Báo cáo khoa học: Functional characterization of artemin, a ferritin homolog synthesized in Artemia embryos during encystment and diapause doc

Functional characterization of artemin, a ferritin homolog synthesized in Artemia embryos during encystment and diapause Tao Chen 1,2, *, Tania S. Villeneuve 1, *, Katy A. Garant 1 , Reinout Amons 3 and Thomas H. MacRae 1 1 Department of Biology, Dalhousie University, Halifax, NS, Canada 2 The College of Animal Science and Technology, Hunan Agricultural University, Changsha, Hunan, China 3 Molecular Cell Biology, LUMC, Leiden University, the Netherlands Embryos of the crustacean Artemia franciscana may undergo oviparous development, which involves cessa- tion of development as gastrulae, encystment, and dia- pause, the last of these characterized by extremely low metabolic activity [1–4]. The encysted embryos (cysts) are exceptionally resistant to physiologic stress [4–6], undoubtedly due in part to the regulated synthesis of small heat shock proteins (sHSPs) [7,8]. As an exam- ple, the Artemia sHSP p26, present in large amounts in cysts, functions as a molecular chaperone in vitro [9–11]. In addition, p26 confers thermotolerance on transformed bacteria and transfected mammalian cells and inhibits apoptosis [7,9–12]. Another abundant protein found in Artemia cysts is artemin, which represents 10–15% of the postribosomal protein pool [13]. Artemin was purified and sequenced by Edman degradation, revealing similarity to ferritin but with high content of histidine and cysteine ⁄ cystine [14,15]. Monomers of artemin are 26 kDa in molecular mass and, like ferritin, they form rosette-like oligomers Keywords Artemia franciscana; artemin; diapause; ferritin; molecular chaperone Correspondence T. H. MacRae, Department of Biology, Dalhousie University, Halifax, NS, B3H 4J1, Canada Fax: +1 902 494 3736 Tel: +1 902 494 6525 E-mail: tmacrae@dal.ca *These authors contributed equally to this work (Received 2 September 2006, revised 8 November 2006, accepted 19 December 2006) doi:10.1111/j.1742-4658.2007.05659.x Oviparously developing embryos of the crustacean Artemia franciscana encyst and enter diapause, exhibiting a level of stress tolerance seldom seen in metazoans. The extraordinary stress resistance of encysted Artemia embryos is thought to depend in part on the regulated synthesis of artemin, a ferritin superfamily member. The objective of this study was to better understand artemin function, and to this end the protein was synthesized in Escherichia coli and purified to apparent homogeneity. Purified artemin consisted of oligomers approximately 700 kDa in molecular mass that dis- sociated into monomers and a small number of dimers upon SDS ⁄ PAGE. Artemin inhibited heat-induced aggregation of citrate synthase in vitro,an activity characteristic of molecular chaperones and shown here to be shared by apoferritin and ferritin. This is the first report that apoferritin ⁄ ferritin may protect cells from stress other than by iron sequestration. Stably trans- fected mammalian cells synthesizing artemin were more resistant to heat and H 2 O 2 than were cells transfected with vector only, actions also shared by molecular chaperones such as the small heat shock proteins. The data indicate that artemin is a structurally modified ferritin arising either from a common ancestor gene or by duplication of the ferritin gene. Divergence, including acquisition of a C-terminal peptide extension and ferroxidase cen- ter modification, eliminated iron sequestration, but chaperone activity was retained. Therefore, because artemin accumulates abundantly during devel- opment, it has the potential to protect embryos from stress during encyst- ment and diapause without adversely affecting iron metabolism. Abbreviations sHSP, small heat shock protein. FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS 1093 of approximately 600 kDa consisting of 24 subunits. Artemin cDNA has been cloned and sequenced, confirming that the protein is a ferritin homolog [16]. Artemin oligomers appear not to bind iron [15], a finding corroborated by molecular modeling, which indicates that the internal metal-binding cavity charac- teristic of ferritin is filled by a C-terminal peptide exten- sion of 35 amino acid residues [16,17]. Only oviparously developing Artemia embryos produce artemin [18], and it is degraded in larvae along with artemin mRNA [14,16,19,20]. Artemin is extremely heat-stable and binds RNA at high temperature in vitro, suggesting a role in RNA protection [13], as was proposed for a 19S protein shown to be artemin [19,20]. However, in spite of these results, artemin function during Artemia embryo development remained uncertain. It is shown in this article that artemin prevents heat- induced denaturation of citrate synthase in vitro,a capability shared by apoferritin and ferritin, and con- fers stress tolerance on transfected mammalian cells. Artemin and ferritin may have arisen from the same ancestor gene or by duplication of the ferritin gene. Subsequent divergence yielded a protein that no longer sequesters metal but retains activities characteristic of molecular chaperones. Thus, the results suggest that artemin, which accumulates in large amounts, protects oviparously developing Artemia embryos from stress without affecting iron metabolism. Results Artemin synthesis and purification Coomassie Blue-stained gels of protein extracts from Artemia cysts contained a major artemin polypeptide that migrated at approximately 32 kDa, a molecular mass greater than that observed in earlier work [13]. This protein reacted strongly with antibody to artemin on western blots, as did a second, lighter-staining band, of approximately twice the molecular mass (Fig. 1A,B, lane 1). Protein extracts from anhydrotetra- cycline-induced Escherichia coli transformed with an artemin cDNA-containing expression vector also exhibited two antibody-reactive polypeptides of equiv- alent relative sizes, both of which were missing from bacteria transformed with vector lacking artemin cDNA (Fig. 1A,B, lanes 2 and 3). These polypeptides were larger than artemin in cyst extracts, due to the 6xHN tag, and they migrated more slowly than antici- pated on the basis of calculated molecular mass, as observed for artemin from Artemia. Purification on TALON yielded a single polypeptide as indicated by Coomassie Blue staining, although silver staining detected three polypeptides, including a light band twice the mass of the major band, and a smaller poly- peptide (Fig. 1A, lanes 4 and 5). All three polypeptides detected by silver staining reacted with antibody to artemin (Fig. 1B, lane 4), and the major band (single arrowhead) was identified as artemin by mass spectometry. Artemin oligomerization As determined by sucrose density gradient centrifuga- tion and chromatography on Sepharose 6B, bacterially produced artemin consists of oligomers approximately 700 kDa in molecular mass (24 monomers) and a les- ser amount of smaller aggregates (Fig. 2A,B). Purifica- tion had little effect on oligomer mass (Fig. 2C), indicating that the protein retained its native confor- mation and was suitable for use in chaperone assays. Electron microscopy of negatively stained samples revealed well-defined particles 14–16 nm in diameter, A B Fig. 1. Purification of artemin. Protein samples were electrophore- sed in SDS polyacrylamide gels that were stained with Coomassie Blue (lanes 1–4) and silver (lane 5) (A), or blotted onto nitrocellulose and stained with antibody to artemin (B). Lane 1, protein extract from Artemia cysts; lane 2, protein extract from E. coli transformed with vector containing artemin cDNA; lane 3, protein extract from E. coli transformed with vector only; lane 4, purified artemin; lane 5, purified artemin. Lanes 1–3 received 5 lg of protein, and lanes 4 and 5 received approximately 1 lg of protein. à, artemin in cyst extract; *, artemin in E. coli extract; single arrowhead, artemin monomer; double arrowhead, artemin doublet. Molecular mass markers · 10 )3 are on the left. Function of an Artemia ferritin homolog T. Chen et al. 1094 FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS although particles as large as 18 nm were sometimes observed, and confirmed the oligomeric status of puri- fied artemin (Fig. 2D). In contrast to apoferritin, and more so for ferritin, which contains iron (Fig. 2E,F), the absence of electron-dense central regions within negatively stained particles of artemin indicated the lack of metal storage capacity. Artemin, apoferritin and ferritin inhibit citrate synthase denaturation Artemin, apoferritin and ferritin protected citrate syn- thase against denaturation at 43 °C in a concentration- dependent manner (Fig. 3A–C). Maximal protection was obtained at a chaperone ⁄ substrate molar ratio of 2 : 1, and increasing this ratio to 4 : 1 had little effect on activity (not shown). At a molar ratio of 0.5 : 1, protection was marginal. BSA and IgG at 600 nm (molar ratio of 4 : 1) failed to prevent heat-induced denaturation of citrate synthase, indicating the absence of nonspecific protection (Fig. 3D). Artemin confers stress resistance on stably transfected mammalian cells Artemin promoted survival of stably transfected mam- malian cells upon exposure to thermal and oxidative stress (Fig. 4A,B). For example, approximately 90% of artemin-containing cells endured a 30 min heat shock, as opposed to only 17% of those lacking the protein (Fig. 4A). In comparison, 55% of cells containing arte- min survived incubation in 0.50 mm H 2 O 2 for 45 min, whereas only 15% without artemin were viable (Fig. 4B). The presence of artemin in transfected but not nontransfected cells was verified by probing of western blots (Fig. 4C,D). Occasionally, an antibody- reactive polypeptide comigrated with artemin on western blots containing cell-free extract from non- transfected cells, but when present, it stained very lightly and was considered to result from nonspecific antibody cross-reactivity. The production of aB-crys- A B C DEF Fig. 2. Oligomer formation by artemin. (A) Artemin-containing bac- terial extracts were centrifuged on sucrose gradients, and samples obtained after fractionation were electrophoresed in SDS polyacryl- amide gels, blotted onto nitrocellulose, stained with antibody to artemin (inset), and scanned. The pixel density in arbitrary units of each protein band was plotted against fraction number. The bottom of the gradient is on the left, and fractions are numbered. All lanes, with the exception of lanes 5 and 6, which received 0.5 lL, were loaded with 6 lL of sample. (B) Artemin-containing bacterial extracts were fractionated in Sepharose CL-6B, and samples were probed with antibody to artemin, as described for sucrose gradients (inset). (C) Purified artemin was centrifuged in sucrose gradients, and the A 280 of each fraction was plotted against fraction number. Molecular mass markers carbonic anhydrase (29 kDa) BSA (66 kDa), alcohol dehydrogenase (150 kDa), b-amylase (200 kDa), apoferritin (443 kDa) and thyroglobulin (669 kDa) are indicated by numbered open circles. Purified artemin (D), apoferritin (E) and ferri- tin (F) were negatively stained with uranyl acetate, and examined by electron microscopy. The bar represents 100 nm. T. Chen et al. Function of an Artemia ferritin homolog FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS 1095 tallin, HSP27, HSP60, HSP70 and HSP90 was not enhanced in transfected cells, indicating that protection against stress was not due to induction of these molecular chaperones by artemin (Fig. 5). Discussion Encysted Artemia embryos exhibit a level of stress tolerance almost unknown among other metazoans, and unusual characteristics such as the capacity to sur- vive extended anoxia [4–6] and repeated dehydra- tion ⁄ rehydration [21] require exceptional biochemical adaptation. Resistance to stress is provided by the cyst wall, which is impervious to most substances and offers structural support [22,23]. Another protective mechanism entails drastic reduction of metabolic activ- ity, thus limiting resource utilization and macromolec- ular degradation to a rate sustainable for years [3,4]. Trehalose and molecular chaperones, including the sHSPs, are produced in excess and are thought to shield encysted Artemia embryos [2,3,7,8]. At least three sHSPs occur in oviparous Artemia embryos (unpublished data), of which p26 has been described [2,9,11,12]. Another plentiful cyst protein is artemin, an oligomeric, heat-stable ferritin homolog that binds mRNA at high temperature in vitro, but that has an uncertain role in vivo [13,15,16]. A 6xHN-tagged derivative of artemin was synthes- ized in E. coli and purified by affinity chromatography, a relatively mild procedure. Purified artemin occurred mainly as large oligomers similar to those in protein extracts from transformed bacteria and Artemia cysts [15], indicating minimal disruption of structure during chromatography. Artemin dimers were observed upon electrophoresis, perhaps reflecting the presence of sta- ble disulfide bonds formed during oligomerization by the many cysteines that characterize the protein [16]. Artemin effectively prevented heat-induced denatura- tion of citrate synthase at artemin ⁄ substrate molar A B D C Fig. 4. Artemin confers stress tolerance upon mammalian cells. (A) Mammalian cells transfected with the artemin cDNA-containing expression vector (nonshaded bars) or with vector only (shaded bars) were heated at 46 °C for the indicated times, and viability was determined by crystal violet staining. The data represent the mean ± standard error of three independent experiments. Inset, stained flasks of cells transfected with vectors containing (left) and lacking (right) artemin cDNA that were heated for 30 min. (B) Mam- malian cells transfected with the artemin cDNA-containing expres- sion vector (nonshaded bars) or with vector only (shaded bars) were exposed to either 0.25 m M (1, 2) or 0.50 mM (3, 4) H 2 O 2 (exposures follow the same order at each time), and then proc- essed as described. Inset, crystal violet-stained flasks of cells trans- fected with vectors containing (left) and lacking (right) artemin cDNA that were exposed to 0.50 m M H 2 O 2 for 30 min. Protein samples obtained from transfected cells prior to stressing were electrophoresed in SDS polyacrylamide gels and either stained with Coomassie Blue (C) or blotted onto nitrocellulose and probed with antibody to artemin (D). 1, cells transfected with empty vector; 2, cells transfected with the artemin cDNA-containing vector. Fig. 3. Artemin, apoferritin and ferritin prevent heat-induced dena- turation of citrate synthase. (A) Purified artemin was incubated at 43 °C with 150 n M citrate synthase, and solution turbidity was measured at 360 nm. Artemin concentrations were: 1, 0.0 n M; 2, 75 n M; 3, 150 nM; 4, 225 nM; 5, 300 nM. Experiments were done in duplicate, and the standard error ranged from 0 to 0.005. The assays were repeated in duplicate for apoferritin (B) and ferritin (C) at the concentrations used for artemin, with standard error ranging, respectively, from 0 to 0.004 and from 0 to 0.003. (D) Citrate syn- thase at 150 n M was incubated at 43 °C in the absence of other proteins (1) and in the presence of either BSA (2) or IgG (3) at 600 n M. Function of an Artemia ferritin homolog T. Chen et al. 1096 FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS ratios of 2 : 1, and similar results were obtained for apoferritin and ferritin. Moreover, like the Artemia sHSP p26 [9,11], artemin functions in the absence of ATP, presumably a benefit during diapause, when metabolism is reduced. Transfected mammalian cells synthesizing artemin were substantially more tolerant of heat and oxidative stress than cells transfected with vector only. However, artemin accumulation was relat- ively low as judged by staining intensity on western blots and in comparison to the protein in cyst extracts, suggesting that it protected transfected cells in a way other than by indiscriminate binding to denaturing proteins. In this vein, Artemia p26 inhibits apoptosis in transfected mammalian cells [12]. On the basis of these observations, artemin has the potential to protect Artemia embryos during physiologic insult, either by influencing discrete cellular processes, as a molecular chaperone with a broad substrate range, or by a com- bination of these capabilities. Moreover, the results expand the potential protective role of apoferritin ⁄ ferritin in stressed cells, which was previously restricted to defense against oxidative damage through modula- tion of iron availability [24–28], although there was a previous report that ferritin mRNA translation is enhanced by heat shock [29]. Artemia organisms contain H-ferritin, of which arte- min is a homolog (Fig. 6A,B). The Artemia genes for artemin and ferritin may have originated from a com- mon ancestor, or one gene may have arisen by duplica- tion of the other. Of these options, the ubiquitous phylogenetic distribution of ferritin, in contrast to the apparent restricted dispersal of artemin, implies that a ferritin gene duplicate diverged to yield artemin. Sub- sequent mutation of the ferritin gene duplicate elimin- ated the ability of its product to oxidize and sequester iron, but chaperone activity was maintained, ultimately generating artemin, a novel developmentally regulated protein with a potentially important role in Artemia embryogenesis, stress tolerance and diapause. In the proposed evolutionary scheme, artemin and ferritin retained significant sequence identity and sim- Fig. 6. Sequence comparison of artemin and ferritin. (A) CLUSTALW was used to compare ferritin sequences. ON, Oncometopia nigri- cans, accession number AAU95196; HC, Homalodisca coagulata, AAT01076; AF, Artemia franciscana, AAL55398; ON2, Oncorhyn- chus nerka, AAK08117; SS, Salmo salar, AAB34575; TN, Tetraodon nigroviridis, CAF92096; RC, Rana catesbeiana, AAA49525; XL, Xen- opus laevis, AAB20316; AJ, Apostichopus japonicus, AAY89589. (B) CLUSTALW was used to compare Artemia ferritin and artemin. Boxed residues, ferritin di-iron ferroxidase center; boxed and sha- ded residues, pore gate paired residues; shaded cysteines may function in oligomer formation and protein stabilization; the artemin C-terminal extension is shaded. *, identical residues; :, conserved substitution; ., semiconserved substitution; –, no residue. Fig. 5. Artemin does not induce synthesis of stress proteins in transfected cells. Protein samples from 293H cells prior to stress- ing were electrophoresed in SDS polyacrylamide gels, blotted onto nitrocellulose, and stained with antibody to aB-crystallin (aB-cry), HSP27, HSP60, HSP70 and HSP90. 1, 20 lg of nontransfected 293H cell lysate; 2, 20 lg of lysate from 293H cells transfected with empty vector; 3, 20 lg of lysate from 293H cells transfected with vector containing artemin cDNA; 4, 0.1 lg of purified a-crystal- lin (top panel) and 20 lg of lysate from heat-shocked HeLa cells (lower four panels). T. Chen et al. Function of an Artemia ferritin homolog FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS 1097 ilar oligomer size, but differences accrued. A major change was the acquisition by artemin of a C-terminal extension thought to fold inward and fill the cavity enclosed by the oligomer shell. The equivalent space in ferritin sequesters iron [17,30–33], and each H-ferritin subunit possesses a dinuclear ferroxidase center com- posed of six conserved residues arranged as catalytic sites A and B, which, by using Fe 2+ and oxygen as substrates, produce a hydrous ferric oxide mineral within the protein cavity. Artemin lacks all but one of the residues constituting the di-iron ferroxidase center, although these residues are conserved in Artemia ferr- itin (Fig. 6A,B), and as a consequence artemin does not catalyze iron mineralization. Because metal bind- ing is prevented, artemin is unlikely to disrupt iron homeostasis during embryo encystment and early dia- pause, when cysts are metabolically active [34], and the concentration of the protein is high. Embryos would also be compromised if intracellular iron were unavail- able when diapause was broken, because inorganic constituents of metabolic pathways would not be able to traverse the cyst shell. Despite the modifications to the ferroxidase center residues, artemin retains three of four pore gate paired residues, with the fourth residue being a conserved substitution (Fig. 6A,B). The chaperone activity of artemin relative to apo- ferritin ⁄ ferritin is essentially unaffected by the struc- tural differences between the proteins, but artemin is available in large quantities during development. The advantage to oviparously developing embryos of arte- min accumulation is that chaperoning capacity is greatly increased, thus improving stress endurance. Accumulation may occur because the artemin gene is under control of a regulatory element that promotes transcription during oviparous development. Other possibilities include enhanced mRNA stability and ⁄ or high translation rate, low susceptibility to proteolytic degradation as a result of disulfide bridges arising from increased cysteine content, or a combination of these factors. It is of interest, in the context of artemin sta- bility, that disulfide bond formation in Hsp16.3, an sHSP from Mycobacterium tuberculosis, disrupts chap- erone activity, and this was offered as an explanation for the low number of cysteines in molecular chaper- ones [35]. This idea does not hold for artemin, and may reflect mechanistic differences between artemin and the sHSPs. To summarize, apoferritin and ferritin possess the ability to inhibit heat-induced protein denaturation, an activity characteristic of molecular chaperones and that suggests for the first time that their ability to protect cells subject to stress extends beyond iron sequestra- tion and prevention of oxidative damage. Moreover, the data support the proposal that artemin arose from ferritin by gene duplication; subsequent divergence eliminated a role in iron homeostasis, but left the chaperone activity intrinsic to apoferritin ⁄ ferritin unchanged. Accumulation of artemin in large amounts by one or more undetermined mechanisms, but per- haps dependent on the stabilization of artemin by disulfide bonds, has the potential to increase stress resistance in oviparously developing Artemia embryos. Thus, artemin has the characteristics of a novel molecular chaperone, and it will be interesting to determine how it functions in vivo, because it may have both RNA and protein substrates. Experimental procedures Construction of expression plasmids Artemin cDNA (accession number AY062896) was ampli- fied by PCR, employing primers 5¢-GTGGTCGACATGGC AACAGAAGGTGCAAG-3¢ and 5¢-GGGATCCAACTTG GACGGGCAACTC-3¢, respectively, containing Sal1 and BamH1 restriction sites, inserted into the TA cloning vector pCR2.1 (Invitrogen, San Diego, CA, USA), and used to transform E. coli DH5a. The artemin cDNA insert was excised from the TA vector with SalI and Bam HI, and fol- lowing electrophoresis in agarose, was recovered with the GFX PCR and Gel band purification kit (Amersham Bio- sciences, Piscataway, NJ, USA). Artemin cDNA was then cloned into the 6xHN-tagged prokaryotic expression vector pPROTet.E133 (BD Biosciences Clontech, Mississauga, ON, Canada) and transformed into E. coli strain BL21PRO (BD Biosciences Clontech). For expression in mammalian cells, artemin cDNA was amplified by PCR with primers 5¢-GATCCTCGAGTTAACTATAGAAGACACGGG-3¢ and 5¢-AGCTCCTAGGGCAACAGAAGGTGCAAG-3¢, inserted into pCR2.1, and used to transform E. coli DH5a. The insert was recovered from pCR2.1 with BamHI and XbaI, and cloned into the eukaryotic expression vector pcDNA3.1(+) (Invitrogen). cDNA was sequenced at the DNA Sequencing Facility, Centre for Applied Genomics, Hospital for Sick Children, Toronto, ON, Canada. Artemin purification Transformed E. coli cells were grown at 37 °C with shaking in Difco Luria Broth Base, Miller (LB) (Becton, Dickinson and Co., Sparks, MD, USA) containing spectinomycin (Sig- ma, Oakville, ON, Canada) at 50 lgÆmL )1 and chloram- phenicol (Sigma) at 34 l gÆmL )1 . Anhydrotetracycline (BD Biosciences Clontech) was added to 400 ngÆmL )1 when cul- ture A 600 reached approximately 0.5, and incubation was continued for 8 h at 37 °C, after which protein extracts were prepared [10]. Induction of bacteria under reduced Function of an Artemia ferritin homolog T. Chen et al. 1098 FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS aeration improved artemin yield over that obtained in well-aerated flasks. Artemin was purified on BD TALON Metal Affinity Resin (BD Biosciences Clontech), following the manufacturer’s instructions and using an equilibration ⁄ washing buffer of 50 mm Na 2 HPO 4 , 500 mm NaCl, and 10 mm imidazole (pH 7.5). Artemin purification was evalu- ated by electrophoresis in SDS polyacrylamide gels fol- lowed by either Coomassie Blue or silver staining [36]. Artemin purification was also monitored by blotting pro- teins resolved in SDS polyacrylamide gels onto nitrocellu- lose membranes and probing with antibody to artemin followed by horseradish peroxidase-conjugated goat anti- rabbit IgG (Jackson ImmunoResearch, Mississauga, ON, Canada) [12]. The primary antibody to purified bacterially produced artemin was prepared in rabbit using TitreMax Gold Adjuvant (Sigma). Rabbits were obtained from Charles River Canada (St Constant, Quebec, Canada) and cared for in accordance with guidelines in ‘Guide to the Care and Use of Experimental Animals’ available from the Canadian Council on Animal Care. The identity of anti- body-reactive purified proteins as artemin was confirmed by mass spectometry [36]. Artemin oligomerization Bacterial extracts containing artemin were applied to con- tinuous 10 mL, 10–40% sucrose gradients in 0.1 m Tris ⁄ glycine buffer (pH 7.4), and centrifuged at 200 000 g for 15 h at 4 °C in a Beckman SW41 Ti rotor. Fractions of 0.74 mL were collected, and samples were electrophore- sed in 12.5% SDS polyacrylamide gels, blotted onto nitro- cellulose membranes, and probed with antibody to artemin. Bacterial extracts containing artemin were also chromatographed at 10 mLÆh )1 in Sepharose CL-6B (S igma) columns (1.0 cm diameter · 50 cm length) equilibrated with 0.1 m Tris ⁄ glycine (pH 7.4). One-milliliter fractions were collected and probed with antibody to artemin after SDS ⁄ PAGE and blotting onto nitrocellulose membranes. Artemin band densities on blots were measured at 400 dots per inch with an UMAX Astra 1200S scanner (Dallas, TX, USA) and plotted against fraction numbers [12]. Purified artemin was centrifuged on sucrose gradients as described above, with the protein being detected by reading the A 280 of fractions. The monomer molecular mass of bacterially produced artemin, including the 6xHN tag, was determined by generunner (version 3.05) (Hast- ings Software Inc., Hastings on Hudson, NY, USA) to be 28.8 kDa, and this value was used to calculate oligomer subunit number. Molecular mass markers (Sigma) of 14.2 kDa (a-lactalbumin), 29 kDa (carbonic anhydrase), 66 kDa (BSA), 150 kDa (alcohol dehydrogenase), 200 kDa (b-amylase), 443 kDa (apoferritin) and 669 kDa (thyro- globulin) were centrifuged separately in gradients or chro- matographed in Sepharose CL-6B columns, and the A 280 values of fractions were determined. Artemin purified on TALON Metal Affinity Resin, horse spleen apoferritin (Sigma) and horse spleen ferritin (Sigma) was applied to formvar-coated copper grids, and samples were stained with 1% uranyl acetate in H 2 O. The grids were examined with a Philips Tecnai transmission electron microscope, and images were captured using analysis Ò , version 2.1 (Soft Imaging System Corp., Lakewood, CO, USA). Inhibition of citrate synthase denaturation by artemin, apoferritin and ferritin Citrate synthase (Sigma) at 150 nm in 40 mm Hepes ⁄ KOH buffer (pH 7.5) was heated at 43 °C as done previously to determine sHSP chaperone activity [9–11], although in this case the assay was performed in the absence and presence of artemin, apoferritin and ferritin. Molarity was based on monomer molecular mass for all proteins. The purified arte- min was centrifuged (1500 g at room temperature for 2 min, using Spectrofuge 16M centrifuged with an 18 place rotor) through Pierce Protein Desalting Spin columns equil- ibrated with 40 mm Hepes ⁄ KOH buffer (pH 7.5) to remove imidazole. Solution turbidity was monitored at 360 nm with a Perkin Elmer (Montreal, QB, Canada) Lambda 3B UV ⁄ VIS spectrophotometer. Stress resistance of mammalian cells containing artemin Mammalian 293H kidney cells were transfected in the pres- ence of Lipofectamine 2000 (Invitrogen) [12] with the expression vector pcDNA3.1(+) (Invitrogen) either con- taining or lacking artemin cDNA, and stable transfectants were selected in Geneticin (Invitrogen). Artemin synthesis was detected by probing western blots containing protein extracts of transfected cell lines resolved in SDS polyacryla- mide gels. To prepare protein extracts, cells grown to con- fluence in 100 mm tissue culture dishes were rinsed with NaCl ⁄ P i (140 mm NaCl, 2.7 mm KCl, 8.0 mm Na 2 HPO 4 , 1.5 mm KH 2 PO 4 , pH 7.4), recovered by scraping in 100 lL of whole cell extraction buffer (25 mm Na 2 HPO 4 , 400 mm NaCl, 0.5% SDS, 0.04 mgÆmL )1 each of soybean trypsin inhibitor, leupeptin and pepstatin A, 0.08 mgÆmL )1 phenyl- methylsulfonyl fluoride, pH 7.2), transferred to an Eppen- dorf tube, and placed in a boiling H 2 O bath for 10 min. The homogenate was cooled, and centrifuged for 10 min in a microcentrifuge; the supernatant was either used immedi- ately or frozen at ) 20 °C. Protein concentrations were determined with the Bio-Rad (Mississauga, ON, Canada) protein assay. To test stress resistance, transfected 293H cells were seeded at 5 · 10 5 cellsÆmL )1 in 30 mm dishes and incubated at 37 °C for 24 h under 5% CO 2 in DMEM (Invitrogen) containing 10% fetal bovine serum (Invitrogen) and 1% T. Chen et al. Function of an Artemia ferritin homolog FEBS Journal 274 (2007) 1093–1101 ª 2007 The Authors Journal compilation ª 2007 FEBS 1099 antibiotic–antimycotic (Invitrogen). The dishes were sealed with Parafilm, heated at 46 ° C for up to 1 h, and then incubated at 37 °C for 24 h, with cell viability being deter- mined by use of crystal violet (Sigma), except that cells were incubated for 1 week before staining [12]. The results were plotted as the average, with standard error, of three experiments. Stably transfected cells prepared as described above were also exposed to either 0.25 or 0.50 mm H 2 O 2 (Sigma) for up to 1 h, and then incubated at 37 °C for 24 h before determination of viability. Artemin production was confirmed by electrophoresis of transfected cell protein extract in SDS polyacrylamide gels, blotting onto nitrocel- lulose, and reacting with antibody. Protein extracts from transfected mammalian cells were probed with antibodies to aB-crystallin, HSP27, HSP60, HSP70 and HSP90 (Stressgen, Victoria, BC, Canada) to deter- mine whether artemin induced their synthesis. aB-crystallin (Stressgen) and HeLa cell lysates (Stressgen) were used to confirm antibody activity. Acknowledgements This work was supported by the Heart and Stroke Foundation of Nova Scotia, the Natural Sciences and Engineering Research Council of Canada, the Nova Scotia Health Research Foundation, and the Canadian Institutes of Health Research. 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