Mass spectrometry analysis of recombinant human ZP3 expressed in glycosylation-deficient CHO cells

The zona pellucida is an extracellular matrix that mediates taxon-specific fertilization in which human sperm will not bind to mouse eggs. The mouse zona pellucida is composed of three glycoproteins (ZP1, ZP2, ZP3). The primary structure of each has been deduced from the cDNA nucleic acid sequence, and each has been analyzed by mass spectrometry. However, determination of the secondary structure and processing of the human zona proteins have been hampered by the paucity of biological material. To investigate if taxon-specific sperm-egg recognition was ascribable to structural differences in a zona protein required for matrix formation, recombinant human ZP3 was expressed in CHO-Lec3.2.8.1 cells and compared to mouse ZP3. With nearly complete coverage, LC-QTOF mass spectrometry was used to determine the cleavage of an N-terminal signal peptide (amino acids 1-22) and the release of secreted ZP3 from a C-terminal transmembrane domain (amino acids 379-424). The resultant N-terminal glutamine was cyclized to pyroglutamate (pyrGln(23)), and several C-terminal peptides were detected, including one ending at Asn(350). The disulfide bond linkages of eight cysteine residues in the conserved zona domain were ascertained (Cys(46)/Cys(140), Cys(78)/Cys(99), Cys(217)/Cys(282), Cys(239)/Cys(300)), but the precise linkage of two additional disulfide bonds was indeterminate due to clustering of the remaining four cysteine residues (Cys(319), Cys(321), Cys(322), Cys(327)). Three of the four potential N-linked oligosaccharide binding sites (Asn(125), Asn(147), Asn(272)) were occupied, and clusters of O-glycans were observed within two regions, amino acids 156-173 and 260-281. Taken together, these data indicate that human and mouse ZP3 proteins are quite similar, and alternative explanations of taxon-specific sperm binding warrant exploration.     

Determination of the disulfide structure and N-glycosylation sites of the extracellular domain of the human signal transducer gp130

gp130 is the common signal transducing receptor subunit for the interleukin-6-type family of cytokines. Its extracellular region (sgp130) is predicted to consist of five fibronectin type III-like domains and an NH2-terminal Ig-like domain. Domains 2 and 3 constitute the cytokine-binding region defined by a set of four conserved cysteines and a WSXWS motif, respectively. Here we determine the disulfide structure of human sgp130 by peptide mapping, in the absence and presence of reducing agent, in combination with Edman degradation and mass spectrometry. Of the 13 cysteines present, 10 form disulfide bonds, two are present as free cysteines (Cys(279) and Cys(469)), and one (Cys(397)) is modified by S-cysteinylation. Of the 11 potential N-glycosylation sites, Asn(21), Asn(61), Asn(109), Asn(135), Asn(205), Asn(357), Asn(361), Asn(531), and Asn(542) are glycosylated but not Asn(224) and Asn(368). The disulfide bonds, Cys(112)-Cys(122) and Cys(150)-Cys(160), are consistent with known cytokine-binding region motifs. Unlike granulocyte colony-stimulating factor receptor, the connectivities of the four cysteines in the NH2-terminal domain of gp130 (Cys(6)-Cys(32) and Cys(26)-Cys(81)) are consistent with known superfamily of Ig-like domains. An eight-residue loop in domain 5 is tethered by Cys(436)-Cys(444). We have created a model predicting that this loop maintains Cys(469) in a reduced form, available for ligand-induced intramolecular disulfide bond formation. Furthermore, we postulate that domain 5 may play a role in the disulfide-linked homodimerization and activation process of gp130.     

Disulfide bond structure and N-glycosylation sites of the extracellular domain of the human interleukin-6 receptor

The high affinity interleukin-6 (IL-6) receptor is a hexameric complex consisting of two molecules each of IL-6, IL-6 receptor (IL-6R), and the high affinity converter and signaling molecule, gp130. The extracellular "soluble" part of the IL-6R (sIL-6R) consists of three domains: an amino-terminal Ig-like domain and two fibronectin-type III (FN III) domains. The two FN III domains comprise the cytokine-binding domain defined by a set of 4 conserved cysteine residues and a WSXWS sequence motif. Here, we have determined the disulfide structure of the human sIL-6R by peptide mapping in the absence and presence of reducing agent. Mass spectrometric analysis of these peptides revealed four disulfide bonds and two free cysteines. The disulfides Cys102-Cys113 and Cys146-Cys157 are consistent with known cytokine-binding domain motifs, and Cys28-Cys77 with known Ig superfamily domains. An unusual cysteine connectivity between Cys6-Cys174, which links the Ig-like and NH2-terminal FN III domains causing them to fold back onto each other, has not previously been observed among cytokine receptors. The two free cysteines (Cys192 and Cys258) were detected as cysteinyl-cysteines, although a small proportion of Cys258 was reactive with the alkylating agent 4-vinylpyridine. Of the four potential N-glycosylation sites, carbohydrate moieties were identified on Asn36, Asn74, and Asn202, but not on Asn226.     

Characterization of Alzheimer's beta -secretase protein BACE. A pepsin family member with unusual properties

The cerebral deposition of amyloid beta-peptide is an early and critical feature of Alzheimer's disease. Amyloid beta-peptide is released from the amyloid precursor protein by the sequential action of two proteases, beta-secretase and gamma-secretase, and these proteases are prime targets for therapeutic intervention. We have recently cloned a novel aspartic protease, BACE, with all the known properties of beta-secretase. Here we demonstrate that BACE is an N-glycosylated integral membrane protein that undergoes constitutive N-terminal processing in the Golgi apparatus. We have used a secreted Fc fusion-form of BACE (BACE-IgG) that contains the entire ectodomain for a detailed analysis of posttranslational modifications. This molecule starts at Glu(46) and contains four N-glycosylation sites (Asn(153), Asn(172), Asn(223), and Asn(354)). The six Cys residues in the ectodomain form three intramolecular disulfide linkages (Cys(216)-Cys(420), Cys(278)-Cys(443), and Cys(330)-Cys(380)). Despite the conservation of the active site residues and the 30-37% amino acid homology with known aspartic proteases, the disulfide motif is fundamentally different from that of other aspartic proteases. This difference may affect the substrate specificity of the enzyme. Taken together, both the presence of a transmembrane domain and the unusual disulfide bond structure lead us to conclude that BACE is an atypical pepsin family member.     

Primary structure and glycan moiety characterization of PD-Ss, type 1 ribosome-inactivating proteins from Phytolacca dioica L. seeds, by precursor ion discovery on a Q-TOF mass spectrometer

Seeds from Phytolacca dioica L. contain at least three N-glycosylated PD-Ss, type 1 ribosome-inactivating proteins (RIPs), which were separated and purified to homogeneity by conventional chromatographic techniques. ESI-Q-TOF mass spectrometry provided the accurate M(r) of native PD-S1 and PD-S3 (30957.1 and 29785.1, respectively) and the major form PD-S2 (30753.8). As the amino acid sequence of PD-S2 was already known, its disulfide pairing was determined and found to be Cys34-Cys262 and Cys88-Cys110. Further structural characterization of PD-S1 and PD-S3 (N-terminal sequence determination up to residue 30, amino acid analysis and tryptic peptide mapping) showed that the three PD-Ss shared the entire protein sequence. To explain the different chromatographic behaviour, their glycosylation patterns were characterized by a fast and sensitive mass spectrometry-based approach, applying a precursor ion discovery mode on a Q-TOF mass spectrometer. A standard plant paucidomannosidic N-glycosylation pattern [Hex(3), HexNAc(2), deoxyhexose(1), pentose(1)] was found for PD-S1 and PD-S2 on Asn120. Furthermore, a glycosylation site carrying only a HexNAc residue was identified on Asn112 in PD-S1 and PD-S3. Finally, considering the two disulfide bridges and the glycan moieties, the experimental M(r) values were in agreement with the mass values calculated from the primary structure. The complete characterization of PD-Ss shows the high potential of mass spectrometry to rapidly characterize proteins, widespread in eukaryotes, differing only in their glycosylation motifs.     

On the post-translational modifications at the C-terminal domain of the major cysteine proteinase (cruzipain) from Trypanosoma cruzi

Cruzipain, the major cysteine proteinase from Trypanosoma cruzi, has a 130 amino acid-long C-terminal domain, which, although microheterogeneous in SDS-PAGE, has a single N-terminal amino acid sequence. Most of the Thr residues present at the beginning of this sequence are modified; the nature of this modification is still unknown, but O-glycosylation and phosphorylation seem both to be absent. The only potential site for N-glycosylation (Asn 254) is glycosylated in vivo. Most of the eight Cys residues are involved in disulfide bridges. The results are consistent with cruzipain being made of two well-defined domains, a catalytic one with high homology to cathepsin L, and a C-terminal domain, linked to the former by a 'hinge' corresponding to the Pro- and Thr-rich region at its N-terminus.     

Neighboring cysteine residues in human fucosyltransferase VII are engaged in disulfide bridges, forming small loop structures.

Among alpha 3-fucosyltransferases (alpha3-FucTs) from most species, four cysteine residues appear to be highly conserved. Two of these cysteines are located at the N-terminus and two at the C-terminus of the catalytic domain. FucT VII possesses two additional cysteines in close proximity to each other located in the middle of the catalytic domain. We identified the disulfide bridges in a recombinant, soluble form of human FucT VII. Potential free cysteines were modified with a biotinylated alkylating reagent, disulfide bonds were reduced and alkylated with iodoacetamide, and the protein was digested with either trypsin or chymotrypsin, before characterization by high-performance liquid chromatography/electrospray ionization mass spectrometry. More than 98% of the amino acid sequence for the truncated enzyme (beginning at amino acid 53) was verified. Mass spectrometry analysis also demonstrated that both potential N-linked sites are occupied. All six cysteines in the FucT VII sequence were shown to be disulfide-linked. The pairing of the cysteines was determined by proteolytic cleavage of nonreduced protein and subsequent analysis by mass spectrometry. The results demonstrated that Cys(68)-Cys(76), Cys(211)-Cys(214), and Cys(318)-Cys(321) are disulfide-linked. We have used this information, together with a method of fold recognition and homology modeling, using the (alpha/beta)(8)-barrel fold of Escherichia coli dihydrodipicolinate synthase as a template to propose a model for FucT VII.     

Mass spectrometric analysis of posttranslational modifications of a carrot extracellular glycoprotein

Expression of extracellular dermal glycoprotein (EDGP) is induced by biotic or abiotic stress. The amino acid sequence alignment showed that EDGP shared significant homology with proteins from legumes, tomato, Arabidopsis, wheat, and cotton. These proteins are involved in signal transduction or stress response systems. Most of the Cys residues in these proteins are conserved, suggesting that they share similar tertiary structures. Surface plasmon resonance (SPR) analysis shows that EDGP binds a soybean 4-kDa hormone-like peptide (4-kDa peptide) in vitro and reduction of EDGP decreased significantly the binding activity, implying that posttranslational modifications are important for its function. Therefore, we investigated the posttranslational modifications in EDGP using mass spectrometry. As the result, six disulfide bonds in EDGP were identified: Cys(70)-Cys(158), Cys(84)-Cys(89), Cys(97)-Cys(113), Cys(100)-Cys(108), Cys(201)-Cys(426), and Cys(332)-Cys(378). In addition, the N-terminal glutamine was cyclized into pyroglutamic acid. All four putative glycosylation sites were occupied by N-linked glycans, which have similar masses of m/z 1171. Finally, measuring the mass of the native protein showed that the posttranslational modifications of EDGP (pI 9.5) involved only disulfide bonds, N-terminal modification, and glycosylation.     

Positions of disulfide bonds and N-glycosylation site in juvenile hormone binding protein

The juvenile hormone binding protein (JHBP) from Galleria mellonella hemolymph is a glycoprotein composed of 225 amino acid residues. It contains four Cys residues forming two disulfide bridges. In this study, the topography of the disulfide bonds as well as the site of glycan attachment in the JHBP molecule from G. mellonella was determined, using electrospray mass spectrometry. The MS analysis was performed on tryptic digests of JHBP. Our results show that the disulfide bridges link Cys10 and Cys17, and Cys151 and Cys195. Of the two potential N-glycosylation sites in JHBP, Asn4, and Asn94, only Asn94 is glycosylated. This site of glycosylation is also found in the fully biologically active recombinant JHBP expressed in the yeast Pichia pastoris.     

Mass spectrometry study of ecto-5'-nucleotidase from bull seminal plasma.

The structure of ecto-5'-nucleotidase from bull seminal plasma, containing a glycosyl-phosphatidylinositol anchor, was studied using mass spectrometry. MALDI-MS analysis of intact protein indicated a mass of 65 568.2 Da for the monomeric form, and it also showed a heterogeneous population of glycoforms with the glycosidic moiety accounting for approximately 6000 Da. MALDI-MS analysis showed that Asn53, Asn311, Asn333 and Asn403 were four sites of N-glycosylation. GC-MS analysis provided information on the glycosidic structures linked to the four asparagines. Asn53, Asn311 and Asn333 were linked to high-mannose saccharide chains, whereas the glycan chains linked to Asn403 contained a heterogeneous mixture of oligosaccharides, the high-mannose type structure being the most abundant and hybrid or complex type glycans being minor components. By combining enzymatic and/or chemical hydrolysis with GC-MS analysis, detailed characterization of the glycosyl-phpsphatidylinositol anchor was obtained. MALDI spectral analysis indicated that the glycosyl-phosphatidylinositol core contained EtN(P)Man3GlcNH2-myo-inositol(P)-glycerol, principally modified by stearoyl and palmitoyl residues or by stearoyl and myristoyl residues to a minor extent. Moreover, 1-palmitoylglycerol and 1-stearoylglycerol outweighed 2-palmitoylglycerol and 2-stearoylglycerol. The combination of chemical and enzymatic digestions of the protein with the mass spectral analysis yielded a complete pattern of S-S bridges. The protein does not contain free thiols and its eight cysteines are linked by intramolecular disulfide bonds, the pairs being: Cys51-Cys57, Cys353-Cys358, Cys365-Cys387 and Cys476-Cys479. This work resolves details of the structure of ecto-5'-nucleotidase, with particular regard to the localization and composition of the glycidic moiety, number and localization of the disulfide bridges and characterization of the glycosyl-phosphatidylinositol anchor.     

Structural conservation of mouse and rat zona pellucida glycoproteins. Probing the native rat zona pellucida proteome by mass spectrometry

The mammalian zona pellucida is an egg extracellular matrix to which sperm bind. Mouse zonae are composed of three glycoproteins (ZP1, ZP2, and ZP3), while rat zonae contain four (ZP1, ZP2, ZP3, and ZP4/ZPB). Mouse sperm bind to zonae comprised solely of mouse ZP2 and ZP3. In this report, we show that rat sperm also bind to these zonae, indicating that ZP2 and ZP3 contain a "minimum structure(s)" to which rodent sperm can bind, and ZP1 and ZP4/ZPB are dispensable in these two rodents. These data are consistent with our mass spectrometric analysis of the native rat zona pellucida proteome (defined as the fraction of the total rat proteome to which the zonae glycoproteins contribute) demonstrating that the rat zonae glycoproteins share a high degree of conservation of structural features with respect to their mouse counterparts. The primary sequences of the rat zonae proteins have been deduced from cDNA. Each zona protein undergoes extensive co- and post-translational modification prior to its secretion and incorporation into an extracellular zona matrix. Each has a predicted N-terminal signal peptide that is cleaved off once protein translation begins and an anchoring C-terminal transmembrane domain from which the mature protein is released. Mass spectrometric analysis with a limited amount of native material allowed determination of the mature N-termini of rat ZP1 and ZP3, both of which are characterized by cyclization of glutamine to pyroglutamate; the N-terminus of ZP2 was identified by Edman degradation. The mature C-termini of ZP1 and ZP3 end two amino acids upstream of a conserved dibasic residue that is part of, but distinct from, the consensus furin cleavage sequence, while the C-terminus of ZP2 was not determined. Each zona protein contains a "zona domain" with eight conserved cysteine residues that is thought to play a role in the polymerization of the zona proteins into matrix filaments. Partial disulfide bond assignment indicates that the intramolecular disulfide patterns in rat ZP1, ZP2, and ZP3 are identical to those of their corresponding mouse counterparts. Last, nearly all potential N-glycosylation sites are occupied in the rat zonae glycoproteins (three of three for ZP1, six or seven of seven for ZP2, and four or five of six for ZP3). In comparison, potential O-glycosylation sites are numerous (59-83 Ser/Thr residues), but only two regions were observed to carry O-glycans in rat ZP3.     

Studies on the N-glycosylation of the subunits of oligosaccharyl transferase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, oligosaccharyl transferase (OT) consists of nine different subunits. Three of the essential gene products, Ost1p, Wbp1p, and Stt3p, are N-linked glycoproteins. To study the function of the N-glycosylation of these proteins, we prepared single or multiple N-glycosylation site mutations in each of them. We established that the four potential N-glycosylation sites in Ost1p and the two potential N-glycosylation sites in Wbp1p were occupied in the mature proteins. Interestingly, none of the N-glycosylation sites in these two proteins was conserved, and no defect in growth or OT activity was observed when the N-glycosylation sites were mutated to block N-glycosylation in either subunit. However, in the third glycosylated subunit, Stt3p, there are two adjacent potential N-glycosylation sites (N(535)NTWN(539)NT) that, in contrast to the other subunits, are highly conserved in eukaryotic organisms. Mass spectrometric analysis of a tryptic digest of Stt3p showed that the peptide containing the two adjacent N-glycosylation sites was N-glycosylated at one site. Furthermore, the glycan chain identified as Man(8)GlcNAc(2) is found linked only to Asn(539). Mutation experiments were carried out at these two sites. Four single amino acid mutations blocking either N-glycosylation site (N535Q, T537A, N539Q, and T541A) resulted in strains that were either lethal or extremely temperature sensitive. However, other mutations in the two N-glycosylation sites N(535)NTWN(539)NT (N536Q, T537S, N540Q, and T541S), did not exhibit growth defects. Based on these studies, we conclude that N-glycosylation of Stt3p at Asn(539) is essential for its function in the OT complex.     

High degree of homology between primary structure of human lysosomal acid phosphatase and human prostatic acid phosphatase

Alignment of the amino-acid sequences of the human lysosomal acid phosphatase (LAP) and human prostatic acid phosphatase (PAP) yielded an extensive homology between the two mature polypeptide chains. In the overlapping part, which extends over the entire PAP sequence and the N-terminal 90% of the LAP sequence, the identity is 49.1%. The LAP has an additional C-terminal sequence, which is encoded by the last exon of the LAP gene. This sequence contains the transmembrane domain of LAP, which is lacking in the secretory PAP. All six cysteine residues as well as 20 out of 27 (LAP) and 26 (PAP) proline residues present in the overlapping part of the proteins are conserved, suggesting that they are involved in stabilization of the tertiary structure of both proteins. Only two out of 8 N-glycosylation sites in LAP and 3 in PAP are conserved, suggesting that the dense N-glycosylation of LAP is related to its function in lysosomes.     

The status of half-cystine residues and locations of N-glycosylated asparagine residues in human eosinophil peroxidase

The determination by protein chemistry methods of the half-cystine status in human eosinophil peroxidase (EPO) is reported. EPO is two-chained and has a total of 14 half-cystine residues. Cys141 and Cys152 form an intrachain bridge in the light chain of EPO. Disulfide bridges connect Cys253 and Cys263, Cys257 and Cys287, Cys359 and Cys370, Cys570 and Cys635, and Cys676 and Cys701, forming five intrachain disulfide bridges in the heavy chain of EPO. Cys291 and Cys455 are found to be unpaired, containing free sulfhydryl groups. The pattern of disulfide bridges is in agreement with that predicted from the X-ray crystallographic structure of canine myeloperoxidase (MPO) (Zeng, J., and Fenna, R. E. (1992) J. Mol. Biol. 226, 185-207) to be general for the class of mammalian peroxidases, including EPO, MPO, lactoperoxidase (LPO), and thyroid peroxidase (TPO). Of four candidate sites in EPO for attachment of glucosamine-based carbohydrate, Asn327 and Asn363 are occupied, whereas Asn700 and Asn708 are unsubstituted. Furthermore, a discrepancy in the literature regarding the sequence of residues 645-659 is resolved.     

Determination of disulfide bond assignments and N-glycosylation sites of the human gastrointestinal carcinoma antigen GA733-2 (CO17-1A, EGP, KS1-4, KSA, and Ep-CAM)

The GA733-2 antigen is a cell surface glycoprotein highly expressed on most human gastrointestinal carcinoma and at a lower level on most normal epithelia. It is an unusual cell-cell adhesion protein that does not exhibit any obvious relationship to the four known classes of adhesion molecules. In this study, the disulfide-bonding pattern of the GA733-2 antigen was determined using matrix-assisted laser desorption/ionization mass spectrometry and N-terminal sequencing of purified tryptic peptides treated with 2-[2'-nitrophenylsulfonyl]-3-methyl-3-bromoindolenine or partially reduced and alkylated. Numbering GA733-2 cysteines sequentially from the N terminus, the first three disulfide linkages are Cys1-Cys4, Cys2-Cys6, and Cys3-Cys5, which is a novel pattern for a cysteine-rich domain instead of the expected epidermal growth factor-like disulfide structure. The next three disulfide linkages are Cys7-Cys8, Cys9-Cys10, and Cys11-Cys12, consistent with the recently determined disulfide pattern of the thyroglobulin type 1A domain of insulin-like growth factor-binding proteins 1 and 6. Analysis of glycosylation sites showed that GA733-2 antigen contained N-linked carbohydrate but that no O-linked carbohydrate groups were detected. Of the three potential N-linked glycosylation sites, Asn175 was not glycosylated, whereas Asn88 was completely glycosylated, and Asn51 was partially glycosylated. These data show that the extracellular domain of the GA733-2 antigen consists of three distinct domains; a novel cysteine-rich N-terminal domain (GA733 type 1 motif), a cysteine-rich thyroglobulin type 1A domain (GA733 type 2 motif), and a unique nonglycosylated domain without cysteines (GA733 type 3 motif).     

Isolation of ovocleidin-116 from chicken eggshells, correction of its amino acid sequence and identification of disulfide bonds and glycosylated Asn.

Fractionation of the soluble chicken eggshell matrix by chromatographic methods yielded 13 endogenous proteolytic fragments of the eggshell-specific proteoglycan core protein ovocleidin-116. The N-terminal amino acid sequences of these fragments in general confirmed the recently cDNA-deduced sequence of ovocleidin-116, with one exception. One fragment yielded a completely new sequence and was instrumental in detecting a frame shift error in the nucleotide sequence. The correction yielded a new sequence which was 38 amino acids shorter than before and contained a 57-amino acid long novel C-terminal sequence. The predicted sequence of ovocleidin-116 contained two consensus N-glycosylation sites, only one of which (Asn62) was found to be fully modified. A disulfide bond was identified between Cys31 and 42 implying that Cys329 and 421 form a second disulfide bond. Finally, the yield of fragments indicated that ovocleidin-116 is a major component of the chicken eggshell matrix.     

Ligand binding-dependent limited proteolysis of the atrial natriuretic peptide receptor: juxtamembrane hinge structure essential for transmembrane signal transduction

The atrial natriuretic peptide (ANP) receptor is a 130-kDa transmembrane protein containing an extracellular ANP-binding domain, a single transmembrane sequence, an intracellular kinase-homologous domain, and a guanylate cyclase (GCase) domain. We observed that the receptor, when bound with ANP, was rapidly cleaved by endogenous or exogenously added protease to yield a 65-kDa ANP-binding fragment. No cleavage occurred without bound ANP. This ligand-induced cleavage abolished GCase activation by ANP. Cleavage occurred in an extracellular, juxtamembrane region containing six closely spaced Pro residues and a disulfide bond. Such structural features are shared among the A-type and B-type ANP receptors but not by ANP clearance receptors. The potential role of the hinge structure was examined by mutagenesis experiments. Mutation of Pro(417), but not other Pro residues, to Ala abolished GCase activation by ANP. Elimination of the disulfide bond by Cys to Ser mutations yielded a constitutively active receptor. Pro(417), and Cys(423) and Cys(432) forming the disulfide bond are strictly conserved among GCase-coupled receptors, while other residues are largely variable. The conserved Pro(417) and the disulfide bond may represent a consensus signaling motif in the juxtamembrane hinge structure that undergoes a marked conformational change upon ligand binding and apparently mediates transmembrane signal transduction.     

Trafficking of cholinesterases and neuroligins mutant proteins. An association with autism

Autism encompasses a wide spectrum of disorders arising during brain development. Recent studies reported that sequence polymorphisms in neuroligin-3 (NLGN3) and neuroligin-4 (NLGN4) genes have been linked to autism spectrum disorders indicating neuroligin genes as candidate targets in brain disorders. We have characterized a single mutation found in two affected brothers that substituted Arg451 to Cys in NL3. Our data show that the exposed Cys causes retention of the protein in the endoplasmic reticulum (ER) when expressed in HEK-293 cells. To examine whether the introduction of a Cys in the C-terminal region of other alpha/beta-hydrolase fold proteins could promote the same cellular phenotype, we made homologous mutations in acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) and found a similar processing deficiency and intracellular retention (De Jaco et al., J Biol Chem. 2006, 281:9667-76). NL3, AChE and BChE mutant proteins are recognized as misfolded in the ER, and degraded via the proteasome pathway. A 2D electrophoresis coupled with mass spectrometry based approach was used to analyze proteins co-immunoprecipitating with NL3 and show differential expression of factors interacting with wild type and mutant NL3. We identified several proteins belonging to distinct ER resident chaperones families, including calnexin, responsible for playing a role in the folding steps of the AChE and NLs.     

Post-translational modifications of human thrombin-activatable fibrinolysis inhibitor (TAFI): evidence for a large shift in the isoelectric point and reduced solubility upon activation

Thrombin-activable fibrinolysis inhibitor (TAFI) is distinct from pancreatic procarboxypeptidase B in several ways. The enzymatic activity of TAFIa is unstable and decays with a half-life of a few minutes. During this study, we observed that (i) the isoelectric point (pI) of TAFI shifts dramatically from pH 5 toward pH 8 upon activation and (ii) TAFIa is significantly less soluble than TAFI. The structural bases for these observations were investigated by characterizing all post-translational modifications, including attached glycans and disulfide connectivity. The analyses revealed that all five potential N-glycosylation sites were utilized including Asn22, Asn51, Asn63, Asn86 (located in the activation peptide), and Asn219 (located in the catalytic domain). Asn219 was also found in an unglycosylated variant. Four of the glycans, Asn51, Asn63, Asn86, and Asn219 displayed microheterogeneity, while the glycan attached to Asn22 appeared to be homogeneous. In addition, bisecting GlcNAc attached to the trimannose core was detected, suggesting an origin other than the liver. Monosaccharide composition and LC-MS/MS analyses did not produce evidence for O glycosylation. TAFI contains eight cysteine residues, of which two, Cys69 and Cys383, are not involved in disulfides and contain free sulfhydryl groups. The remaining six cystines form disulfides, including Cys156-Cys169, Cys228-Cys252, and Cys243-Cys257. This pattern is homologous to pancreatic procarboxypeptidase B, and it is therefore unlikely that permutations in the cysteine connectivity are responsible for the enzymatic instability. LC-MS/MS analyses covering more than 90% of the TAFI amino acid sequence revealed no additional modifications. When these results are taken together, they suggest that the inherent instability of TAFIa is not caused by post-translational modifications. However, after activation, TAFIa loses 80% of the attached glycans, generating a large shift in pI and a propensity to precipitate. These changes are likely to significantly affect the properties of TAFIa as compared to TAFI.     

Human alpha 1,3/4 fucosyltransferases. Characterization of highly conserved cysteine residues and N-linked glycosylation sites

Human alpha1,3 fucosyltransferases (FucTs) contain four highly conserved cysteine (Cys) residues, in addition to a free Cys residue that lies near the binding site for GDP-fucose (Holmes, E. H., Xu, Z. , Sherwood, A. L., and Macher, B. A. (1995) J. Biol. Chem. 270, 8145-8151). The participation of the highly conserved Cys residues in disulfide bonds and their functional significance were characterized by mass spectrometry (MS) analyses and site-directed mutagenesis, respectively. Among the human FucTs is a subset of enzymes (FucT III, V, and VI) having highly homologous sequences, especially in the catalytic domain, and Cys residues in FucT III and V were characterized. The amino acid sequence of FucT III was characterized. Peptides containing the four conserved Cys residues were detected after reduction and alkylation, and found to be involved in disulfide bonds. The disulfide bond pattern was characterized by multiple stage MS analysis and the use of Glu-C protease and MS/MS analysis. Disulfide bonds in FucT III occur between Cys residues (Cys(81) to Cys(338) and Cys(91) to Cys(341)) at the N and C termini of the catalytic domain, bringing these ends close together in space. Mutagenesis of highly conserved Cys residues to Ser in FucT V resulted in proteins lacking enzymatic activity. Three of the four mutants have molecular weights similar to wild type enzyme and maintained an ability to bind GDP, whereas the other (Cys(104)) produced a series of lower molecular weight bands when characterized by Western blot analysis, and did not bind GDP. FucTs have highly conserved, potential N-linked sites, and our mass spectrometry analyses demonstrated that both N-linked sites are modified with oligosaccharides.