for cyanogen bromide peptides from both PA2 components. Complete amino acid sequences were obtained for the C-terminal peptides. The PA2 molecule of 230 amino acids contains four imperfect repeat sequences each of approximately 57 amino acids in length.The combined sequence data, together with a comparison of PA2-related polypeptides produced in vitro and in vivo, indicate that PA2 is synthesized without a signal sequence and does not undergo significant post-translational modification. Although both forms of PA2 contain Asn-X-Thr consensus sequences, neither form is glycosylated. Accumulation of PA2 contributes approximately 11% of the sulfur-amino acids in pea seeds (cysteine plus methionine equals 2.6 residues percent). Suppression of levels of PA2 polypeptides and their mRNAs in developing seeds of sulfur-deficient plants is less marked than that for legumin, in spite of the lower content of sulfur-amino acids in legumin.

Structure-activity relationships of the yeast alpha-factor

The yeast Saccharomyces cerevisiae produces a peptide pheromone, termed the alpha-factor, as a prelude to sexual conjugation. Haploid MAT alpha-cells, but not haploid MAT a-cells or MAT a/alpha-diploids, produce this tridecapeptide of the structure: Trp-His-Trp-Leu-Gln-Leu-Lys-Pro-Gly-Gln-Pro-Met-Tyr. Structural analogues of the alpha-factor have been prepared with alterations in many of the residues, derivatized peptides have been synthesized, and truncated and elongated peptides have been studied. These peptides have been analyzed for their biological activities by various assays. Mutants of S. cerevisiae have been isolated that do not respond to alpha-factor or are supersensitive to the pheromone and its analogues. The mating system of S. cerevisiae provides a powerful model in which genetics, biochemistry, and molecular biology can be used to unravel the mysteries of peptide hormone structure and function.

Nucleotide sequence of the extracellular glucoamylase gene STA1 in the yeast Saccharomyces diastaticus

The complete nucleotide sequence of the extracellular glucoamylase gene STA1 from the yeast Saccharomyces diastaticus has been determined. A single open reading frame codes for a 778-amino-acid protein which contains 13 potential N-glycosylation sites. In the 5'- and 3'-flanking regions of the gene, there are striking sequence homologies to the corresponding regions of ADH1 for alcohol dehydrogenase and MAT alpha 2 for mating type control in the yeast Saccharomyces cerevisiae. The putative precursor begins with a hydrophobic segment that presumably acts as a signal sequence for secretion. The presumptive signal sequence showed a significant homology to that of Bacillus subtilis alpha-amylase precursor. The next segment, of ca. 320 amino acids, contains a threonine-rich tract in which direct repeat sequences of 35 amino acids exist, and is bordered by a pair of basic amino acid residues (Lys-Lys) which may be a proteolytic processing signal. The carboxy-terminal half of the precursor is a presumptive glucoamylase which contains several peptide segments showing a high degree of homology with alpha-amylases from widely diverse organisms including a procaryote (B. subtilis) and eucaryotes (Aspergillus oryzae and mouse). Analysis of both the nucleotide sequence of the STA1 gene and the amino acid composition of the purified glucoamylase suggested that the putative precursor is processed to yield subunits H and Y of mature enzyme by both trypsin-like and chymotrypsin-like cleavages.

Inhibition of synthesis of herpesvirus (HSV-1) glycoproteins and endogenous fusion by beta-hydroxynorvaline in BHK-21 cells

Treatment of HSV-infected BHK-21 cells with 5-10 mM of beta-hydroxynorvaline (Hnv), an analog of threonine which blocked attachment of oligosaccharides at the Asn-X-Thr sites, markedly inhibited the synthesis of all viral glycoproteins as well as the major capsid protein. However, the synthesis of host-specific dolichol-linked oligosaccharides was not significantly affected by Hnv. Treatment of cells with 10 mM reduced the yield of virus greater than 95% and completely blocked endogenous fusion. Inhibition of Hnv could be reversed by simultaneous addition of threonine to the culture medium. It is likely that the incorporation of Hnv into HSV polypeptides at Asn-X-Thr (in place of Thr) sites blocked transfer of N-linked oligosaccharides.

Nucleotide sequence of a cloned duck hepatitis B virus genome: comparison with woodchuck and human hepatitis B virus sequences

The nucleotide sequence of an EcoRI duck hepatitis B virus (DHBV) clone was elucidated by using the Maxam and Gilbert method. This sequence, which is 3,021 nucleotides long, was compared with the two previously analyzed hepatitis B-like viruses (human and woodchuck). From this comparison, it was shown that DHBV is derived from an ancestor common to the two others but has a slightly different genomic organization. There was no intergenic region between genes 5 and 8, which were fused into a single open reading frame in DHBV. Genes for the surface and core proteins were assigned to open reading frames 7 and 5/8. Amino acid comparisons showed some structural relationship between gene 6 product and avian reverse transcriptase, suggesting either evolution from a common ancestor or convergence to some particular structure to fulfill a specific function. This should be correlated with the synthesis of an RNA intermediate during DNA replication. This is also taken as an argument in favor of the hypothesis that gene 6 codes for the DNA polymerase that is found within the virion. DNA sequence comparison also showed that the two mammalian hepatitis B viruses are more homologous to each other than they are to DHBV, indicating that DHBV starts to evolve on its own earlier than the two other viruses, as do birds compared with mammals. From this it is proposed that the viruses evolved in a fashion parallel to the species they infect.