Glycoprotein methods protocols - biotechnology
Dimerization of Domains of Mucin 143143From:Methods in Molecular Biology, Vol. 125: Glycoprotein Methods and Protocols: The MucinsEdited by: A. Corfield © Humana Press Inc., Totowa, NJ13Mucin Domains to Explore Disulfide-DependentDimer FormationSherilyn L. Bell and Janet F. Forstner1. IntroductionThe viscoelastic properties needed for the protective functions of secretory mucinsare in part conditional on the capacity of mucin macromolecules to form linear poly-mers stabilized by disulfide bonds. The individual mucin monomers have a distinctivestructure, consisting of a long central peptide region of tandem repeat sequences,flanked by cysteine-rich regions at each end, which are presumed to mediate polymer-ization. Secretory mucins contain approx 60–80% carbohydrate, with extensive O-glycosylation in the central tandem repeat regions, and N-linked oligosaccharides inthe peripheral regions (1).The ability of mucin peptides to form large polymers, combined with their exten-sive posttranslational glycosylation and sulfation, results in complexes that reachmolecular masses in excess of 10,000 kDa (2). This leads to difficulties in resolvingmucins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)or agarose gel electrophoresis, because both unreduced and reduced mucin samplesare capable of only limited movement. A related difficulty inherent in analyzingmucins lies in their strong negative charge owing to sialic acid and sulfate content (3).Migration through polyacrylamide gels becomes more influenced by charge than bymass. The result is that interpretations of the size of mucin from electrophoreticmobility are not as straightforward as with other proteins.Hypotheses concerning the regions of secretory mucins that could be involved inthe initial dimer formation have centered on the terminal cysteine-rich, poorlyglycosylated domains. Indirect evidence that these domains are involved has beenshown by treating mucins with proteolytic enzymes and reducing agents that act onthese regions, and noting a resultant decrease in the size of mucin and gel formation(4–6). Intriguingly, more indirect evidence was found when database searches identi-fied a functionally unrelated protein, von Willebrand factor (vWF) (which also formsS-S–dependent polymers), to exhibit a mucinlike pattern of cysteine residue align- 144 Bell and Forstnerments in its N- and C-terminal domains (7,8). The function of vWF to cause aggrega-tion of platelets is also dependent on its ability to polymerize via disulfide bonds.More important for the present report, the DNA encoding vWF was expressed suc-cessfully in heterologous cells and shown to undergo S-S–dependent dimer andmultimer formation (9,10). The minimum component necessary for initial dimeriza-tion is a region of approx 150 amino acids at the C-terminal end (11). A similar vWF“motif” has now been recognized near the C-terminus of several secretory mucins,including frog integumentary mucin FIM-B.1 (7), bovine submaxillary mucin (12),porcine submaxillary mucin (13), human and rat intestinal mucin MUC2 (8,14), humantracheobronchial mucin MUC5AC (15), human gall bladder mucin MUC5B (16,17),and human gastric mucin MUC6 (18). Since these mucins are known to form oligo-mers in vivo, their shared C-terminal motif may also be involved in forming mucindimers. Dimerization of mucin molecules represents the crucial first step in the trans-formation of individual mucin molecules into gel structures.In this chapter, we describe a domain construct and expression approach used toexamine directly whether the C-terminal domain of rat intestinal Muc2 is capable ofdimerization through its cysteine residues. This method avoids, to a large extent, thenumerous difficulties of dealing with full-length mucins, since the domain peptide isexpressed as a relatively small, less glycosylated monomer or dimer. The principle isthat DNA encoding the domain of interest is ligated to a known epitope sequence (fordetection by a suitable antibody), and to a signal peptide sequence (to ensure secretion), byrecombinant polymerase chain reaction (PCR) strategies. The resulting construct is ligatedto an expression vector for transfection into heterologous cells. Once the expected peptidehas been translated and processed, dimerization by disulfide bond formation is shown bycomparing the sizes of immunoreactive, thiol-reduced and nonreduced products in cellsand media by SDS-PAGE and Western blotting. The use of specific antibodies to variousregions of the domain can provide assurance that the domain is expressed in an intact formor, alternatively, has been proteolytically processed during dimerization. After establish-ing the dimerization capability of the isolated C-terminal domain of rat Muc2, we describemethods for examining the role of glycosylation in dimerization by manipulating the sys-tem with inhibitors of glycosylation and/or deglycosylating enzymes.2. Materials2.1. Synthesis of Domain Constructs Using Recombinant PCR1. PCR reagents: buffer, dNTPs, MgCl2, Taq DNA polymerase obtained from Perkin Elmer(Foster City, CA).2. Primer 1 (see Fig. 1). This is a sense primer containing an XbaI site to facilitate cloning,and also encoding part of the rat Muc2 signal peptide:a. 5'-CGTCTAGAATGGGGCTGCCACTAGCTCGCCTGGTGGCT-3'.3. Primer 2 (see Fig. 1). The antisense primer containing signal peptide and “linker”sequence to be paired with primer 1:a. 5'-CACAGTTAGATTCCAGCCCTTGGCTAAGGCCAGGACTAGGCACACAG-3'.4. Primer 3 (see Fig. 1). This primer specifies the “linker” sequence and primes the 5' end ofthe target domain of rat Muc2:a. 5'-GGCTTGGAATCTAACTGTGAAGTTGCTGC-3'. Dimerization of Domains of Mucin 1455. Primer 4 (see Fig. 1). The antisense primer encoding the 3' end of rat Muc2 with anadditional SacI site sequence for cloning:a. 5'-CGAGCTCCTATCACTTCCTTCCTAGAAGCCG-3'.6. Clone MLP-3500, which encodes the C-terminal 1121 amino acids of rat Muc2, was usedas the DNA template (8).7. Outer primers 5:a. 5'-CGTCTAGAATGGGGCTGC-3'.8. Antisense primer 6 (see Fig. 1):a. 5'-CGAGCTCCTATCACTTCC-3'.9. Thermal cycler such as Perkin Elmer DNA Thermal Cycler 480.2.2. Ligation of Construct to Transfection Vector1. Invitrogen TA cloning kit (Invitrogen, Carlsbad, CA).2. Transfection vector pSVL available from Pharmacia (Uppsala, Sweden).Fig. 1. Schematic showing the synthesis of construct pRMC and its expression in COS cells.The protocol is described in Subheadings 3.1.–3.3. Oligonucleotide primers are designed thatfacilitate the synthesis and joining of DNA sequence coding for the signal peptide and car-boxyl-terminal 534 amino acids of rat Muc2 via recombinant PCR. The resulting construct isthen subcloned into the expression vector pSVL for expression in COS cells. 146 Bell and Forstner3. Restriction enzymes SacI, XbaI with supplied incubation buffer(s).4. T4 DNA ligase and ligase buffer (Boehringer Mannheim, Mannheim, Germany).5. Subcloning efficiency competent DH5α Escherichia coli in 50-µL aliquots.6. Convection incubator maintained at 37˚C.7. DNA maxiprep columns (Qiagen, Chatsworth, CA).8. Luria broth (LB) plates containing 50 µg/mL of ampicillin (19).9. Agarose gels containing 0.5 µg/mL of ethidium bromide.10. HindIII-digested DNA λ markers for size and quantity estimation.11. 1X TAE: 0.04 M Tris base, 1 mM EDTA, 1.14 mL/L glacial acetic acid.2.3. Transfection1. COS-1 or COS-7 cell line obtained from American Type Culture Collection, (Rockville, MD).2. Dulbecco’s modified Eagle’s medium (Gibco-BRL, Gaithersburg, MD) supplementedwith 10% fetal bovine serum (FBS) (CanSera, Etobicoke, ON, Canada) and with 100 U/mLof penicillin and 100 µL of streptomycin (Gibco-BRL).3. Hemocytometer.4. Lipofectamine (Gibco-BRL).5. Cell culture incubator to maintain an atmosphere of 37˚C and 5% CO2.6. Transfection efficiency reporter such as pCMVßGAL or luciferase systems.2.4. Harvesting of Transfected Cells1. 2X Laemmli SDS sample buffer: 125 mM Tris-HCl, pH 6.8, 20% glycerol, 4% SDS,0.005% bromophenol blue, plus or minus 1,4-Dithiothreitol (DTT) to give a final concen-tration of 10 mM.2. Filter concentrators such as Centricon with a molecular weight cutoff of 30 kDa.3. Phenylmethylsulfonylfluoride (PMSF).4. Beckman J2-21 centrifuge, JA-20.1 rotor (Beckman Instruments, Palo Alto, CA).2.5. SDS-PAGE and Western Blot Analysis1. Tris-glycine polyacrylamide gels (precast) (Novex, San Diego, CA).2. Gel electrophoresis apparatus (Novex Xcell II Mini Cell).3. Gel running buffer: 25 mM Tris, 192 mM glycine, 0.1% SDS, pH 8.3.4. Prestained protein standards.5. Transfer buffer: 25 mM Tris, 192 mM glycine, 20% methanol, pH 8.3, store at 4˚C.6. Transfer apparatus such as Mini Trans-Blot (Bio-Rad, Hercules, CA).7. Nitrocellulose membrane, cut to the size of the gel.8. Blotting paper (0.33 mm).9. Tris-buffered saline (TBS): 20 mM Tris base, 137 mM NaCl, final pH 7.6, and TBS with0.1% Tween-20 added (TBST).10. Blocking solution: 3% bovine serum albumin (BSA) in TBS.11. Primary incubation solution: 1:1000 dilution (v/v) of rabbit polyclonal antibody raisedagainst the deglycosylated C-terminal “link” glycopeptide (20) or against synthetic pep-tides D4553 corresponding to a 14 amino acid segment in the mucin domain, and E20-14,corresponding to the C-terminal 14 amino acids of the mucin domain (21) (see Fig. 1) inTBS and 0.1% BSA.12. Secondary incubation solution: 1:10,000 dilution of goat antirabbit IgG alkaline phos-phatase conjugate in TBS and 0.1% BSA.13. Alkaline phosphatase detection system: equilibration solution (100 mM Tris-HCl, 100 mMNaCl, 50 mM MgCl2, pH 9.5; store at 4˚C), reaction solution (same as equilibration but Dimerization of Domains of Mucin 147with the addition of 6.6 mg of 4-nitro blue tetrazolium chloride and 1.65 mg of 5-bromo-4-chloro-3-indolyl-phosphate), and stop solution (10 mM Tris-HCl, 1 mM EDTA, pH 8.0).Nitro blue tetrazolium (NBT) and 5-bromo-4-chloro-3-indolyl-phosphate (BCIP) are avail-able from Boehringer Mannheim at 100 and 50 µg/µL concentrations, respectively.2.6. The Role of Glycosylation1. Tunicamycin (Sigma, St. Louis, MO) in DMEM, filter sterilized.2. 20 mM benzyl 2-acetamido-2-deoxy-α-D-galactopyranoside (benzyl-α-GalNAc) (Sigma),filter sterilized.3. Peptide-N4-(acetyl-ß-glycosaminyl) asparagine amidase (N-glycosidase F, EC 3.5.1.52)(Boehringer Mannheim).4. Nonidet P-40.5. N-acetylneuraminidase from Vibrio cholerae (EC 3.2.1.18), (Boehringer Mannheim).6. Lysis buffer: 50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 1% Nonidet P-40, and 1X Com-plete™ protease inhibitor cocktail (Boehringer Mannheim).7. Protein A-Sepharose (Boehringer Mannheim).8. Immunoprecipitation buffer: 20 mM Tris-HCl, pH 8.0, containing 0.1 M NaCl, 1 mMNa2EDTA, 0.1 mM PMSF, with and without 0.5% Nonidet P-40.9. Neuraminidase incubation buffer: 40 mM Tris-HCl, 4 mM CaCl2, pH 7.8.3. Methods3.1. Synthesis of Rat Muc2 Domain Construct (pRMC)Using Recombinant PCRFigure 1 shows the general scheme of the synthesis of construct pRMC. Two DNAconstructs are synthesized encoding the entire rat Muc2 signal peptide and the C-ter-minal 534 amino acids, respectively. Each construct is synthesized to contain an iden-tical region (Fig. 1, shaded area) at which a single strand of one construct is able tocomplement the other during the annealing cycle of the second PCR step (22), result-ing in a recombinant product (pRMC) that can be cloned, using the incorporated restrictionsites, into a suitable vector such as pSVL. The detailed procedure is as follows:1. Synthesize DNA fragments to be joined via recombinant PCR. The 5' fragment willencode the 534 amino acids of the C-terminal end of rat Muc2 and a 3' SacI restriction sitefor subcloning. This domain of Muc2 can be detected by the antibodies anti-d-link, anti-D4553,and anti-E20-14 (8,20), which span the length of the pRMC peptide product (Fig. 1).2. Make up 100 µL of PCR samples containing 10 µL of 10X polymerase buffer, 2.5 mMMgCl2, 400 µM dNTPs, 200 ng of primers 1 and 2 or 3 and 4, 1 ng of template DNA (withonly primers 3 and 4), and 5 U of Taq polymerase. Perform a PCR program in a suitablethermal cycler at 94˚C for 5 min, followed by 30 cycles at 94˚C for 1 min, 60˚C for 1 min,and 72˚C for 1 min, ending with an extension period of 72˚C for 7 min.3. Couple the signal peptide and RMuc2 PCR products. Make up 100-µL reaction mixturescontaining 10 µL of 10X polymerase buffer, 2.5 mM MgCl2, 400 µM dNTPs, and approx50 ng of each PCR product as templates. Denature the DNA in a thermal cycler for 1 minat 94˚C, followed by a 5-min incubation at 55˚C to allow both templates to anneal. Add 5U of Taq polymerase, and incubate the reaction at 72˚C to allow extension of the linkedtemplates. Add 2 µg each of primers 5 and 6, and continue the PCR with 30 cycles at 94˚Cfor 30 s, 50˚C for 1 min and 72˚C for 2 min, ending with a 72˚C extension for 7 min. 148 Bell and Forstner4. Check the PCR product for size and purity by electrophoresis on a 1% agarose gel. Theconstruct may be verified by sequencing at this point or after insertion into a vector.3.2. Ligation of Construct to the Transfection VectorTA cloning may be necessary if direct ligation is unsuccessful. In cases wheresubcloning is impeded by incomplete restriction enzyme digestion, a specialized vec-tor containing 3' deoxythymidine (T) overhangs can be utilized to ligate the PCR prod-uct generated by Taq polymerase (which leaves complementary deoxyadenosine or A,overhangs). The ligated TA overhangs can then be digested without difficulty. Alter-natively, it can be assumed from the beginning that TA cloning will be used, whichwould result in a small change in primer design. Specifically, it would not be neces-sary to incorporate SacI and XbaI sites into the beginning primers, because these sitesalready exist in the TA cloning vector.1. Using fresh recombinant PCR product (less than 1 d old), perform TA cloning as outlinedin the protocol of the supplier (Invitrogen). Pick isolated white colonies and check for thecorrect insert by restriction digest of DNA minipreps with SacI and XbaI.2. Add 5 µL of recombinant PCR product or positive TA clone DNA to 3 µL of H2O, 1 µL ofincubation buffer, and at least 1 U of SacI to a microfuge tube. Incubate for 1 h in a 37˚ Cwater bath. Add at least 1 U of XbaI to the reaction and incubate for a further hour at37˚C. Inactivate the enzymes according to the manufacturer’s instructions.3. Purify the SacI/XbaI restriction digest product. We used electroelution from the agarosefragment (19) in TAE buffer for 1 h followed by precipitation in 0.1 vol of 3 M sodiumacetate and 2 vol of ethanol. Electrophorese on a 1% agarose gel and quantitate the sample.Use sufficient quantities of pSVL and insert DNA to result in an approx 1:3 molar ratio,respectively. Bring the ligation reaction to 8 µL with H2O, and then add 1 µL of the 10Xligation buffer supplied with the enzyme and 1 µL (1 U) of T4 DNA ligase. Incubate theligation reaction at 16˚C for 16 h.4. Thaw competent DH5α cells on ice, and to each 50-µL aliquot add 2 µL of the ligationreaction mixture (or H2O for mock-transformation control), stir gently with a pipet tip,and incubate the cells on ice for 30 min.5. Heat shock the reactions in a 37˚C water bath for 20 s, and then incubate them on ice for 2 min.6. Add 950 µL of LB to each reaction tube and incubate on a shaker at 37˚C for 1 h.7. Add 50 and 200 µL from each tube of transformed cells to warmed LB plus ampicillinplates. Distribute the cells over the plate evenly with a sterile glass spreader, allow platesto dry, and leave in a 37˚C convection incubator for 16 hr.8. After confirming that no growth has occurred on control plates of mock-transformed DH5α,pick at least five positive, isolated colonies. Check DNA minipreps of these positive colo-nies by restriction digestion with SacI and XbaI. The positive clones may be sequenced atthis point to confirm that the correct sequence is contained within the plasmid.9. Prepare large-scale amounts of transfection-quality construct pRMC and control vectorpSVL by resin purification such as with the Qiagen Maxiprep kit.3.3. TransfectionThe transfection system uses the expression vector pSVL, which is ideally suitedfor transfection into COS cells owing to the production of T-antigen, which increasespSVL expression (23). Dimerization of Domains of Mucin 1491. Twenty-four hours before the transfection, seed COS cells into 3.5-cm tissue culturedishes at a density of 8 × 106 cells, as counted by a hemocytometer.2. Pilot experiments are necessary to establish the ideal ratio of DNA to Lipofectamine. Forthis system, add 2 µg of pRMC or pSVL control to 100 µL of DMEM. Add 10 µL ofLipofectamine to 100 µL of DMEM in a separate tube.3. Add the contents of the first tube containing DNA to the second tube containingLipofectamine, and incubate the tubes at room temperature for 20 min. While the incuba-tion is ongoing, remove the medium from all dishes of COS cells and wash with 1 mL ofDMEM (containing no antibiotics or FBS). Drain the dishes thoroughly.4. Add 800 µL of DMEM to the DNA-Lipofectamine mixture, pipet to mix, and add to thedish of COS cells. Leave all dishes in the tissue culture incubator at 37˚C for 5 h.5. Replace the mixture with 2 mL of DMEM plus 10% FBS (do not include antibiotics).Leave the dish in the tissue culture incubator for 43 h.6. Remove serum-containing DMEM and wash COS cells thoroughly with DMEM withoutserum. Replace medium with 2 mL of DMEM without serum and incubate at 37˚C foranother 24 h. If it is critical to collect conditioned media from the entire incubation pe-riod, OptiMEM (Gibco-BRL) may be substituted for DMEM to avoid later problems as-sociated with SDS-PAGE involving FBS (e.g., distortion of lower mobility proteinsduring electrophoresis and antibody crossreactivity).3.4. Harvesting of Transfected Cells1. After establishing the success of transfection with a reporter system, remove COS cellstransfected with pRMC and pSVL control vector from the incubator, collect all medium,and wash the cells with 1 mL of phosphate-buffered saline (PBS).2. Add 60 µL of 2X Laemmli sample buffer to each dish, and quickly scrape the lysates intomicrofuge tubes. The lysates may be stored at –70˚C at this time.3. Add PMSF to the conditioned medium to give a concentration of 0.1 mM, and concen-trate using spin columns, such as Centricons, to attain a final volume of approx 100 µL.Medium may be stored at –70˚C at this time.3.5. SDS-PAGE and Western AnalysisThe transfected COS cells are harvested for separation of lysate components onpolyacrylamide gels. The percentage of polyacrylamide used will vary with the size ofthe expected product and the desired separation. Most experiments in the present studywere performed using 8% gels.1. Boil the cell lysates for 3 min to reduce viscosity, divide each sample equally betweentwo microfuge tubes, and add DTT to one of each pair of samples to give a final concen-tration of 10 mM. Boil the samples for a further 2 min, briefly spin to collect droplets, andload each sample into a well of an 8% polyacrylamide gel.2. Add 15 µL of 2X Laemmli sample buffer to 15-µL aliquots of conditioned media (twosamples for each vector transfected) and reduce one of each pair with DTT (final concen-tration 10 mM). Boil, spin, and pipet samples into wells of an 8% polyacrylamide gel.3. Add prestained molecular mass standards to one well and 2x Laemmli sample buffer toany remaining empty wells (see Note 3).4. Electrophorese at 125 V for 100 min, or until the bromophenol blue dye reaches the bot-tom of the gel.5. Transfer the separated proteins from the gel to a nitrocellulose membrane at a constantvoltage of 100 V for 1 h using buffer conditions suitable for the transfer apparatus; the 150 Bell and ForstnerBio-Rad Mini Trans-blot requires buffer maintained at 4˚C. The transfer setup is detailedin ref. 19.6. Remove and submerge the membrane in blocking solution. Incubate on a rotator at roomtemperature for 1 h.7. Wash the membrane five times in TBST at room temperature, 5 min per wash.8. Incubate the membrane for 16 h at 4˚C in primary incubation solution. We have used theanti-d-link, anti-D4553, and anti-E20-14 antibodies (see Subheading 2.5., step 11) inour experiments, with equivalent results. Wash the membrane as described in step 5.9. Incubate the membrane for 1 h in secondary incubation solution. Wash the membrane asin step 5.10. After a 3-min equilibration, develop the membrane in reaction solution containing NBTand BCIP until specific bands are visible. Keep all membranes protected from light dur-ing the color development.11. Submerge the membrane in stop solution and dry on blotting paper. Compare control andexperimental samples (Fig. 2) for the appearance of immunospecific bands that increasein mobility (i.e., decrease in size) upon treatment with DTT (see Note 4).3.6. The Role of Glycosylation3.6.1. Tunicamycin1. Repeat Subheadings 3.1.–3.5. with the addition of 0.5 µg/mL of tunicamycin to theDMEM used in Subheading 3.3. steps 5 and 6. Harvest and analyze cells and condi-tioned media as outlined previously (see Note 5).3.6.2.N-Glycosidase F1. Repeat Subheadings 3.1.–3.3. inclusive.2. Collect and concentrate the conditioned medium as described in Subheading 3.4., step 3.Add 1 U of N-glycosidase F (or H2O as a control) to each 15-µL aliquot and incubate 16 h at37˚C in a water bath. Analyze by SDS-PAGE as described in Subheading 3.5., steps 2–10.3. Wash the COS cells in PBS, scrape the cells into 500 µL of PBS and microfuge the suspensionat 4˚C to form a pellet. Resuspend the pellet in 50 µL of 1% SDS, and boil 1 min to denature.4. Dilute the suspension with Nonidet P-40 to give final concentrations of SDS and NonidetP-40 of 0.1 and 0.5 %, respectively. N-glycosidase F is deactivated by higher concentra-tions of SDS.5. Add 1 U of N-glycosidase F to each cell lysate and medium sample (H2O to controls) andincubate for 16 h at 37˚C in a water bath.6. Vacuum-dry the samples and reconstitute in 60 µL of 2X Laemmli sample buffer.7. Analyze the conditioned media and cell lysates by SDS-PAGE and Western blotting (Fig.3) as described in Subheading 3.5. (see Note 6).3.6.3. Benzyl-α-GalNAc1. Repeat Subheadings 3.1.–3.5. with the addition of benzyl-α-GalNAc at a final concen-tration of 2 mM to each test dish of COS cells. Harvest and analyze cell lysates andconditioned media as outlined previously (see Note 7).3.6.4.N-Acetylneuraminidase1. Repeat Subheadings 3.1.–3.3. inclusive.2. Collect and concentrate the conditioned medium as described in Subheading 3.4., step 3.Add 10 mU of N-acetylneuraminidase from V. cholerae (or H2O as a control) to each 50-µL Dimerization of Domains of Mucin 151aliquot and incubate 16 h at 37˚C in a water bath. Analyze by SDS-PAGE as described inSubheading 3.5., steps 2–10.3. Harvest COS cells in 200 µL of lysis buffer and microfuge at 4˚C to remove cell debris.4. Incubate the remaining supernatant with 4 µL of antibody (anti-D4553 was used in thisstudy) for 1.5 h at 4˚C followed by a further 2.5-h incubation at the same temperaturewith the addition of 50 µL of Protein A-Sepharose. Isolate the immunoprecipitates bycentrifugation at 4˚C for 5 min followed by washing the resultant pellet three times with500 µL of immunoprecipitation buffer (the third wash without Nonidet P-40).5. Analyze the conditioned media and cell immunoprecipitates by SDS-PAGE and Westernblotting (Fig. 3) as described in Subheading 3.5. (see Note 8).4. Notes1. The procedures outlined above for specific domain expression in heterologous cells dem-onstrate that the C-terminal region of rat Muc2 can form disulfide-dependent dimers, andthat N-glycosylation plays a significant role in dimerization. A general schematic reflect-ing the results using these methods is shown in Fig. 4. The reader is referred to Perez-Vilar et al. (25) for an earlier application of domain expression to show dimerization ofthe C-terminal domain of porcine submaxillary mucin. The domain approach was neces-sitated by the virtual impossibility of studying detailed structural changes in molecules aslarge and as viscous as typical full-length secretory mucins. As has been true for theelucidation of vWF physiology and disease-associated mutants of vWF, the expression ofindividual mucin domains and their secretory fate holds promise of enlarging our under-standing of structure-function relationships of mucins in vivo.2. Since the mucin domain forms dimers, the present approach could be extended to includelarger constructs and/or other domains, e.g., constructs encoding both C- and N-terminaldomains, to test whether dimers can expand into larger S-S–linked oligomers ormultimers. Various embellishments are also possible, including the addition to constructsof commercially available tag epitopes at selected areas of the domain and immunopre-cipitation of translated products with specific antiepitope antibodies. Crosslinking agentscould also be added to examine the possibility that nonmucin proteins bind to specificmucin domains. The kinetics of posttranslational modifications of the domain and itssecretion could be studied by performing pulse-chase experiments using radioactive pre-cursors added to transfected cells. The strategic use of inhibitors during cell incubationshas the potential to reveal information about the pre- and post-Golgi movement of mucindomains along the secretory pathway. Finally, truncation or site-specific mutagenesis ofthe initial domain constructs could be introduced to explore the role of selected aminoacids, such as cysteines required for dimerization, or N-glycosylation sites involved inmediating the correct folding for dimerization.3. We have noted some discrepancies in mobility versus size correlations of different com-mercial batches and sources of molecular mass standards, particularly in the range above150 kDa. Thus domain product sizes should be viewed as relative rather than absolute.4. Figure 2A,B shows the results of this protocol for cell lysates and media. Nonspecificbands may be present and can be eliminated from consideration using the pSVL vectorcontrol lanes for comparison (Fig. 2A,B, lanes 1 and 3). The sizes of specific bandsreported herein are given with reference to Bio-Rad prestained molecular mass standards.(Estimated dimer sizes are higher, by up to 30 kDa using Novex standards, whereas mo-nomeric size approximations remain unchanged.) In cell lysates (Fig. 2A), an immuno-specific band is seen at approx 150 kDa (lane 2), which is replaced with an 88-kDa band 152 Bell and Forstneron thiol reduction (lane 4). These represent the mucin domain dimer and reduced mono-mer, respectively. In nonreduced samples there is also a band at 73 kDa. The identity ofthis species is not yet clear, but may represent misfolded or immature monomeric domainproducts. It converts to 88 kDa with reduction (Fig. 2A, lane 4). In the cell medium (Fig.2B), lane 2 shows a secreted immunospecific product with a mobility equivalent to about165 kDa (dimer) that disappears on reduction, leaving a 100-kDa species (lane 4) (mono-mer). Note that the apparent molecular masses of the secreted (medium) dimer and mono-mer are larger than the corresponding cell lysate species (i.e., 165 vs 150 for the dimer,100 vs 88 for the reduced monomer). The explanation lies in a late glycosylation step, asnoted in Subheading 3.6.5. As seen in Fig. 2C, cells treated with tunicamycin exhibit a 45-kDa product (presumablya highly folded, nonglycosylated monomer) under nonreducing conditions (lane 1), and a62-kDa band on reduction (lane 2). The calculated size of the expected translation prod-uct is 59 kDa (534 amino acids), which corresponds well with the observed 62-kDa band.No dimer form appears, indicating a prerequisite for N-glycosylation in domain dimeriza-tion. Because no bands were observed in the medium (not presented), it is clear that thenonglycosylated product cannot be secreted.6. Results are shown in Fig. 3A, in which a mobility shift of the untreated control (lane 1)from 165 kDa to a band at 145 kDa (lane 2) is observed after treatment with N-glycosi-dase F.Fig. 2. SDS PAGE analysis of COS-1 cell translation products after transfection with pRMC, asoutlined in Subheading 3.5. Western blots were performed using antibody to the deglycosylated“link” glycopeptide. “+” lanes refer to samples reduced with 10 mM DTT. (A) Thirty microli-ters of pRMC- (lanes 2 and 4) and pSVL control-transfected (lanes 1 and 3) cell lysates.Reduction produces a shift in mobility from a 150-kDa dimer to an 88-kDa monomer. (B) Asimilar shift, but of larger species, is seen using 30-µL aliquots of concentrated conditionedmedia from pRMC- (lanes 2 and 4) and pSVL-transfected (lanes 1 and 3) COS-1 cells. DTTreduction of samples (denoted +) causes a shift from 165 to 100 kDa. (C) Culture of COS-1cells in 0.5 µL/mL of tunicamycin followed by SDS-PAGE analysis of the cell lysates revealsa highly mobile band, presumably tightly folded, at 45 kDa (lane 1), which yields a 62-kDadeglycosylated monomer on reduction. No dimer product is detected in lysates, and no mono-mer or dimer is detected in conditioned medium (not shown). [...]... heterologous cells dem- onstrate that the C-terminal region of rat Muc2 can form disulfide-dependent dimers, and that N-glycosylation plays a significant role in dimerization. A general schematic reflect- ing the results using these methods is shown in Fig. 4. The reader is referred to Perez- Vilar et al. (25) for an earlier application of domain expression to show dimerization of the C-terminal domain... its N- and C-terminal domains (7,8). The function of vWF to cause aggrega- tion of platelets is also dependent on its ability to polymerize via disulfide bonds. More important for the present report, the DNA encoding vWF was expressed suc- cessfully in heterologous cells and shown to undergo S-S–dependent dimer and multimer formation (9,10). The minimum component necessary for initial dimeriza- tion... to Bio-Rad prestained molecular mass standards. (Estimated dimer sizes are higher, by up to 30 kDa using Novex standards, whereas mo- nomeric size approximations remain unchanged.) In cell lysates (Fig. 2A), an immuno- specific band is seen at approx 150 kDa (lane 2), which is replaced with an 88-kDa band Dimerization of Domains of Mucin 143 143 From: Methods in Molecular Biology, Vol. 125: Glycoprotein. .. approach was neces- sitated by the virtual impossibility of studying detailed structural changes in molecules as large and as viscous as typical full-length secretory mucins. As has been true for the elucidation of vWF physiology and disease-associated mutants of vWF, the expression of individual mucin domains and their secretory fate holds promise of enlarging our under- standing of structure-function relationships... domain and its secretion could be studied by performing pulse-chase experiments using radioactive pre- cursors added to transfected cells. The strategic use of inhibitors during cell incubations has the potential to reveal information about the pre- and post-Golgi movement of mucin domains along the secretory pathway. Finally, truncation or site-specific mutagenesis of the initial domain constructs could... peptide: a. 5'-CGTCTAGAATGGGGCTGCCACTAGCTCGCCTGGTGGCT-3'. 3. Primer 2 (see Fig. 1). The antisense primer containing signal peptide and “linker” sequence to be paired with primer 1: a. 5'-CACAGTTAGATTCCAGCCCTTGGCTAAGGCCAGGACTAGGCACACAG-3'. 4. Primer 3 (see Fig. 1). This primer specifies the “linker” sequence and primes the 5' end of the target domain of rat Muc2: a. 5'-GGCTTGGAATCTAACTGTGAAGTTGCTGC-3'. ... repeat sequences, flanked by cysteine-rich regions at each end, which are presumed to mediate polymer- ization. Secretory mucins contain approx 60–80% carbohydrate, with extensive O- glycosylation in the central tandem repeat regions, and N-linked oligosaccharides in the peripheral regions (1). The ability of mucin peptides to form large polymers, combined with their exten- sive posttranslational glycosylation... constructs and/or other domains, e.g., constructs encoding both C- and N-terminal domains, to test whether dimers can expand into larger S-S–linked oligomers or multimers. Various embellishments are also possible, including the addition to constructs of commercially available tag epitopes at selected areas of the domain and immunopre- cipitation of translated products with specific antiepitope antibodies.... 5'-GGCTTGGAATCTAACTGTGAAGTTGCTGC-3'. Dimerization of Domains of Mucin 151 aliquot and incubate 16 h at 37˚C in a water bath. Analyze by SDS-PAGE as described in Subheading 3.5., steps 2–10. 3. Harvest COS cells in 200 µL of lysis buffer and microfuge at 4˚C to remove cell debris. 4. Incubate the remaining supernatant with 4 µL of antibody (anti-D4553 was used in this study) for 1.5 h at 4˚C followed by a further 2.5-h incubation... immunoreactive, thiol-reduced and nonreduced products in cells and media by SDS-PAGE and Western blotting. The use of specific antibodies to various regions of the domain can provide assurance that the domain is expressed in an intact form or, alternatively, has been proteolytically processed during dimerization. After establish- ing the dimerization capability of the isolated C-terminal domain of rat . benzyl 2-acetamido-2-deoxy-α-D-galactopyranoside (benzyl-α-GalNAc) (Sigma),filter sterilized.3. Peptide-N 4-( acetyl-ß-glycosaminyl) asparagine amidase (N-glycosidase. addition of 6.6 mg of 4-nitro blue tetrazolium chloride and 1.65 mg of 5-bromo-4-chloro-3-indolyl-phosphate), and stop solution (10 mM Tris-HCl, 1 mM EDTA, pH