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1 Department of Biochemistry, Molecular Biology, and Biophysics, and 2 Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, Minnesota 55455, USA
Reprint requests to: Bruce L. Martin or Ian M. Armitage, Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, 6-155 Jackson Hall, 321 Church Street, SE, Minneapolis, MN 55455, USA; e-mail: marti285{at}umn.edu or armitage{at}msi.umn.edu; fax: (612) 625-2163.
(RECEIVED October 21, 2004; FINAL REVISION January 11, 2005; ACCEPTED January 14, 2005)
| Abstract |
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actin. Independently using antibodies against metallothionein-3, creatine kinase, and heat-shock protein 84 showed that all three proteins were coimmunoprecipitated from whole mouse brain homogenates with each of the three antibodies. Mixing purified samples of metallothionein and human brain creatine kinase also generated a complex that could be immunoprecipitated either by anti-metallothionein-3 or anticreatine kinase antibody. These data are consistent with metallothionein-3 being present in the mouse brain as part of a multiprotein complex providing new functional information for understanding the role of metallothionein-3 in neuronal physiology. Keywords: metallothionein-3; partner proteins; mass spectrometry; proteomics; mouse brain
Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.041113005.
| Introduction |
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Metallothionein-3 was originally isolated from brain as a growth inhibitory factor (GIF) using bioassays demonstrating the inhibition of neuronal cell growth, a property not exhibited by MT1 and MT2. Many reports indicate that mRNA for MT3 is downregulated in tissue from Alzheimers patients (Tsuji et al. 1992; Naruse et al. 1994; Carrasco et al. 1999; Yu et al. 2001), although other studies have not confirmed this result (Erickson et al. 1994; Amoureux et al. 1997). The growth-inhibiting activity was established to reside in the amino terminal
-domain (Sewell et al. 1995; Uchida and Ihara 1995), with mutations at proline residues (positions 7 and 9) found to abolish the inhibitory activity of MT3 (Sewell et al. 1995). Although the inhibitory activity was reportedly independent of metal ion binding (Sewell et al. 1995), inhibitory activity was reported to decrease in zinc-deficient mice (Palmiter 1995). MT3 and Zn2+ were co-localized in neurons of mice, which overexpress MT3 (Masters et al. 1994; Erickson et al. 1995). These studies support unique functions for MT3 attributable to the unique structural features of the protein.
The unique properties of MT3 support the hypothesis that there are specific interacting proteins which mediate the functionality of MT3. In this paper, immunological approaches have been used to identify specific MT3 partner proteins using mass spectrometry.
| Experimental procedures |
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Immobilized antibody preparation
Metallothionein-3 antibody (
-MT3) was immobilized on Affi-Gel Hz, Bio-Rad. Purified
-MT3 (20 mg/mL) was desalted with a 3 kDa spin filter and diluted into Affi-Gel Hz coupling buffer (Bio-Rad) at pH 5.5. The solution was oxidized for 1 h at room temperature using NaIO4. The NaIO4/
-MT3 mix was desalted using a 3 kDa spin filter to remove sodium salts. The Affil-Gel Hz gel was washed with coupling buffer. The oxidized, desalted
-MT3 (1 mL) was then mixed with the Affi-Gel Hz (2 mL) and incubated at room temperature for 24 h. After incubation the coupled antibody was washed with 0.5 M NaCl in 10 mM MOPS. The immunoaffinity matrix was stored in the cold room.
Immunoaffinity isolation
Immunoaffinity chromatography was done using
-MT3 antibody immobilized on Affigel-Hz (Bio-Rad Laboratories). Antibody had been first affinity purified using MT3 immobilized on Affigel-15 (Bio-Rad Laboratories). Brain extract was prepared from two mouse brains (Swiss Webster, Pel-Freez) with a combined wet weight of 0.78 g that were homogenized in 1.6 mL 0.01 M NaCl in 10 mM MOPS (pH 7.3). The homogenate was centrifuged (1 h at 3000 rpm) to pellet cell material, and the extract was pipetted off and filtered through a 0.45 micron syringe filter. The collected brain extract was applied to the immobilized antibody (2 mL) in 10 mM MOPS (pH 7.3) with 10 mM NaCl and containing PMSF and a cocktail of protease inhibitors (Sigma-Aldrich; catalog no. P8849). The slurry of immobilized antibody with brain extract was incubated overnight at 4°C with rocking. The beads were collected by centrifugation and washed three times for 20 min with 3 mL 10 mM MOPS (pH 7.3), with 10 mM NaCl with centrifugation after each wash. The resin was then washed with 10 mM MOPS (pH 7.3), with 150 mM NaCl until no protein was found identified by the Bradford method (Bradford 1976) in the recovered washes after centrifugation. Proteins bound to the immobilized antibody were then eluted with 1 mL 3 M NaSCN. The proteins in the eluant were recovered by centrifugation and shown to contain MT3 by slot blotting using
-MT3 antiserum. The eluant was concentrated using centrifugal concentrators with 3000 Da cutoff membrane and analyzed by SDS-polyacrylamide gel electrophoresis with silver staining (Fig. 1
).
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Mass spectrometry analysis
Samples were spotted on a MALDI MS target and peptide fragments were recorded using a QSTAR XL instrument (Applied Biosystems, ABI). Full mass scans were collected in positive ion mode between m/z 500 and 3500 of the peptide mixtures for each sample. The software program Analyst QS (ABI) was then used to produce a peptide peak list for each mass spectrum. For the final searching, the list was trimmed to about 25 m/z peaks. Each peak list of measured peptide masses was then used to search the National Center for Biotechnology Information (NCBI) sequence database for protein identifications using Mascot (http://www.matrixscience.com). Searches were performed allowing for variable modifications that include methionine oxidation and carbamidomethyl addition, as well as allowing for up to one missed cleavage site on the peptide. Some peptides were further characterized by tandem mass spectrometry using the same instrument. Once selected, peptides are guided into the collision chamber whereupon they are fragmented by collision induced dissociation with argon. Data was analyzed using Analyst QS and Mascot.
Slot blot detection
Triplicate 200 µL samples of whole-brain homogenate were applied under gentle vacuum to Trans-Blot nitrocellulose membrane (Bio-Rad) using a slot blot apparatus (Bio-Rad). Each well was washed with 200 µL aliquots of TBS. After removing TBS with gentle suction, the membrane was removed from the apparatus and washed two times with TTBS (TBS with 0.05% Tween-20) and once with TBS. The membrane was divided into three identical pieces and with replicate samples incubated independently with different antibody solutions:
-creatine kinase BB (
-CK),
-heat-shock protein 84 (
-HSP84), and
-MT3. Incubations were allowed to proceed overnight at 4°C with rocking. After incubation, the antibody solutions were removed and the membranes rinsed twice with TTBS and once with TBS. Detection was done using the reagents of the Amplified Alkaline Phosphatase Immun-Blot Kit from Bio-Rad Laboratories. Briefly, the membrane was incubated for 1 h with the secondary antibody (biotinylated goat-antirabbit antibody in TBS); the membrane again washed as described, and the membrane incubated for 1 h with a conjugate of sterptavidin with biotinylated-alkaline phosphatase in TTBS. After a final washing procedure, the presence of antigen was detected by monitoring the color development after exposure to the substrate for the alkaline phosphatase-conjugated secondary antibody. For alkaline phosphatase, the substrate used was comprised of a mix of bromochloroindolyl chloride phosphate (BCIP) and nitroblue tetrazolium (NBT). BCIP is hydrolyzed by alkaline phosphatase and reacts with NBT to stain the membrane at the site of alkaline phosphatase. This reaction occurs only at sites of bound primary antibody. The reaction was stopped by the dilution of substrate solution with water.
Immunoprecipitation
Immunoprecipitation reactions were performed independently with antibodies directed against MT3 (
-MT3), HSP84 (
-HSP84), and the brain form of creatine kinase (
-CK). Antiserum (10 µL) was added to 50 µL brain extract and the solution was rocked overnight at 4°C. Following the incubation, 50 µL protein A sepharose, prewashed with 10 mM MOPS, was added and incubated for 1 h. Each sample was centrifuged and the supernatant was removed. Then the beads were washed three times with 10 mM MOPS and then SDS sample buffer was added to the pellet prior to boiling.
Creatine kinase assay
A creatine kinase assay kit was obtained from Diagnostics Chemical Ltd. and used to assess the amount of creatine kinase activity. The assay is based on the formation of NADPH, and can be monitored at 340 nm. Creatine kinase catalyzes the reaction of ADP with creatine phosphate to yield creatine and ATP. The reaction is coupled to the hexokinase-catalyzed formation of glucose-6-phosphate from glucose and ATP. Finally, glucose-6-phosphate is oxidized to glucono-6-lactone-6-phosphate by glucose-6-phosphate dehydrogenase. The oxidation is coupled to the formation of NADPH from NADP+.
| Results |
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-MT3 antibody immobilized on Affigel-Hz (Bio-Rad Laboratories) was done to isolate specific MT3 binding proteins. Brain extract from Swiss-Webster mice was used as the source of proteins and the recovered samples from the immunoaffinity chromatography were separated by SDS-polyacrylamide gel electrophoresis with silver staining (Fig. 1
actin. The positions of these proteins are shown by the arrows in the figure. Unassigned protein bands shown were analyzed, but insufficient information was recovered for their identification by comparison to the MASCOT database.
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-domain, and evidence from earlier work indicates that Cd4- or Zn4-
-MT3 are very similar. The presence of zinc in MT3 may not be an absolute requirement inasmuch as CK did interact and coimmunoprecipitate with Cd7-MT3 (Fig. 3
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Functional consequences of MT3 interactions
Unfortunately, MT3 has no simple functional assay to assess changes in activity. One MT3 partner, creatine kinase, has a specific and important biological activity which can be assayed spectrophotometrically with monitoring of product formation at 340 nm. Using a creatine kinase assay kit from Diagnostics Chemical Ltd, no effect on creatine kinase activity was found with the addition of recombinant Cd7-MT3 to the assay mixture (0.57 µg/mL MT3 and 0.075 µg/mL CK). A MT3 knockout mouse strain has been reported in the literature (Erickson et al. 1997), and may prove valuable for insight about the relationship of MT3 with these partner proteins. Will the complexes remain in the same form in the absence of MT3? Immunocytochemical approaches will also be invaluable in establishing whether these same proteins are co-localized in the same regions of the brain.
Summary
MT3 was isolated using immunoaffinity chromatography. Select mouse brain proteins were copurified with the corollary conclusion that these proteins exist as cellular macromolecular complex. The identities of the interacting proteins was established using mass spectrometry an database comparisons. A working model for the functional consequences of these interactions is proposed, and will provide a basis for future directions. It is noteworthy that MT3 and all the identified partner proteins are induced by the same physiological processes, such as oxidative stress and neural injury. These links support the premise that these interactions will prove significant in understanding the roles of these proteins in the brain.
| Acknowledgments |
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