Online mass spectrometric analysis of proteins/peptides following electrolytic cleavage of disulfide bonds

The disulfide bond bridge is an important post-translational modification for proteins. This study presents a structural analysis of biologically active peptides and proteins containing disulfide bonds using electrochemistry (EC) online combined with desorption electrospray ionization mass spectrometry (DESI-MS), in which the sample undergoes electrolytic disulfide cleavage in an electrochemical flow cell followed by MS detection. Using this EC/DESI-MS method, the disulfide-containing peptides can be quickly identified from enzymatic digestion mixtures, simply based on the abrupt decrease in their relative ion abundances after electrolysis. Peptide mass mapping and tandem MS analysis of the ions of the resulting free peptide chains can possibly establish the disulfide linkage pattern and sequence the precursor peptides. In this regard, the method provides much more chemical information than previous analogous electrochemical analyses. In addition, derivatization of thiols by selective selenamide reagents is useful for easy recognition of reduced peptide ions and the number of their free thiols. Furthermore, electrolytic reduction of proteins (e.g., α-lactalbumin) leads to increased charges on the detected protein ions, revealing the role of disulfide bonds on maintaining protein conformation. This electrochemical mass spectrometric method is fast (completed in few minutes) and does not need chemical reductants, potentially having valuable applications in proteomics research.     

Development of disulfide peptide mapping and determination of disulfide structure of recombinant human osteoprotegerin chimera produced in Escherichia coli

Recombinant human osteoprotegerin chimera is a 90-kDa protein containing a human IgG Fc domain fused to human osteoprotegerin. The molecule is a dimer linked by two intermolecular disulfide bonds and contains eleven intramolecular disulfide bonds per monomer. A cysteine-rich region in osteoprotegerin contains nine disulfide bridges homologous to the cysteine-rich signature structure of the tumor necrosis factor receptor/nerve growth factor receptor superfamily. In this report, we have developed peptide mapping procedures suitable to generate disulfide-containing peptides for disulfide structure assignment of the fusion molecule. The methods employed included proteolytic digestion using endoproteinases Glu-C and Lys-C in combination followed by LC-MS analyses. Disulfide linkages of peptide fragments containing a single disulfide bond were assigned by sequence analysis via detection of (phenylthiohydantoinyl) cystine and/or by MS analysis. Disulfide bonds of a large, core fragment containing three peptide sequences linked by four disulfides were assigned after generation of smaller disulfide-linked peptides by a secondary thermolysin digestion. Disulfide structures of peptide fragments containing two disulfide bonds were assigned using matrix-assisted laser desorption ionization mass spectrometry with postsource decay. Both the inter- and intramolecular disulfide linkages of the chimeric dimer were confirmed.     

Intrachain disulfide bond in the core hinge region of human IgG4.

IgG is a tetrameric protein composed of two copies each of the light and heavy chains. The four-chain structure is maintained by strong noncovalent interactions between the amino-terminal half of pairs of heavy-light chains and between the carboxyl-terminal regions of the two heavy chains. In addition, interchain disulfide bonds link each heavy-light chain and also link the paired heavy chains. An engineered human IgG4 specific for human tumor necrosis factor-alpha (CDP571) is similar to human myeloma IgG4 in that it is secreted as both disulfide bonded tetramers (approximately 75% of the total amount of IgG) and as tetramers composed of nondisulfide bonded half-IgG4 (heavy chain disulfide bonded to light chain) molecules. However, when CDP571 was genetically engineered with a proline at residue 229 of the core hinge region rather than serine, CDP571 (S229P), or with an IgG1 rather than IgG4 hinge region, CDP571(gamma 1), only trace amounts of nondisulfide bonded half-IgG tetramers were observed. Trypsin digest reversephase HPLC peptide mapping studies of CDP571 and CDP571(gamma 1) with on-line electrospray ionization mass spectroscopy supplemented with Edman sequencing identified the chemical factor preventing inter-heavy chain disulfide bond formation between half-IgG molecules: the two cysteines in the IgG4 and IgG1 core hinge region (CPSCP and CPPCP, respectively) are capable of forming an intrachain disulfide bond. Conformational modeling studies on cyclic disulfide bonded CPSCP and CPPCP peptides yielded energy ranges for the low-energy conformations of 31-33 kcal/mol and 40-42 kcal/mol, respectively. In addition, higher torsion and angle bending energies were observed for the CPPCP peptide due to backbone constraints caused by the extra proline. These modeling results suggest a reason why a larger fraction of intrachain bonds are observed in IgG4 rather than IgG1 molecules: the serine in the core hinge region of IgG4 allows more hinge region flexibility than the proline of IgG1 and thus may permit formation of a stable intrachain disulfide bond more readily.     

Use of recombinant DNA derived human relaxin to probe the structure of the native protein

This report describes the physical, chemical, and biological characterization of recombinant human relaxin (rhRlx) used as a probe to establish the disulfide pairing in native human relaxin. This strategy is necessary since native human relaxin is only available in the nanogram range. The relaxin molecule is composed of two nonidentical peptide chains, an A-chain 24 amino acids in length and a B-chain of 29 amino acids, linked by two disulfide bridges with an additional disulfide linkage in the A-chain. Native relaxin isolated from human corpora lutea was compared to rhRlx by reversed-phase chromatography, partial sequence analysis, mass spectroscopy, and bioassay. The potency of rhRlx was established by its ability to stimulate cAMP from primary human uterine endometrial cells. Native relaxin isolated from human corpora lutea was equipotent to chemically synthesized relaxin, which in turn was equipotent to rhRlx. A tryptic map was developed for rhRlx to confirm the complete amino acid sequence and assignment of the disulfide bonds. The three disulfide bonds (CysA10-CysA15, CysA11-CysB11, and CysA24-CysB23) were assigned by mass spectrometric analysis of the tryptic peptides and by comparison to chemically synthesized peptides disulfide linked in the two most probable configurations. In addition, the observed amino acid composition and sequence of rhRlx was in agreement with that predicted from the cDNA sequence with the exception that the A-chain amino terminal was pyroglutamic acid. The migration of rhRlx upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis was consistent with a monomeric structure, and the identity of the band was demonstrated by immunoblotting.     

Determination of the disulfide bond arrangement of human respiratory syncytial virus attachment (G) protein by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

The attachment protein or G protein of the A2 strain of human respiratory syncytial virus (RSV) was digested with trypsin and the resultant peptides separated by reverse-phase high-performance liquid chromatography (HPLC). One tryptic peptide produced a mass by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) corresponding to residues 152-187 with the four Cys residues of the ectodomain (residues 173, 176, 182, and 186) in disulfide linkage and absence of glycosylation. Sub-digestion of this tryptic peptide with pepsin and thermolysin produced peptides consistent with disulfide bonds between Cys173 and Cys186 and between Cys176 and Cys182. Analysis of ions produced by post-source decay of a peptic peptide during MALDI-TOF-MS revealed fragmentation of peptide bonds with minimal fission of an inter-chain disulfide bond. Ions produced by this unprecedented MALDI-induced post-source fragmentation corroborated the existence of the disulfide arrangement deduced from mass analysis of proteolysis products. These findings indicate that the ectodomain of the G protein has a non-glycosylated subdomain containing a "cystine noose."     

Determination of the glycosylation patterns, disulfide linkages, and protein heterogeneities of baculovirus-expressed mouse interleukin-3 by mass spectrometry.

The primary structure of mouse interleukin-3 (IL-3) expressed by recombinant baculovirus-infected silkworm (Bombyx mori) larvae was analyzed by subjecting isolated IL-3 derived peptides to liquid secondary ion mass spectrometry. Two species of IL-3 were isolated from the silkworm hemolymph by reverse-phase high-pressure liquid chromatography. The major component has M(r)20-22 x 10(3) as determined by SDS-PAGE. Liquid secondary ion mass spectrometric analysis was carried out on the reduced tryptic and endopeptidase lysyl-C peptides of glycosylated and deglycosylated IL-3. These studies provided evidence that (1) Asn-16 is heterogeneously glycosylated with four different oligosaccharides, (2) Asn-86 is either nonglycosylated or has attached to it one oligosaccharide, (3) the N-glycosylation sites Asn-44 and Asn-51 are not glycosylated, and (4) there is no O-glycosylation. Liquid secondary ion mass spectrometric analysis of the unreduced tryptic peptides provided evidence for disulfide linkages between Cys-140 and Cys-79 or Cys-80 and between Cys-17 and Cys-79 or Cys-80. In comparison to the major component, a minor IL-3 species (M(r) 17-19 x 10(3) by SDS-PAGE) isolated from the hemolymph showed no difference with respect to the glycosylation pattern or the disulfide linkages, but it was cleaved between Ala-127 and Ser-128, and only a disulfide linkage between Cys-140 and Cys-79 or Cys-80 held the molecule together.(ABSTRACT TRUNCATED AT 250 WORDS)     

Renaturation and purification of rhGM-CSF with ion-exchange chromatography

The renaturation and purification of recombinant human granulocyte macrophage colony stimulation factor (rhGM-CSF) expressed in Escherichia coli with strong anion-exchange chromatography (SAX) were studied. The effects of pH values, ratios of concentrations of GSH/GSSG, and urea concentrations in the mobile phase on the renaturation and purification of rhGM-CSF with SAX were investigated, respectively. The results show that the above three factors have remarkable influences on the efficiency of renaturation and mass recovery of rhGM-CSF. The addition of GSH/GSSG in the mobile phase can improve the formation of correct disulfide bonds in rhGM-CSF so that its renaturation yield increases. In addition, to enhance the mass recovery of rhGM-CSF with SAX, the low concentration of urea was added in the mobile phase to prevent denatured protein aggregation. Under the optimal conditions, rhGM-CSF was renatured with simultaneous purification on SAX column within 30 min only by one step. After that its specific bioactivity, mass recovery, and purity reached 1.66 x 10(7) IU x mg, 58.8%, and 96.2%, respectively.     

Automated in-solution protein digestion using a commonly available high-performance liquid chromatography autosampler

A completely automated peptide mapping liquid chromatography/mass spectrometry (LC/MS) system for characterization of therapeutic proteins in which a common high-performance liquid chromatography (HPLC) autosampler is used for automated sample preparation, including protein denaturation, reduction, alkylation, and enzymatic digestion, is described. The digested protein samples are then automatically subjected to LC/MS analysis using the same HPLC system. The system was used for peptide mapping of monoclonal antibodies (mAbs), known as a challenging group of therapeutic proteins for achieving complete coverage and quantitative representation of all peptides. Detailed sample preparation protocols, using an Agilent HPLC system, are described for Lys-C digestion of mAbs with intact disulfide bonds and tryptic digestion of mAbs after reduction and alkylation. The automated procedure of Lys-C digestion of nonreduced antibody, followed by postdigestion disulfide reduction, produces both the nonreduced and reduced digests that facilitate disulfide linkage analysis. The automated peptide mapping LC/MS system has great utility in preparing and analyzing multiple samples for protein characterization, identification, and quantification of posttranslational modifications during process and formulation development as well as for protein identity and quality control.     

Combined use of ESI-MS and UV diode-array detection for localization of disulfide bonds in proteins: application to an alpha-L-fucosidase of pea.

A simplified strategy is described for the assignment of disulfide bonds in proteins of medium to high molecular mass (10-30 kDa). The method combines the use of high-performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC-ESI-MS) and HPLC with UV diode-array detection (HPLC diode array). The denatured protein is subjected to proteolysis and the peptide mixture is divided into three fractions: (i) underivatized peptides, (ii) ethylpyridylated peptides, and (iii) reduced and ethylpyridylated peptides. The three peptide ensembles are then subjected to chromatographic and spectroscopic analysis. A systematic methodology is described to analyze the large amount of data obtained. The method was applied to the localization of disulfide bonds in alpha-L-fucosidase from pea. The two disulfide bonds were located between residues Cys64 and Cys109 and between Cys162 and Cys169, while Cys127 was free.     

Fragmentation Follows Structure: Top-Down Mass Spectrometry Elucidates the Topology of Engineered Cystine-Knot Miniproteins

Over the last decades the field of pharmaceutically relevant peptides has enormously expanded. Among them, several peptide families exist that contain three or more disulfide bonds. In this context, elucidation of the disulfide patterns is extremely important as these motifs are often prerequisites for folding, stability, and activity. An example of this structure-determining pattern is a cystine knot which comprises three constrained disulfide bonds and represents a core element in a vast number of mechanically interlocked peptidic structures possessing different biological activities. Herein, we present our studies on disulfide pattern determination and structure elucidation of cystine-knot miniproteins derived from Momordica cochinchinensis peptide MCoTI-II, which act as potent inhibitors of human matriptase-1. A top-down mass spectrometric analysis of the oxidised and bioactive peptides is described. Following the detailed sequencing of the peptide backbone, interpretation of the MS³ spectra allowed for the verification of the knotted topology of the examined miniproteins. Moreover, we found that the fragmentation pattern depends on the knottin’s folding state, hence, tertiary structure, which to our knowledge has not been described for a top-down MS approach before.     

Disulfide bond structure of the atrial natriuretic peptide receptor extracellular domain: conserved disulfide bonds among guanylate cyclase-coupled receptors

The disulfide bond structure of the extracellular domain of rat atrial natriuretic peptide (ANP) receptor (NPR-ECD) has been determined by mass spectrometry (MS) and Edman sequencing. Recombinant NPR-ECD expressed in COS-1 cells and purified from the culture medium binds ANP with as high affinity as the natural ANP receptor. Reaction with iodoacetic acid yielded no S-carboxymethylcysteine, indicating that all six Cys residues in NPR-ECD are involved in disulfide bonds. Electrospray ionization MS of NPR-ECD deglycosylated by peptide-N-glycosidase F gave a molecular mass of 48377.5+/-1.6 Da, which was consistent with the presence of three disulfide bonds. Liquid chromatography MS analysis of a lysylendopeptidase digest yielded three cystine-containing fragments with disulfide bonds Cys(60)-Cys(86), Cys(164)-Cys(213) and Cys(423)-Cys(432) based on their observed masses. These bonds were confirmed by Edman sequencing of each of the three fragments. No evidence for an inter-molecular disulfide bond was found. The six Cys residues in NPR-ECD, forming a 1-2, 3-4, 5-6 disulfide pairing pattern, are strictly conserved among A-type natriuretic peptide receptors and are similar in B-type receptors. We found that in other families of guanylate cyclase-coupled receptors, the Cys residues involved in 1-2 and 5-6 disulfide pairs are conserved in nearly all, suggesting an important contribution of these disulfide bonds to the receptor's structure and function.     

Two-step selective formation of three disulfide bridges in the synthesis of the C-terminal epidermal growth factor-like domain in human blood coagulation factor IX.

The 45-residue C-terminal EGF-like domain in human blood coagulation factor IX has been synthesized by a 2-step method to form selectively 3 disulfide bridges. Four out of 6 cysteines are blocked with either trityl or 4-methyl-benzyl, and the remaining 2 cysteines are blocked with acetamidomethyl (Acm). In the first step, 4 free cysteinyl thiols are released concurrently with the removal of all protecting groups except Acm and are oxidized to form 1 of the 3 possible isomers containing 2 pairs of disulfides. In the second step, iodine is used to remove the Acm groups to yield the third disulfide bridge. This approach reduces the number of possible disulfide bridging patterns from 15 to 3. To determine the optimal protecting group strategy, 3 peptides are synthesized, each with Acm blocking 1 of the 3 pairs of cysteines involved in disulfide bridges: Cys5 to Cys16 (Cys 1-3), Cys12 to Cys26 (Cys 2-4), or Cys28 to Cys41 (Cys 5-6). Only the peptide having the Cys 2-4 pair blocked with Acm forms the desired disulfide isomer (Cys 1-3/5-6) in high yield after the first step folding, as identified by proteolytic digestion in conjunction with mass spectrometric peptide mapping. Thus, the choice of which pair of cysteines to block with Acm is critically important. In the case of EGF-like peptides, it is better to place the Acm blocking groups on one of the pairs of cysteines involved in the crossing of disulfide bonds.     

Location of the Disulfide Bonds of the Sweetness-suppressing Polypeptide Gurmarin

The sweetness-suppressing polypeptide gurmarin has been isolated from the leaves of Gymnema sylvestre and consists of 35 amino acid residues including three intramolecular disulfide bonds. The primary structure has already been determined. The positions of the disulfide bonds were located, by a combination of mass spectrometric analysis and sequencing of cystine-containing pep tides obtained by thermolysin-catalyzed hydrolysis of gurmarin, to be at Cys3–Cys18, Cys10–Cys23, and Cys17–Cys33.     

Structural characterization of human hemoglobin crosslinked by bis(3,5-dibromosalicyl) fumarate using mass spectrometric techniques.

Diaspirin crosslinked hemoglobin (DCLHb) was analyzed by mass spectrometric-based techniques to identify the protein modifications effected by the crosslinking reaction with bis(3,5-dibromosalicyl) fumarate. DCLHb consists of two principal components. These components were isolated by size-exclusion chromatography and identified by measurement of their molecular weight using electrospray mass spectrometry and subsequent peptide mass mapping and mass spectrometric sequence analysis of their individual digests. Three major RP-HPLC fractions were observed from the major hemoglobin in DCLHb. Their MWs matched the MW of heme, intact hemoglobin beta-chain, and two hemoglobin alpha-chains crosslinked by a fumarate moiety, respectively. The minor HPLC peaks of DCLHb were also separated, and characterized by mass spectrometric methods. These minor components revealed additional details of the structural nature of covalent modification of DCLHb.     

Assignment of the disulfide bonds in the sweet protein brazzein

The thermostable sweet protein brazzein consists of 54 amino acid residues and has four intramolecular disulfide bonds, the location of which is unknown. We found that brazzein resists enzymatic hydrolysis at enzyme/substrate ratios (w/w) of 1:100-1:10 at 35–40°C for 24–48 h. Brazzein was hydrolyzed using thermolysin at an enzyme/substrate ratio of 1:1 (w/w) in water, pH 5.5. for 6 h and at 50°C. The disulfide bonds were determined, by a combination of mass spectrometric analysis and amino acid sequencing of cystine-containing peptides, to be between Cys4-Cys52, Cys16-Cys37, Cys22-Cys47, and Cys26-Cys49. These disulfide bonds contribute to its thermostability. © 1996 John Wiley & Sons, Inc.     

Recombinant production and structural studies of the Aplysia water-borne protein pheromone enticin indicates it has a novel disulfide stabilized fold

Enticin is one of three Aplysia proteins released during egg laying that act in concert with the pheromone attractin to attract other Aplysia and stimulate mating behavior. Whereas the enticin cDNA predicts a 69-residue mature protein, enticin isolated from the albumen gland was found to be posttranslationally processed in vivo by cleavage at Arg(50) residue to generate a smaller 49-residue mature peptide. The Arg(50) cleavage site is conserved in enticin from both Aplysia californica and Aplysia brasiliana. In order to generate sufficient enticin for structural studies, recombinant full-length protein was produced in a soluble form in Escherichia coli using a cold shock promoter-based protein expression system. The enticin cDNA was cloned into the bacterial vector pCold III, and efficiently expressed, as determined by amino acid microsequence and immunoblot analyses. Recombinant enticin, which contained an additional N-terminal 13-residue translation-enhancing element, was purified by reversed-phase HPLC and compared to enticin isolated from the albumen gland. The three disulfide bonds in enticin were characterized by endoproteinase Glu-C proteolysis followed by mass spectrometric characterization of the fragments. The cysteine pairing, for both recombinant and native enticin, was I-II, III-IV, and V-VI, confirming that the protein produced in the bacterial system was correctly folded. The circular dichroism spectrum of the recombinant protein indicated it was predominantly alpha-helical. While this was consistent with fold recognition server results indicating a fold for enticin similar to that of attractin, the disulfide bonding pattern differs. A model for enticin was prepared based on its helical structure and these disulfide constraints.     

Determination of the disulfide structure of sillucin, a highly knotted, cysteine-rich peptide, by cyanylation/cleavage mass mapping

The disulfide structure of sillucin, a highly knotted, cysteine-rich, antimicrobial peptide, isolated from Rhizomucor pusillus, has been determined to be Cys2--Cys7, Cys12--Cys24, Cys13--Cys30, and Cys14--Cys21 by disulfide mass mapping based on partial reduction and CN-induced cleavage enabled by cyanylation. The denatured 30-residue peptide was subjected to partial reduction by tris(2-carboxyethyl)phosphine hydrochloride at pH 3 to produce a mixture of partially reduced sillucin species; the nascent sulfhydryl groups were immediately cyanylated by 1-cyano-4-(dimethylamino)pyridinium tetrafluoroborate. The cyanylated species, separated and collected during reversed phase high-performance liquid chromatography, were treated with aqueous ammonia, which cleaved the peptide chain on the N-terminal side of cyanylated cysteine residues. The CN-induced cleavage mixture was analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry before and after complete reduction of residual disulfide bonds in partially reduced and cyanylated species to mass map the truncated peptides to the sequence. Because the masses of the CN-induced cleavage fragments of both singly and doubly reduced and cyanylated sillucin are related to the linkages of the disulfide bonds in the original molecule, the presence of certain truncated peptide(s) can be used to positively identify the linkage of a specific disulfide bond or exclude the presence of other possible linkages.     

人表皮生长因子肽谱及一级结构的质谱法分析

采用液相色谱－质谱联用法（ＬＣ－ＭＳ）,分别对基因工程表达的人表皮生长因子胰蛋白酶和Ｖ８蛋白酶的酶解产物进行了肽质谱分析,获得各肽段的分子量信息。并对大部分肽段用串联质谱（ＭＳ－ＭＳ）分析了肽的序列,获得其氨基酸序列信息。此外,还测定了分子中二硫键的数目。均获得满意的结果。这一方法的建立将有助于天然的,人工合成的及基因工程表达的蛋白质或多肽的鉴定和结构分析,其精确度及分辨率都超过常规方法。     

Structural Characterization of the Recombinant P40 Heavy Chain Subunit Monomer and Homodimer of Murine IL-12

Interleukin-12 (IL-12) is a heterodimeric cytokine that consists of two structurally unrelated subunits, P35 and P40. However, when expressed alone in Chinese hamster ovary (CHO) cells, murine P40 showed two species of different molecular weights under nonreducing conditions, a monomeric form of 45 kDa and a homodimer of >97 kDa. Under reducing conditions the two forms migrated as an identical array of species of 40-45 kDa. The monomer was separated from the homodimer under nonreducing conditions by heparin affinity chromatography and the disulfide bond structures of both species were determined by peptide mapping, Edman sequencing, and mass spectrometry. The peptide maps of the two species were identical except for a single peak that changed retention time. Sequencing showed that this peak contained two peptides of identical sequences in both maps. Mass spectrometric analysis of the peak from the >97-kDa species revealed an ion of double the expected mass, thus indicating that the peptide pair had dimerized. Mass analysis of the peak from the 40- to 45-kDa species showed that the peptide pair contained a mass difference that corresponded to that of an extra cysteine and which disappeared upon reduction. Amino acid analysis confirmed that the monomeric form of rmP40 is modified by a reducible cysteine. Structural analysis of the remainder of the cysteine-containing peaks showed that both species of rmP40 contained the same set of intramolecular disulfide bonds. The murine P40 homodimer arises from formation of a single intermolecular disulfide bond at Cys¹⁷⁵. In the monomeric P40, however, this cysteine is capped by an additional cysteine. Purified rmP40 monomer and homodimer inhibited the IL-12-dependent induction of interferon-γ, but neither appeared capable of inducing IL-12-like biological activity.     

Structural characterization of recombinant soluble rat neuroligin 1: mapping of secondary structure and glycosylation by mass spectrometry

Neuroligins (NLs) are a family of transmembrane proteins that function in synapse formation and/or remodeling by interacting with beta-neurexins (beta-NXs) to form heterophilic cell adhesions. The large N-terminal extracellular domain of NLs, required for beta-NX interactions, has sequence homology to the alpha/beta hydrolase fold superfamily of proteins. By peptide mapping and mass spectrometric analysis of a soluble recombinant form of NL1, several structural features of the extracellular domain have been established. Of the nine cysteine residues in NL1, eight are shown to form intramolecular disulfide bonds. Disulfide pairings of Cys 117 to Cys 153 and Cys 342 to Cys 353 are consistent with disulfide linkages that are conserved among the family of alpha/beta hydrolase proteins. The disulfide bond between Cys 172 and Cys 181 occurs within a region of the protein encoded by an alternatively spliced exon. The disulfide pairing of Cys 512 and Cys 546 in NL1 yields a structural motif unique to the NLs, since these residues are highly conserved. The potential N-glycosylation sequons in NL1 at Asn 109, Asn 303, Asn 343, and Asn 547 are shown occupied by carbohydrate. An additional consensus sequence for N-glycosylation at Asn 662 is likely occupied. Analysis of N-linked oligosaccharide content by mass matching paradigms reveals significant microheterogeneous populations of complex glycosyl moieties. In addition, O-linked glycosylation is observed in the predicted stalk region of NL1, prior to the transmembrane spanning domain. From predictions based on sequence homology of NL1 to acetylcholinesterase and the molecular features of NL1 established from mass spectrometric analysis, a novel topology model for NL three-dimensional structure has been constructed.