Amino acid distributions around O-linked glycosylation sites

Expression of GARP, a major surface glycoprotein of Trypanosoma congolense, on the surface of Trypanosoma brucei: characterization and use as a selectable marker

Procyclic and epimastigote forms of Trypanosoma congolense express an immunodominant glutamic acid/alanine-rich protein (GARP) that covers the parasite surface. Although GARP shows no sequence similarity to procyclins from T. brucei, the general characteristics of the two sets of surface glycoproteins suggest that they have analogous functions, in much the same way that variant surface glycoproteins with unrelated primary sequences fulfil the same function in bloodstream form trypanosomes. Since T. brucei and T. congolense do not follow the same pathway through the tsetse fly, one possible function of procyclins might be to direct parasites to the correct compartments. As a first step towards testing this hypothesis, we have produced stably transformed procyclic forms of T. brucei in which the GARP coding region has been integrated into a procyclin expression site. GARP can be detected on the surface of these transgenic trypanosomes, uniformly distributed within the endogenous procyclin coat, but there are differences in post-translational modification when it is expressed in T. brucei rather than in T. congolense. The fact that GARP is readily accessible to antibodies which were raised against a bacterial fusion protein led us to examine its potential as a selectable surface marker for transfection. We have established a rapid and simple procedure for isolating stable transformants that provides an alternative to conventional methods of selection for antibiotic resistance.

Mapping the mouse ZP3 combining site for sperm by exon swapping and site-directed mutagenesis

During fertilization in mice, sperm bind to mouse ZP3 (mZP3), a M(r) approximately 83,000 glycoprotein present in the ovulated egg extracellular coat, or zona pellucida. Sperm recognize and bind to specific serine/threonine-linked (O-linked) oligosaccharides present at the mZP3 combining site for sperm. Binding to mZP3 induces sperm to undergo a form of exocytosis, the acrosome reaction. To map the mZP3 combining site for sperm, we examined the effect of exon swapping and site-directed mutagenesis on the glycoprotein's two activities, sperm binding and induction of the acrosome reaction. Stably transfected embryonal carcinoma cell lines were established that synthesized recombinant glycoproteins and secreted them into the culture medium. The glycoproteins were partially purified from culture medium and assayed for sperm-binding and acrosome reaction-inducing activities. Results of these assays suggest that glycosylation of one or more of five serine residues, clustered together in a polypeptide region encoded by mZP3 gene exon 7, is required for activity. Interestingly, this polypeptide region exhibits considerable sequence divergence during evolution and may be related to the proposed role for oligosaccharides in species-specific gamete adhesion during mammalian fertilization.

Structure, biosynthesis, and function of salivary mucins

The glandular secretions of the oral cavity lining the underlying buccal mucosa are highly specialized fluids which provide lubrication, prevent mechanical damage, protect efficiently against viral and bacterial infections, and promote the clearance of external pollutants. This mucus blanket contains large glycoproteins termed mucins which contribute greatly to the viscoelastic nature of saliva and affect its complex physiological activity. The protein core of mucins consists of repetitive sequences, rich in O-glycosylated serine and threonine, and containing many helix-breaking proline residues. These features account for the extended, somewhat rigid structure of the molecule, a high hydrodynamic volume, its high buoyant density, and high viscosity. The oligosaccharide moiety of salivary mucins accounts for up to 85% of their weight. The oligosaccharide side chains exhibit an astonishing structural diversity. The isolation, composition, structure, molecular characteristics, and functional relevance of salivary mucins and their constituents is discussed in relation to recent advancements in biochemistry and molecular biology.

Combination of high-performance anion-exchange chromatography and electrospray mass spectrometry for analysis of the in vitro O-glycosylated mucin motif peptide

Reversed-phase high-performance liquid chromatography (HPLC) and high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection were developed for the study of products obtained from the in vitro O-glycosylation of a mucin motif peptide, TTSAPTTS, the most representative sequence encoded by the human gene MUC5C. After incubation of the peptide, which is rich in clustered hydroxyamino acids, by both human colonic and gastric microsomal homogenates, the glycosylated products were separated by HPLC and HPAEC and analysed by electrospray mass spectrometry (ES-MS). The combination of HPAEC and ES-MS was the approach used for evaluating the differences between the polypeptide N-acetylgalactosaminyltransferase activity in different digestive tissues.

Novel inducible antibacterial peptides from a hemipteran insect, the sap-sucking bug Pyrrhocoris apterus

Insects belonging to the recent orders of the endopterygote clade (Lepidoptera, Diptera, Hymenoptera and Coleoptera) respond to bacterial challenge by the rapid and transient synthesis of a battery of potent antibacterial peptides which are secreted into their haemolymph. Here we present the first report on inducible antibacterial molecules in the sap-sucking bug Pyrrhocoris apterus, a representative species of the Hemiptera, which predated the Endoptergotes by at least 50 million years in evolution. We have isolated and characterized from immune blood of this species three novel peptides or polypeptides: (i) a 43-residue cysteine-rich anti-(Gram-positive bacteria) peptide which is a new member of the family of insect defensins; (ii) a 20-residue proline-rich peptide carrying an O-glycosylated substitution (N-acetylgalactosamine), active against Gram-negative bacteria; (iii) a 133-residue glycine-rich polypeptide also active against Gram-negative bacteria. The proline-rich peptide shows high sequence similarities with drosocin, an O-glycosylated antibacterial peptide from Drosophila, and also with the N-terminal domain of diptericin, an inducible 9 kDa antibacterial peptide from members of the order Diptera, whereas the glycine-rich peptide has similarities with the glycine-rich domain of diptericin. We discuss the evolutionary aspects of these findings.

The smallest gene of the orbivirus, epizootic hemorrhagic disease, is expressed in virus-infected cells as two proteins and the expression differs from that of the cognate gene of bluetongue virus

The smallest gene (S10) of the virus of epizootic hemorrhagic disease of deer (EHD, serotype 2) is expressed as two proteins in virus-infected cells. By contrast, the non-structural proteins (NS3 and NS3A) encoded in the smallest gene of bluetongue (BT) viruses are difficult to detect in virus-infected cells. The nucleotide sequence of S10 of EHDV-2 contains two in-frame initiation codons which allow for translation of proteins of mol. wt. 25503 and 23921 analogous to NS3 and NS3A of BT viruses. The S10 genes of BT viruses are highly conserved (82%-99%); the nucleotide sequence similarity of S10 of EHDV-2 and BT viruses is about 64%. Some structural features of NS3 and NS3A are conserved in the two viruses, despite the divergence in the amino acid sequences of the proteins. The hydrophobic domains of the proteins and the putative transmembrane sequences are conserved, as are potential glycosylation sites in the proteins. A cluster of proline residues, which is conserved at residues 36-50 in all of the published sequences of NS3 of BT viruses, is conserved exactly in the alignment of the sequence of NS3 of EHDV-2 with that of the BT viruses. An explanation for the differences in expression of NS3/NS3A in EHD and BT viruses was not evident in comparing the nucleotide sequences of S10 of the viruses.

UDPgalactose:glycoprotein-N-acetyl-D-galactosamine 3-beta-D-galactosyltransferase activity synthesizing O-glycan core 1 is controlled by the amino acid sequence and glycosylation of glycopeptide substrates

In order to investigate the role of the peptide moiety of glycoproteins in the control of O-glycan biosynthesis, UDPgalactose:glycoprotein-N-acetyl-D-galactosamine 3-beta-D-galactosyltransferase (core 1 beta 3-Gal-T) from rat liver was tested for its specificity towards GalNAc-containing glycopeptide substrates. Series of glycopeptides have been synthesized by solid-phase synthesis, protected with an acetyl group on the amino terminal and an amide group on the carboxy terminal, based on variations of the repeat sequences of human intestinal mucin. Most glycopeptides were excellent substrates for core 1 beta 3-Gal-T compared to benzyl alpha-D-galactosamine as indicated by their relatively high Vmax/Km. The enzyme preferred threonine alpha-D-galactosamine Thr(GalNAc) to serine alpha-D-galactosamine. Pro on the carboxy-terminal side adjacent to Thr(GalNAc) was inhibitory. Negatively charged amino acids on either side showed a low Km; substrates with negatively charged amino acids on the amino-terminal side were highly efficient substrates, suggesting charge-charge interactions between enzyme and substrate. Gal beta 1-3GalNAc alpha residues adjacent to Thr(GalNAc) reduced the activity. Product analysis using glycopeptide substrates with three adjacent GalNAc residues showed incorporation of one, two and a small amount of three Gal residues per molecule with an uneven distribution of the potential di-galactosylated isomers. These studies indicate that, in addition to initial glycosylation, the second step in the glycosylation pathways of O-glycans is also controlled by the structure and glycosylation of the peptide core of substrates.

A comparison of serine and threonine O-glycosylation by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase

O-glycosylated proteins are ubiquitous in eukaryotes and are responsible for a variety of biological functions. O-glycosylation is initiated by the addition of N-acetylgalactosamine to serine or threonine residues, though it is not clear how specific residues are selected for modification. We have compared serine and threonine glycosylation using peptide substrates based on sequences from erythropoietin (EPO) and von Willebrand factor (HVF) that are glycosylated in vivo. UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase was derived from rat parotid, submandibular, and sublingual glands, liver and kidney as well as from human colostrum. The threonine-containing substrates were glycosylated to a much greater extent than those containing serine for all the enzyme sources. Changes in reaction pH, donor concentration, or divalent cation were unable to increase glycosylation of serine. When the incubation time was extended, serine in the EPO-based peptide was found to incorporate GalNAc at a low level, in contrast to the serine-containing HVF peptide, which did not glycosylate at all. By circular dichroism, the non-glycosylating peptide was the only one of the series that did not exhibit random coil structure. Our data suggest that although the structural and sequence requirements for O-glycosylation of serine and threonine residues are similar, serine sites are glycosylated less effectively than are threonine sites in vitro.

A review of the molecular and cellular biology of butyrophilin, the major protein of bovine milk fat globule membrane

The molecular and cellular biology of the milk protein butyrophilin is reviewed. Butyrophilin constitutes more than 40% by weight of the total protein associated with the fat globule membrane of bovine milk. Closely related proteins are abundant in the fat globule membranes of many other species. Butyrophilin is synthesized as a peptide of 526 amino acids with an amino-terminal hydrophobic signal sequence of 26 amino acids, which is cleaved before secretion in association with the fat globule membrane. Hydropathy analysis and in vitro translation of butyrophilin mRNA indicate that the protein associates with membranes in a type I orientation via a single stretch of 27 hydrophobic amino acids in the approximate middle of the sequence. Evidence that butyrophilin is incorporated into fat globule membrane as a transmembrane protein and as a cytoplasmically oriented peripheral component is discussed. The carboxy-terminal sequence of butyrophilin is significantly homologous to two other proteins: ret finger protein and the 52-kDa nuclear antigen A of Sjögren's syndrome. Expression of bovine butyrophilin mRNA correlates with the onset of milk fat secretion toward the end of pregnancy and is maintained throughout lactation. The possible function of butyrophilin in the secretion of milk lipid droplets is discussed.

Identification of O-linked oligosaccharide chains in the activation peptides of blood coagulation factor X. The role of the carbohydrate moieties in the activation of factor X

Conversion of factor X to factor Xa results in release of a heavily glycosylated activation peptide. Analysis of protease-digested glycopeptides derived from the activation peptides of bovine and human blood coagulation factor X allowed the identification of sites of the O-linked oligosaccharide chains in these peptides. Glycopeptides were prepared from the activation peptides by digestion with chymotrypsin or Staphylococcus aureus V8 protease. By combined analysis of amino acid sequence and sialic acid content, we found that bovine factor X had an O-linked oligosaccharide chain linked to Thr26, and human factor X had four carbohydrate-attachment sites, namely, O-glycosidic linkages to Thr17 and Thr29, respectively, and N-glycosidic linkages to Asn39 and Asn49, respectively, in their activation peptides. The O-linked carbohydrate-attachment sites were identified since the yields of phenylthiohydantoin derivatives of amino acids that corresponded to their residues were increased during amino acid sequencing after deglycosylation of the glycopeptides with sialidase and O-glycanase. The effect of deglycosylation of bovine factor X1 was investigated with factor-X-activating enzyme from Russell's viper venom or extrinsic Xase (factor VIIa/tissue factor/phospholipid) by examining the activation rates of derivatives of factor X prepared using O-glycanase, sialidase, and/or N-glycanase. The removal of O-linked carbohydrate resulted in a decrease in the rate of activation. It appears that carbohydrate residues in factor X play an important role in the activation of the zymogen.

Identification in the HLA class I region of a gene expressed late in keratinocyte differentiation

A gene designated S has been identified in the class I region of the human major histocompatibility complex. The S gene is located 160 kb telomeric of HLA-C. It is expressed at high levels as 2.2-kb and 2.6-kb mRNAs in human skin. No homologous transcripts were detected in other tissues including placenta, liver, spleen, thymus, and brain. In situ hybridization showed that S gene expression was restricted to the differentiating keratinocytes in the granular layer of the epidermis. The predicted amino acid sequence of the S protein was remarkable for its high content of serine, glycine, and proline. There were significant similarities with the amino acid sequences of loricrin, keratin 1, and keratin 10, all major components of the granular-cell layer. The selective expression of the S gene in the granular-cell layer in the epidermis suggests a role in the developmental program of differentiating keratinocytes. Furthermore, in light of the recognized association of psoriasis vulgaris, a disorder of keratinocyte proliferation, with alleles of HLA-C, this gene may contribute primarily to the pathogenesis of this common disorder.

Components of glycine reductase from Eubacterium acidaminophilum. Cloning, sequencing and identification of the genes for thioredoxin reductase, thioredoxin and selenoprotein PA

The genes encoding thioredoxin reductase (trxB), thioredoxin (trxA), protein PA of glycine reductase (grdA) and the first 23 amino acids of the large subunit of protein PC of glycine reductase (grdC) belonging to the reductive deamination systems present in Eubacterium acidaminophilum were cloned and sequenced. The proteins were products of closely linked genes with 314 codons (thioredoxin reductase), 110 codons (thioredoxin), and 158 codons (protein PA). The protein previously called 'atypically small lipoamide dehydrogenase' or 'electron transferring flavoprotein' could now conclusively be identified as a thioredoxin reductase (subunit mass of 34781 Da) by the alignment with the enzyme of Escherichia coli showing the same typical order of the corresponding domains. The thioredoxin (molecular mass of 11742 Da) deviated considerably from the known consensus sequence, even in the most strongly conserved redox-active segment WCGPC that was now GCVPC. The selenocysteine of protein PA (molecular mass of 16609 Da) was encoded by TGA. The protein was highly similar to those of Clostridium purinolyticum and Clostridium sticklandii involved in glycine reductase. Thioredoxin reductase and thioredoxin of E. acidaminophilum could be successfully expressed in E. coli.

A major surface antigen of procyclic stage Trypanosoma congolense

Five monoclonal antibodies (mAb) were raised that bound to the surface of procyclic stage Trypanosoma congolense with high intensity in immunofluorescence. Immunoblot analysis of trypanosome lysates using 3 of these mAb revealed a diffuse SDS-PAGE band of 36-40 kDa. The purified antigen did not react with Coomassie Blue or silver stains, but did stain blue with Stains-all, indicating acidity. For the one mAb tested, the epitope was periodate-sensitive and therefore probably glycan. Although this antigen shares properties with procyclin/PARP, which forms a surface coat on procyclic Trypanosoma brucei, a search in T. congolense for homologues of a procyclin/PARP gene revealed only non-coding sequence of partial similarity. Using a differential screen, a procyclic stage T. congolense cDNA clone was isolated that encoded a putative 256-amino acid protein containing 2 peptides chemically sequenced independently by Beecroft et al. [36]. The protein, termed glutamate and alanine-rich protein (GARP), has potential hydrophobic leader and tail sequences (the latter with potential for replacement by a glycosyl phosphoinositol anchor) and no potential N-linked glycosylation sites. It has no significant sequence homology with known proteins. Antibodies against a translational fusion of GARP bound specifically in Western blots to a band very similar to that detected by the mAb and also to the purified antigen. Immunogold electron microscopy revealed a dense packing of the antigen on the cell surface. It appears that procyclic T. brucei and T. congolense have major surface proteins with structural analogy, but with no sequence homology.

Identification of the predominant glycosaminoglycan-attachment site in soluble recombinant human thrombomodulin: potential regulation of functionality by glycosyltransferase competition for serine474

Thrombomodulin (TM) is an endothelial cell thrombin receptor that converts thrombin from a procoagulant to an anticoagulant enzyme. It has previously been shown that TM is expressed in both a high-M(r) form containing chondroitin sulphate and a low-M(r) form lacking this modification. Site-directed mutagenesis of a soluble human TM derivative (TMD1) was employed to determine the attachment site(s) of this functionally important oligosaccharide on the core protein. Although there are four serine residues within the Ser/Thr-rich domain of TMD1 that might support glycosaminoglycan assembly, our analysis demonstrates that the primary site of attachment is at Ser474, and evidence is presented for low levels of attachment at Ser472. It was possible to improve the overall degree of attachment by mutating Ser472 to glutamic acid (so as to conform Ser474 to the xylosyltransferase acceptor consensus acidic-Gly-Ser-Gly-acidic); however, a significant proportion (approx. 35%) of the total TM still lacked a glycosaminoglycan moiety. Mutants that possess a substitution for Ser474 show an increased mobility of their low-M(r) form on SDS/PAGE compared with native TMD1. Isolation and sequencing of a C-terminal peptide demonstrated that this serine is modified in the low-M(r) form of native TMD1. An apparent 'acceptor consensus overlap' at Ser474 suggests that the mechanism behind the glycosaminoglycan split of TM may involve a competition for substrate between xylosyltransferase and N-acetylgalactosaminyltransferase.

Glycosylation

The majority of candidate recombinant therapeutics are glycoproteins. Four aspects of glycobiology are requisite if the full potential of such reagents is to be realised: an understanding of glycan biosynthesis and its regulation, rapid and sensitive oligosaccharide analytical techniques, determination of structure-function relationships in protein-linked carbohydrate; and manufacturing systems where product glycosylation is manipulable and consistent. The past year has seen significant progress in all of these areas, exemplified by the approval of glycosylation-engineered glucocerebrosidase for clinical use in replacement therapy of Gaucher's disease.

Structure and localization of O- and N-linked oligosaccharide chains on basement membrane protein nidogen

The carbohydrate content of mouse nidogen predicts the occupation of two N- and about seven O-linked acceptor sites. The corresponding oligosaccharides were examined by sequential exoglycosidase digestions. The data indicate N-linked substitutions by several bi-, tri- and tetraantennary complex types of oligosaccharides which are further modified by additional lactosamines and terminal alpha-galactose and/or sialic acid. Mannose-rich oligosaccharides were of low abundance. O-linked structures included a di- and tetrasaccharide core structure that were in addition sialylated and may be similar to structures found in fetuin. Evidence is provided that the two sequence-predicted asparagine acceptors are almost fully substituted. Sequence analysis of tryptic peptides identified Thr-271, Ser-303, Thr-309, Thr-317, Thr-320, Thr-892 and Thr-905 as the most likely sites for galactosamine substitutions. These residues are located in the flexible link connecting the N-terminal globular domains G1 and G2 of nidogen and at the border between the rod and the C-terminal globe G3. Four of them showed Pro in the -1 or +3 position. All these Ser, Thr and Pro residues but not the N-linked attachment sites are identical in human nidogen.

Biochemical characterization of the extracellular domain of the 75-kilodalton tumor necrosis factor receptor

An expression plasmid encoding the extracellular domain of the 75-kDa human tumor necrosis factor (TNF) type 2 receptor (TNF-R2) was constructed and used to generate a stable cell line secreting soluble TNF-R2 (sTNF-R2). Purified sTNF-R2 was resolved by SDS-PAGE into one band of approximate M(r) 43,000, consistent with a molecular weight of 36,000 +/- 4800 obtained by sedimentation equilibrium analysis. The apparent molecular weight observed by gel filtration chromatography was approximately 136,000. Glycosylation analysis revealed that Asn-149 is fully glycosylated, while Asn-171 is incompletely glycosylated (approximately 50%), and that a proline-, serine-, and threonine-rich region (residues 175-234) contains O-linked carbohydrate structures. Scatchard analysis of [125I]TNF-alpha and [125I]TNF-beta binding to sTNF-R2 gave dissociation constants (Kd) of 0.3 and 0.75 nM, respectively, comparable to those observed for intact cell-surface TNF-R2. The sTNF-R2 was found to block the cytotoxicity of both TNF-alpha and TNF-beta in a murine L-M cell assay. The sizes of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes determined by gel filtration chromatography were approximately 322 and 204 kDa, respectively. The stoichiometry of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes were examined by size-exclusion chromatography, sedimentation equilibrium, and cross-linking. The data from these studies suggest that at least two molecules of sTNF-R2 can bind to a single TNF-alpha or TNF-beta trimer.

Sea urchin egg receptor for sperm: sequence similarity of binding domain and hsp70

Fertilization depends on cell surface recognition proteins that interact and thereby mediate binding and subsequent fusion of the sperm and egg. Overlapping complementary DNA's encoding the egg plasma membrane receptor for sperm from the sea urchin Strongylocentrotus purpuratus were cloned and sequenced. Analysis of the deduced primary structure suggests that the receptor is a transmembrane protein with a short cytoplasmic domain. This domain showed no sequence similarity to known protein sequences. In contrast, the extracellular, sperm binding domain of the receptor did show sequence similarity to the heat shock protein 70 (hsp70) family of proteins. Recombinant protein representing this portion of the receptor bound to the sperm protein, binding, and also inhibited fertilization in a species-specific manner; beads coated with the protein became specifically bound to acrosome-reacted sperm. These data provide a basis for detailed investigations of molecular interactions that occur in gamete recognition and egg activation.

Molecular cloning of chicken aggrecan. Structural analyses

The large, aggregating chondroitin sulphate proteoglycan of cartilage, aggrecan, has served as a generic model of proteoglycan structure. Molecular cloning of aggrecans has further defined their amino acid sequences and domain structures. In this study, we have obtained the complete coding sequence of chicken sternal cartilage aggrecan by a combination of cDNA and genomic DNA sequencing. The composite sequence is 6117 bp in length, encoding 1951 amino acids. Comparison of chicken aggrecan protein primary structure with rat, human and bovine aggrecans has disclosed both similarities and differences. The domains which are most highly conserved at 70-80% identity are the N-terminal domains G1 and G2 and the C-terminal domain G3. The chondroitin sulphate domain of chicken aggrecan is smaller than that of rat and human aggrecans and has very distinctive repeat sequences. It has two separate sections, one comprising 12 consecutive Ser-Gly-Glu repeats of 20 amino acids each, adjacent to the other which has 23 discontinuous Ser-Gly-Glu repeats of 10 amino acids each; this latter region, N-terminal to the former one, appears to be unique to chicken aggrecan. The two regions contain a total of 94 potential chondroitin sulphate attachment sites. Genomic comparison shows that, although chicken exons 11-14 are identical in size to the rat and human exons, chicken exon 10 is the smallest of the three species. This is also reflected in the size of its chondroitin sulphate coding region and in the total number of Ser-Gly pairs. The putative keratan sulphate domain shows 31-45% identity with the other species and lacks the repetitive sequences seen in the others. In summary, while the linear arrangement of specific domains of chicken aggrecan is identical to that in the aggrecans of other species, and while there is considerable identity of three separate domains, chicken aggrecan demonstrates unique features, notably in its chondroitin sulphate domain and its keratan sulphate domain. Thus different variants of chondroitin sulphate and keratan sulphate domains may have evolved separately to fulfil specific biochemical and physiological functions.

Identification of a region of mouse zona pellucida glycoprotein mZP3 that possesses sperm receptor activity

The ability of mouse zona pellucida glycoprotein ZP3 (mZP3) to function as a sperm receptor is attributable to certain of its oligosaccharides, not to its polypeptide (P. M. Wassarman, 1990. Development 108, 1-17). Here, purified, radioiodinated mZP3 was digested by either papain or V8 protease, and the glycopeptides produced were fractionated by HPLC and assayed for sperm receptor activity in vitro. Each proteolytic digest of mZP3 contained a heavily glycosylated peptide, approximately 55,000 apparent M(r), that exhibited sperm receptor activity in vitro. To determine the region of mZP3 polypeptide from which the active glycopeptides were derived, Western gel immunoblotting, employing an antiserum directed against a specific mZP3 peptide epitope, and automated amino-terminal amino acid sequencing were employed. Results of these experiments strongly suggest that the active glycopeptides produced by digestion of mZP3 with either papain or V8 protease are derived from the same region of the carboxy-terminal half of the mZP3 polypeptide. These and other findings are discussed in terms of mZP3 structure and function.

Analysis of glycoforms present in two mouse IgG2a monoclonal antibody preparations

Lectins have been used for the determination of the oligosaccharide structures expressed by two monoclonal IgG antibodies, MN12 and RIV6. Dot blot experiments revealed the presence of terminal Fuc alpha (1-->6)GlcNAc, Gal beta (1-->3)GalNAc, Gal beta (1-->4)GlcNAc, Man alpha (1-->6, 1-->3)Man, NeuAc alpha (2-->6)Gal and NeuAc alpha (2-->6)GalNAc on both monoclonal antibodies. MN12 was shown to contain a carbohydrate moiety within the Fc region only. RIV6 contained carbohydrate moieties within both the Fc and Fab regions. Additional O-glycosidic linked carbohydrate chains were detected within the Fc region of both monoclonal antibodies. High mannose structures were also detected on both Mabs.

Porcine oocyte zona pellucida M(r) 55,000 glycoproteins: identification of O-glycosylated domains

The distribution of O-linked oligosaccharides on the M(r) 55,000 glycoproteins, ZP3 alpha and ZP3 beta, of the porcine oocyte zona pellucida was examined. Purified preparations of endo-beta-galactosidase digested ZP3 alpha and ZP3 beta were reduced and carboxamidomethylated and digested with trypsin. When the trypsin digests were mapped by HPLC, each glycoprotein yielded only one N-acetylgalactosamine containing glycopeptide. Purification of the O-glycopeptides was achieved by a two-step protocol. Tryptic digests were applied to jacalin-agarose and specifically-bound O-glycopeptides (alpha OGP and beta OGP) were eluted with buffer containing 50 mM alpha-methylgalactoside as the haptenic sugar. Further purification of each O-glycopeptide was accomplished by reverse phase HPLC. Purified O-glycopeptides were characterized with respect to amino acid and carbohydrate compositions and sequenced by automated Edman degradation; alpha OGP was a 41-residue glycopeptide with three O-linked sugar chains. Sequence comparisons revealed a 75% identity between alpha OGP and a corresponding segment of rabbit rec55 zona protein; beta OGP was a 25-residue glycopeptide characterized by the presence of one N-linked and five O-linked sugar chains and a trypsin-resistant internal arginine residue. Sequence alignments revealed an 80% or greater identity between beta OGP and internal peptides of mouse, hamster and human ZP3 zona proteins. These studies demonstrate that in the case of ZP3 alpha and ZP3 beta, the pig homologues of rabbit rec55 and mouse ZP3, respectively, O-linked oligosaccharides are confined within delimited domains rather than widely dispersed on the polypeptide backbone. Such clustering of O-linked oligosaccharides may represent an essential determinant of the structure and biological activity of zona proteins.

Characterization of the oligosaccharide side chains on kainate binding proteins and AMPA receptors

The amino acid sequences of the kainate binding proteins (KBPs) from frog and chicken brain are homologous with the carboxy terminal half of the rat brain AMPA receptors. In this study, we have characterized the oligosaccharide side chains present on the KBPs from chicken and frog brain, and the AMPA receptors (GluR1, GluR2, and GluR3) from rat brain. Deglycosylation of the asparagine-linked carbohydrates present on the chicken, frog, and rat receptor subunits with N-glycanase, resulted in decreases in the relative molecular weights (M(r)) of 3.4, 3.4, and 5.1 kDa respectively. Thus the percent of asparagine linked carbohydrate (based on M(r) values derived from SDS polyacrylamide gels) of the 49 kDa chicken, the 48 kDa frog, and the 107 kDa receptor rat subunits is 6.9, 7.1, and 4.8 percent respectively. No shifts in the M(r) were detected after treatment with neuraminidase indicating that sialic acid does not appear to be a major component of these receptors. Lectin binding studies demonstrated that both asparagine-linked and serine/threonine-linked oligosaccharides were present in the chicken, frog, and rat proteins. The data indicate that at least one of the asparagine linked oligosaccharide side chains appear to be of the complex or non-bisected hybrid type in all three species. The similarities in the glycosyl moieties of the chicken and frog kainate KBPs and the rat brain AMPA receptors suggests that the homology in the amino acid sequences between these proteins may extend to homology in their oligosaccharide sides chains as well.

Possible relationship between coding recognition amino acid sequence motif or residue(s) and post-translational chemical modification of proteins

1. The "code-sequence" of N-glycosylation site(s), the amino acids located around O-glycosylation site(s), the sequence motifs of several kinases, the sequence motifs of--sulfation, amidation, isoprenylation, myristoylation, palmitoylation and N-acetylation, Aspartic and Asparagine hydroxylation-site, gamma-carboxyglutamate domain, phosphopantetheine attachment site etc. are extensively listed, compared to those reported by "PROSITE" Computer Screen Center and discussed. 2. The structural aspects of protein-DNA recognition are quoted as discussion and conclusion.

Human insulin-like growth factor binding protein-6 is O-glycosylated

Insulin-like growth factor binding protein-6 is abundant in cerebrospinal fluid and has a marked preferential binding affinity for IGF-II over IGF-I. The present study demonstrates that IGFBP-6 is O-glycosylated but not N-glycosylated. Carbohydrate analysis revealed the presence of approximately 20-30 carbohydrate residues/molecule. Galactosamine, galactose and sialic acid were most abundant, with glucosamine and fucose present in lower concentrations. Mannose was not detected. Enzymatic deglycosylation did not alter the high affinity of IGF binding protein-6 for IGF-II (Ka 4.4 +/- 2.2 x 10(11) M-1) or its preference for IGF-II over IGF-I. Glycosylation of IGFBP-6 may affect its secretion, in vivo stability or localization, but does not affect its ligand binding properties.

Specific in vitro O-glycosylation of human granulocyte-macrophage colony-stimulating-factor-derived peptides by O-glycosyltransferases of yeast and rat liver cells

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) is O-glycosylated at residues Ser9 and Thr10 during secretion by yeast and COS-1 cells [Ernst, J.F., Mermod, J.-J. and Richman, L.I. (1992) Eur. J. Biochem. 203, 663-667]. Two types of octapeptides encompassing residues 4-11 (peptide 4-11) and variants thereof, or residues 8-15 (peptide 8-15) of hGM-CSF were tested as substrates for in vitro O-glycosylation using dolichyl-phosphate- D-mannose: protein O-D-mannosyltransferase (Man-transferase) of the yeast Saccharomyces cerevisiae, or UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (GalNAc-transferase) of rat liver cells. Peptide 8-15 was found to be O-glycosylated at residues Ser9 and Thr10 by GalNAc-transferase and, with reduced efficiency, also by Man-transferase. Peptide 4-11 was a good substrate for yeast Man-transferase, leading to mannosylation of only Thr10, whereas it was very poorly O-glycosylated at positions Ser5 and Ser7 by GalNAc-transferase. The observed differences in peptide-acceptor activities indicate that the site of O-glycosylation depends on similar, but not identical protein structural features in yeast and mammalian cells.

Polypeptide N-acetylgalactosaminyltransferase activity in tracheal epithelial microsomes

Pig tracheal epithelium, a site of extensive mucin biosynthesis, contained polypeptide N-acetylgalactosaminyltransferase activity directed towards L-threonine residues. The enzyme preparation was broadly similar in properties to preparations from other tissues, e.g. pig and bovine submaxillary glands, bovine colostrum, BW5147 mouse lymphoma and baby-hamster kidney cells. Enzyme was membrane-bound and was released from microsomal preparations by extraction with Triton X-100. Extracted enzyme had a pH optimum of 7.5, had a requirement for Mn2+ (10 mM) and was inhibited by Na2EDTA. The Km for UDP-N-acetylgalactosamine was 110 microM and that for an octapeptide acceptor (VTPRTPPP) was 3.0 mM at 37 degrees C. Using a range of synthetic peptides of known structure related to TPPP it was established that L-threonine residues were specifically O-glycosylated probably in the alpha-configuration. Synthetic peptides containing the TPPP sequence required a peptide length of five or more for significant acceptor activity. In VTPRTPPP the two threonine residues were similarly glycosylated, as revealed by tryptic cleavage of the glycosylated product and separation of the 3H-labelled fragments. The enzyme preparation also specifically catalysed the transfer of N-acetylgalactosaminyl residues from UDP-N-acetyl[1-3H]galactosamine to bovine submaxillary mucin core protein and to myelin basic protein.