链霉亲和素及其亲和系统的蛋白质进化
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摘要
链霉亲和素/生物素(Streptavidin/Biotin)体系作为目前已知的最高亲和力作用体系,已在生物学研究中获得广泛应用。本文针对Streptavidin/Biotin和Strep-Tactin/Strep-tag两个相关系统的演化,分别从链霉亲和素蛋白的结构改造、亲和肽标签优化等方面进行了较为详细的归纳。通过对链霉亲和素蛋白各种突变体的优缺点的比较,有助于实际应用中选择合适的Streptavidin突变体。本文通过对链霉亲和素蛋白质进化的综述,可帮助更准确地理解市场上各种链霉亲和素蛋白的功能和用途,并为深入研究链霉亲和素蛋白的进化提供参考。
The Streptavidin/Biotin system, which has the highest affinity to date, has been widely used in biological research. In this paper, the evolution of two related systems, Streptavidin/Biotin and Strep-Tactin/Strep-tag, were summarized in detail from the structural modification of the streptavidin protein and the optimization of affinity peptide tags. By comparing the advantages and disadvantages of various mutants of the streptavidin protein, we aim to provide a valuable reference to select a suitable streptavidin mutant in practical applications. Herein we review the evolution of the streptavidin protein, facilitating a more accurate understanding of the function and use of various streptavidin proteins in the market, and provide a reference for further research on the evolution of streptavidin.
The Streptavidin/Biotin system, which has the highest affinity to date, has been widely used in biological research. In this paper, the evolution of two related systems, Streptavidin/Biotin and Strep-Tactin/Strep-tag, were summarized in detail from the structural modification of the streptavidin protein and the optimization of affinity peptide tags. By comparing the advantages and disadvantages of various mutants of the streptavidin protein, we aim to provide a valuable reference to select a suitable streptavidin mutant in practical applications. Herein we review the evolution of the streptavidin protein, facilitating a more accurate understanding of the function and use of various streptavidin proteins in the market, and provide a reference for further research on the evolution of streptavidin.
引文
[1] Green NM.Avidin and streptavidin [J].Methods Enzymol,1990,184:51-67
[2] Weber PC,Ohlendorf DH,Wendoloski JJ,et al.Structural origins of high-affinity biotin binding to streptavidin [J].Science,1989,243(4887):85-88
[3] 刘小兰,齐天琪,林婷,等.纳米金标记-逐步银染法快速检测金黄色葡萄球菌 [J].中国生物化学与分子生物学报(Liu XL,Qi TQ,Lin T,et al.Nanogold labeling-stepwise silver staining method for rapid detection of staphylococcus aureus[J].Chin J Biochem Mol Biol),2012,28(7):671-676
[4] 程振球,章谷生.链霉亲和素的特性及其应用 [J].现代免疫学(Cheng Zhen-Qiu,Zhang Gu-Sheng.Characteristics of streptavidin and its application[J].Curr Immunol),1992,12(1):56-59
[5] Livnah O,Bayer EA,Wilchek M,et al.Three-dimensional structures of avidin and the avidin-biotin complex [J].Proc Natl Acad Sci U S A,1993,90(11):5076-5080
[6] Hendrickson WA,Pahler A,Smith JL,et al.Crystal Structure of core streptavidin determined from multiwavelength anomalous diffraction of synchrotron radiation [J].Proc Natl Acad Sci U S A,1989,86(7):2190-2194
[7] Freitag S,Le Trong I,Chilkoti A,et al.Structural studies of binding site tryptophan mutants in the high-affinity streptavidin-biotin complex [J].J Mol Biol,1998,279(1):211-221
[8] Schmidt TG,Koepke J,Frank R,et al.Molecular interaction between the strep-tag affinity peptide and its cognate target,streptavidin [J].J Mol Biol,1996,255(5):753-766
[9] Hirschi S,Stauffer M,Harder D,et al.Engineering and assembly of protein modules into functional molecular systems [J].Chimia(Aarau),2016,70(6):398-401
[10] Torreggiani A,Fini G.Raman spectroscopic studies of ligand-protein interactions:the binding of biotin analogues by avidin [J].J Raman Spectrosc,1998,29:229-236
[11] Jeschek M,Bahls MO,Schneider V,et al.Biotin-independent strains of Escherichia coli for enhanced streptavidin production [J].Metab Eng,2017,40:33-40
[12] 彭福中.链霉亲和素的原核表达及复性[D].江西师范大学(Peng FZ.Prokaryotic expression and renaturation of streptavidin[D].Jiangxi Normal University,2010
[13] Gallizia A,de Lalla C,Nardone E,et al.Production of a soluble and functional recombinant streptavidin in Escherichia coli [J].Protein Expr Purif,1998,14(2):192-196
[14] Miksch G,Ryu S,Risse JM,et al.Factors that influence the extracellular expression of streptavidin in Escherichia coli using a bacteriocin release protein [J].Appl Microbiol Biotechnol,2008,81(2):319-326
[15] Nagarajan V,Ramaley R,Albertson H,et al.Secretion of streptavidin from Bacillus subtilis [J].Appl Environ Microbiol,1993,59(11):3894-3898
[16] Wu SC,Wong SL.Engineering of a Bacillus subtilis strain with adjustable levels of intracellular biotin for secretory production of functional streptavidin [J].Appl Environ Microbiol,2002,68(3):1102-1108
[17] Noda S,Matsumoto T,Tanaka T,et al.Secretory production of tetrameric native full-length streptavidin with thermostability using Streptomyces lividans as a host [J].Microb Cell Fact,2015,14:5
[18] Casteluber MC,Damasceno LM,da Silveira WB,et al.Cloning and expression of a functional core streptavidin in Pichia pastoris:strategies to increase yield [J].Biotechnol Prog,2012,28(6):1419-1425
[19] Wetzel D,Müller JM,Flaschel E,et al.Fed-batch production and secretion of streptavidin by Hansenula polymorpha:evaluation of genetic factors and bioprocess development [J].J Biotechnol,2016,225:3-9
[20] Sano T,Cantor CR.Expression of a cloned streptavidin gene in Escherichia coli [J].Proc Natl Acad Sci U S A,1990,87(1):142-146
[21] Thompson LD,Weber PC.Construction and expression of a synthetic streptavidin-encoding gene in Escherichia coli [J].Gene,1993,136(1-2):243-246
[22] Chua LH,Tan SC,Liew MWO.Process intensification of core streptavidin production through high-cell-density cultivation of recombinant E.coli and a temperature-based refolding method [J].J Biotechnol,2018,276-277:34-41
[23] ModanlooJouybari R,Sadeghi A,Khansarinejad B,et al.Production of recombinant streptavidin and optimization of refolding conditions for recovery of biological activity [J].Rep Biochem Mol Biol,2018,6(2):178-185
[24] Choi JH,Keum KC,Lee SY.Production of recombinant proteins by high cell density culture of Escherichia coli [J].Chem Eng Sci,2006,61(3):876-885
[25] 訾静,张月娟,万一.核心链霉亲和素在毕赤酵母中的表达及纯化 [J].中国生物制品学杂志(Zi J,Zhang YJ,Wan Y.Expression of core-streptavidin in Pichia pastoris and purification of expressed product [J].Chin J Biol),2018,31(5):485-488
[26] Sano T,Smith CL,Cantor CR.A Streptavidin mutant containing a cysteine stretch that facilitates production of a variety of specific streptavidin conjugates [J].Biotechnology(N Y),1993,11(2):201-206
[27] Wu SC,Wong SL.Engineering soluble monomeric streptavidin with reversible biotin binding capability [J].J Biol Chem,2005,280(24):23225-23231
[28] Clare DA,Valentine VW,Catignani GL,et al.Molecular design,expression,and affinity immobilization of a trypsin-streptavidin fusion protein(1) [J].Enzyme Microb Technol,2001,28(6):483-491
[29] Wu SC,Ng KK,Wong SL.Engineering monomeric streptavidin and its ligands with infinite affinity in binding but reversibility in interaction [J].Proteins,2009,77(2):404-412
[30] Cull MG,Schatz PJ.Biotinylation of proteins in vivo and in vitro using small peptide tags [J].Methods Enzymol,2000,326:430-440
[31] Bauer MS,Milles LF,Sedlak SM,et al.Monomeric streptavidin:a versatile regenerative handle for force spectroscopy [J].BioRxiv,2018.doi:https://doi.org/10.1101/276444
[32] Chu V,Freitag S,Le Trong I,et al.Thermodynamic and structural consequences of flexible loop deletion by circular permutation in the streptavidin-biotin system [J].Protein Sci,1998,7(4):848-859
[33] Lim KH,Huang H,Pralle A,et al.Engineered streptavidin monomer and dimer with improved stability and function [J].Biochemistry,2011,50(40):8682-8691
[34] Meir A,Helppolainen SH,Podoly E,et al.Crystal structure of rhizavidin:insights into the enigmatic high-affinity interaction of an innate biotin-binding protein dimer [J].J Mol Biol,2009,386(2):379-390
[35] Lim KH,Huang H,Pralle A,et al.Stable,high-affinity streptavidin monomer for protein labeling and monovalent biotin detection [J].Biotechnol Bioeng,2013,110(1):57-67
[36] Howarth M,Chinnapen DJ,Gerrow K,et al.A monovalent streptavidin with a single femtomolar biotin binding site [J].Nat Methods,2006,3(4):267-273
[37] Bruneau E,Sutter D,Hume RI,et al.Identification of nicotinic acetylcholine receptor recycling and its role in maintaining receptor density at the neuromuscular junction in vivo [J].J Neurosci,2005,25(43):9949-9959
[38] Chivers CE,Crozat E,Chu C,et al.A streptavidin variant with slower biotin dissociation and increased mechanostability [J].Nat Methods,2010,7(5):391-393
[39] Aslan FM,Yu Y,Mohr SC,et al.Engineered single-chain dimeric streptavidins with an unexpected strong preference for biotin-4-fluorescein [J].Proc Natl Acad Sci U S A,2005,102(24):8507-8512
[40] Skerra A,Schmidt TG.Use of the Strep-Tag and streptavidin for detection and purification of recombinant proteins [J].Methods Enzymol,2000,326:271-304
[41] Schmidt TG,Skerra A.The random peptide library-assisted engineering of a C-terminal affinity peptide,useful for the detection and purification of a functional Ig Fv fragment [J].Protein Eng,1993,6(1):109-122
[42] Schmidt TG,Skerra A.One-step affinity purification of bacterially produced proteins by means of the “Strep tag” and immobilized recombinant core streptavidin [J].J Chromatogr A,1994,676(2):337-345
[43] Voss S,Skerra A.Mutagenesis of a flexible loop in streptavidin leads to higher affinity for the Strep-tag II peptide and improved performance in recombinant protein purification [J].Protein Eng,1997,10(8):975-982
[44] Kornd?rfer IP,Skerra A.Improved affinity of engineered streptavidin for the Strep‐tag II peptide is due to a fixed open conformation of the lid‐like loop at the binding site [J].Protein Sci,2002,11(4):883-893
[45] Schmidt TG,Skerra A.The Strep-tag system for one-step purification and high-affinity detection or capturing of proteins [J].Nat Protoc,2007,2(6):1528-1535
[46] Schmidt TG,Batz L,Bonet L,et al.Development of the Twin-Strep-tag? and its application for purification of recombinant proteins from cell culture supernatants [J].Protein Expr Purif,2013,92(1):54-61
[47] Junttila MR,Saarinen S,Schmidt T,et al.Single-step Strep-tag purification for the isolation and identification of protein complexes from mammalian cells [J].Proteomics,2005,5(5):1199-1203
[48] Yeliseev A,Zoubak L,Schmidt TG.Application of Strep-Tactin XT for affinity purification of Twin-Strep-tagged CB2,a G protein-coupled cannabinoid receptor [J].Protein Expr Purif,2017,131:109-118
[49] Wang L,Fan M,Zeng C,et al.Expression and purification of a rapidly degraded protein,TMEM8B-a,in mammalian cell line [J].Protein Expr Purif,2018,151:38-45
[50] Baumann F,Bauer MS,Milles LF,et al.Monovalent Strep-Tactin for strong and site-specific tethering in nanospectroscopy [J].Nat Nanotechnol,2016,11(1):89-94
[51] Erlich KR,Baumann F,Pippig DA,et al.Strep-Tag II and monovalent Strep-Tactin as novel handles in single-molecule cut-and-paste [J].Small Methods,2017,1(8):1-5
[52] Demonte D,Drake EJ,Lim KH,et al.Structure-based engineering of streptavidin monomer with a reduced biotin dissociation rate [J].Proteins,2013,81(9):1621-1633
[53] Chamma I,Rossier O,Giannone G,et al.Optimized labeling of membrane proteins for applications to super-resolution imaging in confined cellular environments using monomeric streptavidin [J].Nat Protoc,2017,12(4):748-763
[54] Chamma I,Letellier M,Butler C,et al.Mapping the dynamics and nanoscale organization of synaptic adhesion proteins using monomeric streptavidin [J].Nat Commun,2016,7:10773
[55] Ikonomova SP,Le MT,Kalla N,et al.Effect of linkers on immobilization of scFvs with biotin-streptavidin interaction [J].Biotechnol Appl Biochem,2018,65(4):580-585
[56] Wu J,Chen H,Jiang P.Chromophore attachment to fusion proteins of streptavidin and recombinant allophycocyanin alpha subunit [J].Bioenqineered,2018,9(1):108-115
[57] Diamandis EP,Christopoulos TK.The biotin-(strept)avidin system:principles and applications in biotechnology [J].Clin Chem,1991,37(5):625-636
[2] Weber PC,Ohlendorf DH,Wendoloski JJ,et al.Structural origins of high-affinity biotin binding to streptavidin [J].Science,1989,243(4887):85-88
[3] 刘小兰,齐天琪,林婷,等.纳米金标记-逐步银染法快速检测金黄色葡萄球菌 [J].中国生物化学与分子生物学报(Liu XL,Qi TQ,Lin T,et al.Nanogold labeling-stepwise silver staining method for rapid detection of staphylococcus aureus[J].Chin J Biochem Mol Biol),2012,28(7):671-676
[4] 程振球,章谷生.链霉亲和素的特性及其应用 [J].现代免疫学(Cheng Zhen-Qiu,Zhang Gu-Sheng.Characteristics of streptavidin and its application[J].Curr Immunol),1992,12(1):56-59
[5] Livnah O,Bayer EA,Wilchek M,et al.Three-dimensional structures of avidin and the avidin-biotin complex [J].Proc Natl Acad Sci U S A,1993,90(11):5076-5080
[6] Hendrickson WA,Pahler A,Smith JL,et al.Crystal Structure of core streptavidin determined from multiwavelength anomalous diffraction of synchrotron radiation [J].Proc Natl Acad Sci U S A,1989,86(7):2190-2194
[7] Freitag S,Le Trong I,Chilkoti A,et al.Structural studies of binding site tryptophan mutants in the high-affinity streptavidin-biotin complex [J].J Mol Biol,1998,279(1):211-221
[8] Schmidt TG,Koepke J,Frank R,et al.Molecular interaction between the strep-tag affinity peptide and its cognate target,streptavidin [J].J Mol Biol,1996,255(5):753-766
[9] Hirschi S,Stauffer M,Harder D,et al.Engineering and assembly of protein modules into functional molecular systems [J].Chimia(Aarau),2016,70(6):398-401
[10] Torreggiani A,Fini G.Raman spectroscopic studies of ligand-protein interactions:the binding of biotin analogues by avidin [J].J Raman Spectrosc,1998,29:229-236
[11] Jeschek M,Bahls MO,Schneider V,et al.Biotin-independent strains of Escherichia coli for enhanced streptavidin production [J].Metab Eng,2017,40:33-40
[12] 彭福中.链霉亲和素的原核表达及复性[D].江西师范大学(Peng FZ.Prokaryotic expression and renaturation of streptavidin[D].Jiangxi Normal University,2010
[13] Gallizia A,de Lalla C,Nardone E,et al.Production of a soluble and functional recombinant streptavidin in Escherichia coli [J].Protein Expr Purif,1998,14(2):192-196
[14] Miksch G,Ryu S,Risse JM,et al.Factors that influence the extracellular expression of streptavidin in Escherichia coli using a bacteriocin release protein [J].Appl Microbiol Biotechnol,2008,81(2):319-326
[15] Nagarajan V,Ramaley R,Albertson H,et al.Secretion of streptavidin from Bacillus subtilis [J].Appl Environ Microbiol,1993,59(11):3894-3898
[16] Wu SC,Wong SL.Engineering of a Bacillus subtilis strain with adjustable levels of intracellular biotin for secretory production of functional streptavidin [J].Appl Environ Microbiol,2002,68(3):1102-1108
[17] Noda S,Matsumoto T,Tanaka T,et al.Secretory production of tetrameric native full-length streptavidin with thermostability using Streptomyces lividans as a host [J].Microb Cell Fact,2015,14:5
[18] Casteluber MC,Damasceno LM,da Silveira WB,et al.Cloning and expression of a functional core streptavidin in Pichia pastoris:strategies to increase yield [J].Biotechnol Prog,2012,28(6):1419-1425
[19] Wetzel D,Müller JM,Flaschel E,et al.Fed-batch production and secretion of streptavidin by Hansenula polymorpha:evaluation of genetic factors and bioprocess development [J].J Biotechnol,2016,225:3-9
[20] Sano T,Cantor CR.Expression of a cloned streptavidin gene in Escherichia coli [J].Proc Natl Acad Sci U S A,1990,87(1):142-146
[21] Thompson LD,Weber PC.Construction and expression of a synthetic streptavidin-encoding gene in Escherichia coli [J].Gene,1993,136(1-2):243-246
[22] Chua LH,Tan SC,Liew MWO.Process intensification of core streptavidin production through high-cell-density cultivation of recombinant E.coli and a temperature-based refolding method [J].J Biotechnol,2018,276-277:34-41
[23] ModanlooJouybari R,Sadeghi A,Khansarinejad B,et al.Production of recombinant streptavidin and optimization of refolding conditions for recovery of biological activity [J].Rep Biochem Mol Biol,2018,6(2):178-185
[24] Choi JH,Keum KC,Lee SY.Production of recombinant proteins by high cell density culture of Escherichia coli [J].Chem Eng Sci,2006,61(3):876-885
[25] 訾静,张月娟,万一.核心链霉亲和素在毕赤酵母中的表达及纯化 [J].中国生物制品学杂志(Zi J,Zhang YJ,Wan Y.Expression of core-streptavidin in Pichia pastoris and purification of expressed product [J].Chin J Biol),2018,31(5):485-488
[26] Sano T,Smith CL,Cantor CR.A Streptavidin mutant containing a cysteine stretch that facilitates production of a variety of specific streptavidin conjugates [J].Biotechnology(N Y),1993,11(2):201-206
[27] Wu SC,Wong SL.Engineering soluble monomeric streptavidin with reversible biotin binding capability [J].J Biol Chem,2005,280(24):23225-23231
[28] Clare DA,Valentine VW,Catignani GL,et al.Molecular design,expression,and affinity immobilization of a trypsin-streptavidin fusion protein(1) [J].Enzyme Microb Technol,2001,28(6):483-491
[29] Wu SC,Ng KK,Wong SL.Engineering monomeric streptavidin and its ligands with infinite affinity in binding but reversibility in interaction [J].Proteins,2009,77(2):404-412
[30] Cull MG,Schatz PJ.Biotinylation of proteins in vivo and in vitro using small peptide tags [J].Methods Enzymol,2000,326:430-440
[31] Bauer MS,Milles LF,Sedlak SM,et al.Monomeric streptavidin:a versatile regenerative handle for force spectroscopy [J].BioRxiv,2018.doi:https://doi.org/10.1101/276444
[32] Chu V,Freitag S,Le Trong I,et al.Thermodynamic and structural consequences of flexible loop deletion by circular permutation in the streptavidin-biotin system [J].Protein Sci,1998,7(4):848-859
[33] Lim KH,Huang H,Pralle A,et al.Engineered streptavidin monomer and dimer with improved stability and function [J].Biochemistry,2011,50(40):8682-8691
[34] Meir A,Helppolainen SH,Podoly E,et al.Crystal structure of rhizavidin:insights into the enigmatic high-affinity interaction of an innate biotin-binding protein dimer [J].J Mol Biol,2009,386(2):379-390
[35] Lim KH,Huang H,Pralle A,et al.Stable,high-affinity streptavidin monomer for protein labeling and monovalent biotin detection [J].Biotechnol Bioeng,2013,110(1):57-67
[36] Howarth M,Chinnapen DJ,Gerrow K,et al.A monovalent streptavidin with a single femtomolar biotin binding site [J].Nat Methods,2006,3(4):267-273
[37] Bruneau E,Sutter D,Hume RI,et al.Identification of nicotinic acetylcholine receptor recycling and its role in maintaining receptor density at the neuromuscular junction in vivo [J].J Neurosci,2005,25(43):9949-9959
[38] Chivers CE,Crozat E,Chu C,et al.A streptavidin variant with slower biotin dissociation and increased mechanostability [J].Nat Methods,2010,7(5):391-393
[39] Aslan FM,Yu Y,Mohr SC,et al.Engineered single-chain dimeric streptavidins with an unexpected strong preference for biotin-4-fluorescein [J].Proc Natl Acad Sci U S A,2005,102(24):8507-8512
[40] Skerra A,Schmidt TG.Use of the Strep-Tag and streptavidin for detection and purification of recombinant proteins [J].Methods Enzymol,2000,326:271-304
[41] Schmidt TG,Skerra A.The random peptide library-assisted engineering of a C-terminal affinity peptide,useful for the detection and purification of a functional Ig Fv fragment [J].Protein Eng,1993,6(1):109-122
[42] Schmidt TG,Skerra A.One-step affinity purification of bacterially produced proteins by means of the “Strep tag” and immobilized recombinant core streptavidin [J].J Chromatogr A,1994,676(2):337-345
[43] Voss S,Skerra A.Mutagenesis of a flexible loop in streptavidin leads to higher affinity for the Strep-tag II peptide and improved performance in recombinant protein purification [J].Protein Eng,1997,10(8):975-982
[44] Kornd?rfer IP,Skerra A.Improved affinity of engineered streptavidin for the Strep‐tag II peptide is due to a fixed open conformation of the lid‐like loop at the binding site [J].Protein Sci,2002,11(4):883-893
[45] Schmidt TG,Skerra A.The Strep-tag system for one-step purification and high-affinity detection or capturing of proteins [J].Nat Protoc,2007,2(6):1528-1535
[46] Schmidt TG,Batz L,Bonet L,et al.Development of the Twin-Strep-tag? and its application for purification of recombinant proteins from cell culture supernatants [J].Protein Expr Purif,2013,92(1):54-61
[47] Junttila MR,Saarinen S,Schmidt T,et al.Single-step Strep-tag purification for the isolation and identification of protein complexes from mammalian cells [J].Proteomics,2005,5(5):1199-1203
[48] Yeliseev A,Zoubak L,Schmidt TG.Application of Strep-Tactin XT for affinity purification of Twin-Strep-tagged CB2,a G protein-coupled cannabinoid receptor [J].Protein Expr Purif,2017,131:109-118
[49] Wang L,Fan M,Zeng C,et al.Expression and purification of a rapidly degraded protein,TMEM8B-a,in mammalian cell line [J].Protein Expr Purif,2018,151:38-45
[50] Baumann F,Bauer MS,Milles LF,et al.Monovalent Strep-Tactin for strong and site-specific tethering in nanospectroscopy [J].Nat Nanotechnol,2016,11(1):89-94
[51] Erlich KR,Baumann F,Pippig DA,et al.Strep-Tag II and monovalent Strep-Tactin as novel handles in single-molecule cut-and-paste [J].Small Methods,2017,1(8):1-5
[52] Demonte D,Drake EJ,Lim KH,et al.Structure-based engineering of streptavidin monomer with a reduced biotin dissociation rate [J].Proteins,2013,81(9):1621-1633
[53] Chamma I,Rossier O,Giannone G,et al.Optimized labeling of membrane proteins for applications to super-resolution imaging in confined cellular environments using monomeric streptavidin [J].Nat Protoc,2017,12(4):748-763
[54] Chamma I,Letellier M,Butler C,et al.Mapping the dynamics and nanoscale organization of synaptic adhesion proteins using monomeric streptavidin [J].Nat Commun,2016,7:10773
[55] Ikonomova SP,Le MT,Kalla N,et al.Effect of linkers on immobilization of scFvs with biotin-streptavidin interaction [J].Biotechnol Appl Biochem,2018,65(4):580-585
[56] Wu J,Chen H,Jiang P.Chromophore attachment to fusion proteins of streptavidin and recombinant allophycocyanin alpha subunit [J].Bioenqineered,2018,9(1):108-115
[57] Diamandis EP,Christopoulos TK.The biotin-(strept)avidin system:principles and applications in biotechnology [J].Clin Chem,1991,37(5):625-636