Insulin-stimulated phosphorylation of protein phosphatase 1 regulatory subunit 12B revealed by HPLC-ESI-MS/MS
- Kimberly Pham†1,
- Paul Langlais†1,
- Xiangmin Zhang†1, 2,
- Alex Chao1,
- Morgan Zingsheim1 and
- Zhengping Yi1, 2Email author
© Pham et al.; licensee BioMed Central Ltd. 2012
Received: 13 February 2012
Accepted: 31 July 2012
Published: 1 September 2012
Protein phosphatase 1 (PP1) is one of the major phosphatases responsible for protein dephosphorylation in eukaryotes. Protein phosphatase 1 regulatory subunit 12B (PPP1R12B), one of the regulatory subunits of PP1, can bind to PP1cδ, one of the catalytic subunits of PP1, and modulate the specificity and activity of PP1cδ against its substrates. Phosphorylation of PPP1R12B on threonine 646 by Rho kinase inhibits the activity of the PP1c-PPP1R12B complex. However, it is not currently known whether PPP1R12B phosphorylation at threonine 646 and other sites is regulated by insulin. We set out to identify phosphorylation sites in PPP1R12B and to quantify the effect of insulin on PPP1R12B phosphorylation by using high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry.
14 PPP1R12B phosphorylation sites were identified, 7 of which were previously unreported. Potential kinases were predicted for these sites. Furthermore, relative quantification of PPP1R12B phosphorylation sites for basal and insulin-treated samples was obtained by using peak area-based label-free mass spectrometry of fragment ions. The results indicate that insulin stimulates the phosphorylation of PPP1R12B significantly at serine 29 (3.02 ± 0.94 fold), serine 504 (11.67 ± 3.33 fold), and serine 645/threonine 646 (2.34 ± 0.58 fold).
PPP1R12B was identified as a phosphatase subunit that undergoes insulin-stimulated phosphorylation, suggesting that PPP1R12B might play a role in insulin signaling. This study also identified novel targets for future investigation of the regulation of PPP1R12B not only in insulin signaling in cell models, animal models, and in humans, but also in other signaling pathways.
KeywordsPPP1R12B Phosphorylation HPLC-ESI-MS/MS Insulin signaling Label-free Quantification
Protein phosphatase 1 (PP1) is one of the most abundant serine/threonine phosphatases; it is responsible for most protein dephosphorylation [1–3], which regulates diverse biological processes in eukaryotes. Interactions between catalytic subunits of PP1 (PP1c) and the regulatory subunits of PP1 (PP1R) lead to the formation of numerous PP1 complexes that have unique substrate specificities, distinct subcellular localizations, and various regulatory mechanisms [1–3].
Protein phosphatase 1 regulatory subunit 12B (PPP1R12B), also known as myosin phosphatase target subunit 2 (MYPT2), is one of the regulatory subunits of PP1 and is predominantly expressed in cardiac/skeletal muscle and brain [4, 5]. PPP1R12B regulates muscle contraction, cardiac torsion, and sarcomere organization as well as other cellular processes . PPP1R12B contains an RVxF binding motif (residues 53–56), several ankyrin repeats, and a C-terminal leucine zipper domain, all of which are involved in protein-protein interactions [4–7]. In addition, PPP1R12B has 108 serine, 63 threonine, and 16 tyrosine residues, 26 of which have been reported as phosphorylated in the four large phosphorylation databases (http://www.phosphosite.org, phospho.elm.eu.org, http://www.uniprot.org, and http://www.phosida.com). However, only phosphorylation at threonine 646 (Thr646) has been shown to regulate PPP1R12B function . Thr646 was phosphorylated by Rho-kinase in kidney COS7 cells, reducing the activity of the PPP1R12B-PP1cδ complex . Whether Thr646 phosphorylation plays the same inhibitory role in PPP1R12B-PP1cδ complex activity in other cells remains to be determined.
Insulin is a potent anabolic hormone that modulates a wide variety of biological processes. Protein phosphorylation plays a critical role in relaying the insulin signal from initiation at the insulin receptor to the transport of GLUT4 to the plasma membrane. Dysregulated protein phosphorylation events in insulin signaling may contribute to various diseases, such as type 2 diabetes and cardiovascular diseases. Extensive research has been carried out to study the role of kinases in insulin action. However, a mechanism for serine/threonine phosphatase action in insulin signal transduction is largely unknown. In an effort to discover phosphatases that may be involved in insulin signaling, we identified protein phosphatase 1 regulatory subunit 12A (PPP1R12A) as a novel endogenous, insulin stimulated interaction partner of insulin receptor substrate-1 (IRS-1), a well recognized player in insulin signaling, implying that PPP1R12A might play a role in IRS-1 dephosphorylation and insulin signaling . PPP1R12A is an isoform of PPP1R12B with high expression in smooth muscle cells . As mentioned previously, PPP1R12B is predominantly expressed in cardiac/skeletal muscle and brain. Thus, it is possible that PPP1R12B could anchor the catalytic subunit of PP1, PP1cδ, to dephosphorylate IRS-1 in cardiac/skeletal muscle and brain. More recently, we provided a relative global picture of PPP1R12A phosphorylation in CHO/IR cells, and reported that insulin stimulated or suppressed PPP1R12A phosphorylation at multiple sites . It is currently not known whether insulin plays a regulatory role in PPP1R12B phosphorylation. Therefore, in the present study, we used multi-segment high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) to identify and quantify PPP1R12B phosphorylation sites that are regulated by insulin. We utilized the peak area of MS2 generated fragment ions, an approach developed in our laboratory , to quantify relative changes in PPP1R12B phosphorylation after insulin treatment.
PPP1R12B phosphopeptides and predicted potential kinases
CK2α, CK2α’, DMPK, PAK-1, PAK-2, PAK-3, PAK-4, PKAα, PKAβ, p70-S6K
AIM1, PAK-1, PAK-2, PAK-3, PKAα, PKAβ, DMPK, Pim-2h/ DMPK, Pim-2h, RhoK
CK2α, CK2α’, TLK1
ActRIIA, ActRIIB, CLK1, CLK2, DMPK, RAGE-1, TGFR-2
ActRIIA, ActRIIB, TGFR-2
PPP1R12B internal standard peptides
Molecular weight (Da)
Quantified ( m/z )*
Unique fragment ions used to quantify phosphorylation sites
Product ion ( m/z )*
Effect of insulin on PPP1R12B phosphorylation, n = 4
Normalized peak area #
Normalized peak area #
0.00E + 00
Normalized peak area #
Normalized peak area #
Normalized peak area #
Normalized peak area #
0.00E + 00
It has been shown that phosphorylation of PPP1R12B at Thr646 by Rho kinase reduces the activity of the PPP1R12B-PP1cδ complex against smooth muscle myosin light chain in COS7 kidney cells . Whether Thr646 phosphorylation plays the same inhibitory role in PPP1R12B-PP1cδ complex activity in CHO/IR cells remains to be elucidated. A previous report indicated that insulin might stimulate Rho kinase activity . Thus, it is possible that after insulin stimulation, Rho kinase phosphorylates Thr646 in PPP1R12B in CHO/IR cells and serves as a negative regulator of the PPP1R12B-PP1cδ complex. We also observed the phosphorylation of PPP1R12B at the pThr646 proximal site, Ser645, although these 2 phospho sites were not distinguishable based on the MS/MS spectrum, and whether they behave similarly in the regulation of PPP1R12B is unclear at present. Mutation of Thr646 or Ser645 to alanine is on-going to assess the role of PPP1R12B phosphorylation on PP1c activity and insulin signaling.
Ser29 and Thr31 are in close proximity to the PP1c binding motif (K/R-I/V-X-F/W, residues 53–56 in human PPP1R12B) . In addition, the crystalline structure of the PP1 complex between the chicken PP1c δ isoform (also called β isoform) and amino acids 1–299 of protein phosphatase 1 regulatory subunit 12A (PPP1R12A) (also called MYPT1, an isoform of PPP1R12B) has been resolved . It indicates that residues 1–34, which precede the PP1c binding motif in human PPP1R12A (residues 35–38), also interact with PP1cδ. It has been shown that a short peptide (residues 23–38) of PPP1R12A, which contains the PP1c binding motif but lacks the N terminus, binds to PP1c but has no effect on PP1cδ catalytic activity , whereas a peptide containing residues 1–38 of PPP1R12A both interacts with PP1c and increases its phosphatase activity. Hence, it is reasonable to conclude that some structure within residues 1–22 is responsible for the increased catalytic activity. To date, structural information for PPP1R12B is lacking. However, based on the similarity between PPP1R12A and PPP1R12B as well as the insulin-stimulated phosphorylation of Ser29 (pS29), we speculate that pS29 might play a role in regulating PP1cδ activity when it is in a complex with PPP1R12B. Without pS29, PPP1R12B might still bind to PP1cδ through the PP1c binding motif; however, the resulting complex might not have the full phosphatase activity against its substrates. We are in the process of mutating Ser29 to alanine to test the functional consequence of this mutation, such as effect on phosphatase activity.
Ser504 of PPP1R12B exhibited over 11-fold more phosphorylation after insulin treatment. Because it was found, by surface plasmon resonance, that PP1cδ might interact with the PPP1R12A truncation containing residues 304–511 , we speculate that phosphorylation at Ser504 might also be involved in the interplay between PPP1R12B and PP1cδ. The increase in phosphorylation of PPP1R12B at Ser504 represents the strongest fold change of any insulin-stimulated serine or threonine phosphorylation site that we have studied to date using this mass spectrometry technique to quantify protein phosphorylation [11, 26–28]. The strength of the insulin-stimulated PPP1R12B phosphorylation at Ser504 could indicate that it is a major regulatory mechanism responsible for controlling PPP1R12B function in insulin signaling. Mutation of Ser504 to alanine is on-going to assess the function of this phosphorylation site in regulating PPP1R12B and PP1c activity.
Insulin signaling is crucial to many biological processes, such as glycogen synthesis, glucose transport, mitogenesis, and protein synthesis. The intracellular actions of insulin are mediated by controlled protein phosphorylation and dephosphorylation . Insulin activates the insulin receptor, and the activated insulin receptor then phosphorylates tyrosine residues IRS-1, which allows IRS-1 to recruit phosphatidylinositide 3-kinase and leads to phosphorylation of Akt on threonine/serine residues. Activated Akt phosphorylates its substrate proteins, such as AS160, and promotes GLUT4 translocation to the plasma membrane, leading to enhanced glucose uptake. In addition, activated Akt can increase glycogen synthesis by phosphorylating glycogen synthase kinase 3, and decreasing the phosphorylation of glycogen synthase. Moreover, phosphorylated Akt enhances protein synthesis through serine/threonine phosphorylation of mammalian target of rapamycin and ribosomal protein S6 kinase beta-1 . Furthermore, IRS-1 interacts with growth factor receptor binding protein 2, leading to serine/threonine phosphorylation of a number of signaling proteins in the mitogen-activated protein kinase pathway and subsequent promotion of cell survival and mitogenesis . As discussed above, several of the serine/threonine kinases, such as Akt, mammalian target of rapamycin, ribosomal protein S6 kinase beta-1, glycogen synthase kinase 3, and mitogen-activated protein kinase, have been shown to play a role in insulin signaling. However, a mechanism for serine/threonine phosphatase action in insulin signal transduction is not known. The present study identified PPP1R12B, a regulatory subunit of PP1 (which is a serine/threonine protein phosphatase), as a new insulin-signaling protein with site-specific phosphorylation that is regulated by insulin in CHO/IR cells. The results presented in this study will provide targets for future investigations delineating the role of serine/threonine phosphatases in insulin signaling.
The sequencing-grade trypsin and anti-FLAG antibody were purchased from Sigma (St. Louis, MO), and the C18 ZipTip from Millipore (Billerica, MA). Chinese hamster ovary cells overexpressing the insulin receptor (CHO/IR) were a gift from Dr. Feng Liu (University of Texas Health Science Center at San Antonio, TX). Establishment of the CHO/IR cell line was described previously . The cDNA encoding full-length wild-type human PPP1R12B was a gift from Dr. Ryuji Okamoto and Dr. Masaaki Ito (Mie University, Tsu, Mie, Japan).
Cell culture, transfection, immunoprecipitation, and SDS-PAGE
CHO/IR cells were transfected with 5–10 μg of FLAG-tagged PPP1R12B plasmid DNA using Lipofectamine reagent (Invitrogen, Carlsbad, CA), serum starved for 4 h at 37°C, and left untreated or treated with insulin (100 nM) for 15 min at 37°C. The cells were lysed, and cell lysates (1 mg) were diluted in lysis buffer and incubated with 2 μg of anti-FLAG antibody for PPP1R12B purification. The immunoprecipitates were collected with Protein A agarose beads (Sigma, St. Louis, MO). Samples were boiled in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) sample buffer and resolved by 10% 1D-SDS-PAGE. The proteins were then visualized by Coomassie blue staining (Sigma, St. Louis, MO). Please see Additional file 3 for more details.
In-gel digestion and mass spectrometry
In-gel digestion and mass spectrometry were performed as described previously [11, 26, 32]. Briefly, the gel portions containing PPP1R12B were excised, destained, dehydrated, dried, and subjected to trypsin digestion overnight. The resulting peptides were desalted and analyzed by on-line HPLC on a linear trap quadrupole-Fourier transform ion cyclotron resonance (LTQ-FTICR). Please see the Additional file 3 for details.
Phosphorylation sites were located using Scaffold PTM (version 1.0.3, Proteome Software, Portland, OR), a program based on the Ascore algorithm [33, 34]. Sites with Ascores ≥ 13 (P ≤ 0.05) were considered confidently localized [33, 34].
Peak areas for each peptide were obtained by integrating the appropriate reconstructed ion chromatograms with 10 ppm error tolerance for precursor-ion masses acquired using FTICR and 0.5 Dalton for the fragment ions acquired using the LTQ mass analyzer. Relative quantification of each phosphopeptide was obtained by comparing normalized peak-area ratios for control and insulin-treated samples [11, 26, 32].
Statistical significance was assessed by comparing control and insulin-stimulated phosphopeptide peak areas (normalized to PPP1R12B representative peptides as described above) using the paired t-test.
Chinese hamster ovary cells overexpressing human insulin receptor
High-performance liquid chromatography-electrospray ionization-mass spectrometry
Insulin receptor substrate-1
Linear trap quadrupole-Fourier transform ion cyclotron resonance
Tandem mass spectrometry
Protein phosphatase 1 catalytic subunit
Protein phosphatase 1 regulatory subunit 12B.
This research was supported by funds from the National Institutes of Health R01DK081750 (ZY) and an American Diabetes Association Clinical/Translational Research Award 7-09-CT-56 (ZY) as well as the Howard Hughes Medical Institute through the Undergraduate Science Education Program and from the Arizona State University School of Life Sciences (KP).
- Bollen M, Peti W, Ragusa MJ, Beullens M: The extended PP1 toolkit: designed to create specificity. Trends Biochem Sci 2010, 35: 450–458. 10.1016/j.tibs.2010.03.002PubMed CentralView Article
- Cohen PT: Protein phosphatase 1-targeted in many directions. J Cell Sci 2002, 115: 241–256.
- Virshup DM, Shenolikar S: From promiscuity to precision: protein phosphatases get a makeover. Mol Cell 2009, 33: 537–545. 10.1016/j.molcel.2009.02.015View Article
- Grassie ME, Moffat LD, Walsh MP, Macdonald JA: The myosin phosphatase targeting protein (MYPT) family: a regulated mechanism for achieving substrate specificity of the catalytic subunit of protein phosphatase type 1δ. Arch Biochem Biophys 2011, 510: 147–159. 10.1016/j.abb.2011.01.018View Article
- Okamoto R, Kato T, Mizoguchi A, Takahashi N, Nakakuki T, Mizutani H, Isaka N, Imanaka-Yoshida K, Kaibuchi K, Lu Z, et al.: Characterization and function of MYPT2, a target subunit of myosin phosphatase in heart. Cell Signal 2006, 18: 1408–1416. 10.1016/j.cellsig.2005.11.001View Article
- Ito M, Nakano T, Erdodi F, Hartshorne DJ: Myosin phosphatase: structure, regulation and function. Mol Cell Biochem 2004, 259: 197–209.View Article
- Shichi D, Arimura T, Ishikawa T, Kimura A: Heart-specific small subunit of myosin light chain phosphatase activates rho-associated kinase and regulates phosphorylation of myosin phosphatase target subunit 1. J Biol Chem 2010, 285: 33680–33690. 10.1074/jbc.M110.122390PubMed CentralView Article
- Geetha T, Langlais P, Luo M, Mapes R, Lefort N, Chen SC, Mandarino LJ, Yi Z: Label-free proteomic identification of endogenous, insulin-stimulated interaction partners of insulin receptor substrate-1. J Am Soc Mass Spectrom 2011, 22: 457–466. 10.1007/s13361-010-0051-2PubMed CentralView Article
- Matsumura F, Hartshorne DJ: Myosin phosphatase target subunit: Many roles in cell function. Biochem Biophys Res Commun 2008, 369: 149–156. 10.1016/j.bbrc.2007.12.090PubMed CentralView Article
- Chao A, Xiangmin Z, Danjun M, Langlais P, Moulun-Luo LJM, Zingsheim M, Pham K, Dillon J, Zhengping Y: Site-Specific Phosphorylation of Protein Phosphatase 1 Regulatory Subunit 12A Stimulated or Suppressed by Insulin. Journal of Proteomics 2012, 75: 3342–3350. 10.1016/j.jprot.2012.03.043PubMed CentralView Article
- Langlais P, Mandarino LJ, Yi Z: Label-free relative quantification of co-eluting isobaric phosphopeptides of insulin receptor substrate-1 by HPLC-ESI-MS/MS. J Am Soc Mass Spectrom 2010, 21: 1490–1499. 10.1016/j.jasms.2010.05.009PubMed CentralView Article
- Moritz A, Li Y, Guo A, Villen J, Wang Y, MacNeill J, Kornhauser J, Sprott K, Zhou J, Possemato A, et al.: Akt-RSK-S6 kinase signaling networks activated by oncogenic receptor tyrosine kinases. Sci Signal 2010, 3: ra64. 10.1126/scisignal.2000998PubMed CentralView Article
- Pan C, Gnad F, Olsen JV, Mann M: Quantitative phosphoproteome analysis of a mouse liver cell line reveals specificity of phosphatase inhibitors. Proteomics 2008, 8: 4534–4546. 10.1002/pmic.200800105View Article
- Huttlin EL, Jedrychowski MP, Elias JE, Goswami T, Rad R, Beausoleil SA, Villen J, Haas W, Sowa ME, Gygi SP: A tissue-specific atlas of mouse protein phosphorylation and expression. Cell 2010, 143: 1174–1189. 10.1016/j.cell.2010.12.001PubMed CentralView Article
- Hoffert JD, Pisitkun T, Wang G, Shen RF, Knepper MA: Quantitative phosphoproteomics of vasopressin-sensitive renal cells: regulation of aquaporin-2 phosphorylation at two sites. Proc Natl Acad Sci U S A 2006, 103: 7159–7164. 10.1073/pnas.0600895103PubMed CentralView Article
- Zanivan S, Gnad F, Wickstrom SA, Geiger T, Macek B, Cox J, Fassler R, Mann M: Solid tumor proteome and phosphoproteome analysis by high resolution mass spectrometry. J Proteome Res 2008, 7: 5314–5326. 10.1021/pr800599nView Article
- Imami K, Sugiyama N, Kyono Y, Tomita M, Ishihama Y: Automated phosphoproteome analysis for cultured cancer cells by two-dimensional nanoLC-MS using a calcined titania/C18 biphasic column. Anal Sci 2008, 24: 161–166. 10.2116/analsci.24.161View Article
- Linding R, Jensen LJ, Ostheimer GJ, van Vugt MA, Jorgensen C, Miron IM, Diella F, Colwill K, Taylor L, Elder K, et al.: Systematic discovery of in vivo phosphorylation networks. Cell 2007, 129: 1415–1426. 10.1016/j.cell.2007.05.052PubMed CentralView Article
- Linding R, Jensen LJ, Pasculescu A, Olhovsky M, Colwill K, Bork P, Yaffe MB, Pawson T: NetworKIN: a resource for exploring cellular phosphorylation networks. Nucleic Acids Res 2008, 36: D695–699.PubMed CentralView Article
- Sun J, Khalid S, Rozakis-Adcock M, Fantus IG, Jin T: P-21-activated protein kinase-1 functions as a linker between insulin and Wnt signaling pathways in the intestine. Oncogene 2009, 28: 3132–3144. 10.1038/onc.2009.167View Article
- Tanasijevic MJ, Myers MG, Thoma RS, Crimmins DL, White MF, Sacks DB: Phosphorylation of the insulin receptor substrate IRS-1 by casein kinase II. J Biol Chem 1993, 268: 18157–18166.
- Llagostera E, Catalucci D, Marti L, Liesa M, Camps M, Ciaraldi TP, Kondo R, Reddy S, Dillmann WH, Palacin M, et al.: Role of myotonic dystrophy protein kinase (DMPK) in glucose homeostasis and muscle insulin action. PLoS One 2007, 2: e1134. 10.1371/journal.pone.0001134PubMed CentralView Article
- Furukawa N, Ongusaha P, Jahng WJ, Araki K, Choi CS, Kim HJ, Lee YH, Kaibuchi K, Kahn BB, Masuzaki H, et al.: Role of Rho-kinase in regulation of insulin action and glucose homeostasis. Cell metabolism 2005, 2: 119–129. 10.1016/j.cmet.2005.06.011View Article
- Terrak M, Kerff F, Langsetmo K, Tao T, Dominguez R: Structural basis of protein phosphatase 1 regulation. Nature 2004, 429: 780–784. 10.1038/nature02582View Article
- Toth A, Kiss E, Herberg FW, Gergely P, Hartshorne DJ, Erdodi F: Study of the subunit interactions in myosin phosphatase by surface plasmon resonance. Eur J Biochem 2000, 267: 1687–1697. 10.1046/j.1432-1327.2000.01158.xView Article
- Yi Z, Langlais P, De Filippis EA, Luo M, Flynn CR, Schroeder S, Weintraub ST, Mapes R, Mandarino LJ: Global assessment of regulation of phosphorylation of insulin receptor substrate-1 by insulin in vivo in human muscle. Diabetes 2007, 56: 1508–1516. 10.2337/db06-1355View Article
- Langlais P, Yi Z, Mandarino LJ: The Identification of Raptor as a Substrate for p44/42 MAPK. Endocrinology 2011, 152: 1264–1273. 10.1210/en.2010-1271PubMed CentralView Article
- Højlund K, Yi Z, Lefort N, Langlais P, Bowen B, Levin K, Beck-Nielsen H, Mandarino L: Human ATP synthase beta is phosphorylated at multiple sites and shows abnormal phosphorylation at specific sites in insulin-resistant muscle. Diabetologia 2010, 53: 541–551. 10.1007/s00125-009-1624-0View Article
- Siddle K: Signalling by insulin and IGF receptors: supporting acts and new players. J Mol Endocrinol 2011, 47: R1–10. 10.1530/JME-11-0022View Article
- Yamashiro S, Yamakita Y, Totsukawa G, Goto H, Kaibuchi K, Ito M, Hartshorne DJ, Matsumura F: Myosin phosphatase-targeting subunit 1 regulates mitosis by antagonizing polo-like kinase 1. Developmental cell 2008, 14: 787–797. 10.1016/j.devcel.2008.02.013PubMed CentralView Article
- Ando A, Momomura K, Tobe K, Yamamoto-Honda R, Sakura H, Tamori Y, Kaburagi Y, Koshio O, Akanuma Y, Yazaki Y, et al.: Enhanced insulin-induced mitogenesis and mitogen-activated protein kinase activities in mutant insulin receptors with substitution of two COOH-terminal tyrosine autophosphorylation sites by phenylalanine. J Biol Chem 1992, 267: 12788–12796.
- Yi Z, Luo M, Mandarino LJ, Reyna SM, Carroll CA, Weintraub ST: Quantification of phosphorylation of insulin receptor substrate-1 by HPLC-ESI-MS/MS. J Am Soc Mass Spectrom 2006, 17: 562–567. 10.1016/j.jasms.2005.12.010View Article
- Beausoleil SA, Villen J, Gerber SA, Rush J, Gygi SP: A probability-based approach for high-throughput protein phosphorylation analysis and site localization. Nat Biotechnol 2006, 24: 1285–1292. 10.1038/nbt1240View Article
- Zhai B, Villen J, Beausoleil SA, Mintseris J, Gygi SP: Phosphoproteome analysis of Drosophila melanogaster embryos. J Proteome Res 2008, 7: 1675–1682. 10.1021/pr700696aPubMed CentralView Article
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