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Département de Biochimie, Université de Sherbrooke, 3001 12e Avenue Nord, Sherbrooke, Québec J1H 5N4, Canada
Correspondence
Simon Labbé
Simon.Labbe{at}USherbrooke.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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null cells exhibited no CAO activity, while ccc2 mutant cells exhibited decreased levels of amine oxidase activity, and mutations in cox17 and ccs1 did not cause any defects in this activity. Copper-deprived S. cerevisiae cells expressing spao1+ required a functional atx1+ gene for growth on minimal medium containing ethylamine as the sole nitrogen source. Under these conditions, the inability of the atx1 cells to utilize ethylamine correlated with the lack of SPAO1 activity, in spite of the efficient expression of the protein. Cells carrying a disrupted ccc2 allele exhibited only weak growth on ethylamine medium containing a copper chelator. The results of these studies reveal that expression of the heterologous spao1+ gene in S. cerevisiae is required for its growth in medium containing ethylamine as the sole nitrogen source, and that expression of an active Schiz. pombe SPAO1 protein in S. cerevisiae depends on the acquisition of copper through the high-affinity copper transporters Ctr1 and Ctr3, and the copper chaperone Atx1.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). This redox-active metal is a key cofactor for several enzymes, including cytochrome c oxidase, copper, zinc-superoxide dismutase, dopamine-
-hydroxylase and lysyl oxidase. These cupro-enzymes are involved in diverse biological processes such as respiration, free-radical defence, catecholamine formation and maturation of connective tissue (reviewed by Peña et al., 1999). Another important copper-dependent enzyme, copper amine oxidase (CAO), catalyses the oxidation of various amine substrates to their corresponding aldehydes, with the subsequent release of NH3 and H2O2 (reviewed by Brazeau et al., 2004). In prokaryotes and fungi, CAOs allow the organisms to utilize amine substrates as sources of carbon and nitrogen (Samuels & Klinman, 2005). In higher eukaryotes, their biological functions are less well defined, with suggested roles in detoxifying xenobiotic amines, wound healing, metabolism of glucose, cell–cell recognition and cell growth (reviewed by Yu et al., 2003). CAOs are homodimers of 
70–95 kDa monomers. Each monomer contains a copper and an organic cofactor, 2,4,5-trihydroxyphenylalanine quinone (TPQ), which is post-translationally derived from a tyrosine residue that is highly conserved within the protein. The formation of TPQ has been shown to be a self-processing event requiring both copper and oxygen (reviewed by Dawkes & Phillips, 2001). Copper is coordinated by three His residues, conserved in all CAO primary sequences, from bacteria to eukaryotes (Kumar et al., 1996; Li et al., 1998; Matsunami et al., 2004). Currently, the mechanism for copper acquisition by CAOs is unclear.
Copper is also a potentially toxic metal due to its proclivity to engage in Fenton-like reactions that generate highly destructive hydroxyl radicals, thus, most organisms have developed mechanisms to assimilate copper in a highly controlled manner (reviewed by Rees & Thiele, 2004). In the yeast Saccharomyces cerevisiae, high-affinity copper uptake and distribution within cells have been characterized at the molecular level. Copper is first reduced from Cu2+ to Cu+ by the cell-surface reductases Fre1 and Fre2 (Dancis et al., 1990; Georgatsou & Alexandraki, 1994; Hassett & Kosman, 1995; Martins et al., 1998). Following reduction, copper is transported across the plasma membrane by two distinct high-affinity copper transporters, Ctr1 and Ctr3 (Dancis et al., 1994a; Knight et al., 1996; Peña et al., 2000; Puig et al., 2002b). Both proteins function independently in high-affinity copper uptake (Peña et al., 2000). Within the cell, copper is specifically delivered to the late secretory compartment, mitochondria and cytosolic copper, zinc-superoxide dismutase by the copper chaperones Atx1, Cox17, and Ccs1, respectively (reviewed by O'Halloran & Culotta, 2000). Within the secretory pathway, Ccc2 accepts copper from Atx1, and incorporates copper into cupro-proteins, such as the multicopper ferrioxidase Fet3, as they mature in the pathway (Lin et al., 1997; Pufahl et al., 1997). In the mitochondria, copper delivered by Cox17 is incorporated into cytochrome c oxidase (possibly in cooperation with Sco1 and Cox11) (Abajian et al., 2004; Carr & Winge, 2003; Glerum et al., 1996; Horng et al., 2004). Ccs1 delivers copper to copper, zinc-superoxide dismutase in the cytosol (Culotta et al., 1997). Consistent with their function in discrete pathways in intracellular copper distribution, mutations in any one of the copper chaperone genes give rise only to specific defects in their respective pathways, while overproduction of one copper chaperone cannot complement the loss of function in another (Lin et al., 1997). In contrast, mutations in Ctr1 and Ctr3, which act upstream of the copper chaperones, result in pleiotropic phenotypes due to copper deficiency in all compartments (Dancis et al., 1994a, b; Knight et al., 1996).
Curiously, S. cerevisiae is one of the few yeast species that does not have an endogenous CAO (Cai & Klinman, 1994b; Large, 1986). However, heterologous expression of a CAO from another organism in S. cerevisiae produces a functional enzyme (Cai & Klinman, 1994a). Thus, it can serve as an excellent host for the expression and characterization of genes encoding CAOs from other organisms. Furthermore, as elegantly shown by Klinman and colleagues when they studied heterologous expression of the CAO from Hansenula polymorpha (HPAO) in S. cerevisiae, this latter organism must have a ‘molecular pathway’ to activate HPAO, since these authors have been unable to perform in vitro reconstitution studies by addition of copper to the apo-form of HPAO (Cai et al., 1997). In the fission yeast Schizosaccharomyces pombe, two candidate CAO molecules, SPAC2E1P3.04 and SPBC1289.16c, have been annotated from the Schiz. pombe Genome Project. We designated these proteins SPAO1 and SPAO2. Their function and regulation in Schiz. pombe are currently unknown. To begin to characterize these proteins, we expressed the Schiz. pombe CAOs in S. cerevisiae, and determined the requirement of the known copper transporters and chaperones for their activity. Interestingly, SPAO1, but not SPAO2, is capable of catalysing ethylamine oxidation. SPAO1 is localized in the cytosol. Using mutant S. cerevisiae strains, we determined that a functionally active SPAO1 was dependent on the copper transporters Ctr1 and Ctr3 when cells were grown under conditions of copper starvation, Atx1 for delivery of copper within the cell, and Ccc2, whose deletion resulted in partial loss of SPAO1 activity. In contrast, deletion of cox17 and ccs1 had no effect on SPAO1 activity. Taken together, these results define components of the copper homeostatic pathway that are required for the physiological activation of CAOs when heterologously expressed in S. cerevisiae.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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1 leu20 met150 ura30) (Brachmann et al., 1998), and the atx1 (isogenic to BY4741 plus atx1 : : KANr), ccc2 (isogenic to BY4741 plus ccc2 : : KANr), cox17 (isogenic to BY4741 plus cox17 : : KANr) and ccs1 (isogenic to BY4741 plus ccs1 : : KANr) disruption strains. S. cerevisiae isogenic strains BR10 (MATa gal1 trp1-1 his3 ade8 CUPr) (Rymond et al., 1983), SLY12 (MATa gal1 trp1-1 his3 ade8 CUPr ura3 : : KANr leu2 : : HISG mac1 : : LEU2) and MPY20 (MATa gal1 trp1-1 his3 ade8 CUPr ctr1 : : TRP1 ctr3 : : LEU2 leu2 : : KANr) were also used in this work. All strains were cultured in yeast extract/bacto peptone/dextrose (YPD) medium or synthetic complete (SC) medium lacking the appropriate amino acid for plasmid selection (Sherman, 1991). For detection of peroxisomes, cells were grown in synthetic minimal (SD) medium containing 50 mg l–1 methionine, histidine and leucine; 0.2 % oleic acid; and 0.2 % Tween 80 (adjusted to pH 7.0 with NaOH) (Gurvitz et al., 1997; Veenhuis et al., 1987). Copper starvation was carried out by adding the indicated concentration of ammonium tetrathiomolybdate (TTM) (323446; Aldrich) or bathocuproine disulfonic acid (BCS) (14662-5; Aldrich) to cells grown to mid-exponential phase (OD600 1.0). Similarly, copper repletion was performed by the addition of 10 or 100 µM CuSO4 to cells grown at OD600 1.0. After treatments at 30 °C for 8 h, 20 ml samples were withdrawn from the cultures for subsequent detection of CAO activity, steady-state mRNA or protein analysis.
RNA analysis and plasmids.
The spao1+ gene was isolated by PCR using primers that corresponded to the initiator and stop codons of the ORF from Schiz. pombe strain FY254 (Forsburg et al., 1997) genomic DNA. Because the primers contained EcoRI and SalI restriction sites, the purified DNA fragment was digested with these restriction enzymes and cloned into the corresponding sites of p416ADH or p416GPD (Mumberg et al., 1995). Using an identical approach, we cloned spao2+ in both p416ADH and p416GPD. The integrity of the DNA sequence of the EcoRI–SalI fragment from each respective PCR-amplified fragment was verified by dideoxy sequencing. For RNase protection assays, two plasmids were created to generate antisense RNA probes. pSKspao1+ was constructed by inserting a 170 bp NotI–EcoRI fragment from the spao1+ gene into the same sites of pBluescript SK (Stratagene). The antisense RNA hybridizes to the first 170 ribonucleotides of the spao1+ transcript. To construct pSKspao2+, a 162 bp fragment of the spao2+ gene was isolated by PCR and cloned into the BamHI and EcoRI sites of pBluescript SK. This fragment hybridizes to the region between +2083 and +2245 downstream from the initiator codon of spao2+. The riboprobe derived from pKSACT1 (Labbé et al., 1997) was used to probe ACT1 mRNA as an internal control. Analysis of gene expression by the RNase protection protocol was carried out as described previously (Beaudoin & Labbé, 2001). The GFP coding sequence derived from pSP1pccs+I-II-III-IV-GFP (Laliberté et al., 2004) was isolated by PCR using primers designed to generate SpeI and BamHI sites at the 5' and 3' termini of the GFP gene, respectively. The resulting DNA fragment was used to clone the GFP gene into p416ADH or p416GPD vectors at compatible SpeI and BamHI sites. To create green fluorescent protein (GFP) that harbours a C-terminal tripeptide SKL, the primers GFPSKLEND (5'-AAGGAAAAAAGCGGCCGCGGATCCTTATAATTTAGATAGTTCATCCATGCCATGTG-3') and GFPSTART (5'-GGACTAGTATGGGCCGCAGTAAAGGAGAAGAACTTTTC-3') were made, corresponding to the beginning and the end of the GFP gene with three extra amino acid residues (underlined) after the leucine at position 238 (Leu238-Ser-Lys-Leu-Stop) of GFP. The PCR product obtained was digested with SpeI and BamHI and cloned into the corresponding sites of p416ADH or p416GPD. To generate the spao1+–StuI–BspEI allele, a 12 bp StuI–BspEI linker was inserted in-frame and downstream of the last codon of the spao1+ gene by the overlap extension method (Ho et al., 1989). This allele was found to be functional, based on its ability to encode a protein that catalysed the oxidative deamination of ethylamine. We used the restriction sites that StuI and BspEI created within spao1+ to insert a copy of the GFP gene. The plasmid, named p416ADHspao1+-GFP, was used to determine the localization of SPAO1–GFP fusion protein in S. cerevisiae by fluorescence microscopy. Fluorescence and differential interference contrast images of the cells were obtained on an Eclipse E800 epifluorescent microscope (Nikon) equipped with an ORCA ER digital cooled camera (Hamamatsu) as described previously by Beaudoin et al. (2006).
CAO assay.
To determine the presence of CAO activity (Bruun & Houen, 1996), spheroplasts were obtained from transformed cells and lysed as described by Harding et al. (1995). Cell lysates were quantified using the Bradford assay, and equal amounts of cellular protein were subjected to electrophoresis on a 1 % Tris/Tricine calcium lactate agarose gel. A Tris/Tricine calcium lactate stock solution [80 mM Tris base, 24 mM Tricine (T-7911; Sigma), 2 mM calcium lactate, pH 8.5] was utilized to make a 1 % agarose solution. Gels were pre-equilibrated at 4 °C in cold Tris/Tricine (pH 8.5) calcium lactate buffer prior to electrophoresis, which was carried out at 75 V for 1 h. After electrophoresis, gels were blotted on to nitrocellulose membranes (Hybond-ECL; Amersham Biosciences). Transfer of proteins was performed by gravitational pressure for 90 min at 4 °C. The membrane was removed from the gel and placed in 5 mM phosphate buffer, pH 7.2, for 5 min at 4 °C, briefly pressed between two filter papers, and then layered on top of a filter paper that was prewetted with the chemiluminescence detection solution [10 mM ethylamine, 5 mM phosphate buffer, pH 7.2, 2 ml luminol reagent (ECL chemiluminescent detection reagent 2, Amersham Biosciences), and 10 µg ml–1 horseradish peroxidase C]. The assembly was placed in a plastic sheet protector, and exposed to film (ECL hyperfilm, Amersham Biosciences) for 1 min to 1 h. Oxidation of the ethylamine to its corresponding aldehyde by an active CAO on the nitrocellulose membrane releases NH3 and H2O2. The horseradish peroxidase and luminol present in the detection solution cause dismutation of H2O2 into water and oxygen, and subsequent light emission from the oxidation of luminol.
Immunoblotting.
For Western blotting experiments, the protein extracts were resolved by SDS-PAGE, transferred to PVDF Hybond-P membranes (Amersham Biosciences), and the blots were analysed for steady-state levels of GFP and phosphoglycerol kinase (PGK) proteins using antisera B-2 (Santa Cruz Biotechnology) and 22C5-D8 (Molecular Probes), respectively. After a 2 h incubation, the membranes were washed with TBS (10 mM Tris/HCl, pH 7.4, 150 mM NaCl, 1 % bovine serum albumin), incubated with the appropriate horseradish peroxidase-conjugated secondary antibodies (Amersham Biosciences) and visualized by chemiluminescence.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). The SPAC2E1P3.04-encoded protein, denoted SPAO1, displays 48.6 % amino acid sequence identity with an SPBC1289.16c-encoded protein, denoted SPAO2. Both SPAO1 and SPAO2 have predicted functional motifs that are highly similar to other microbial and metazoan CAOs. In SPAO1, a conserved Asn406–Tyr407–Glu408–Tyr409 sequence is present in which Tyr407 may serve as the precursor to TPQ (Fig. 1). The peptidyl Tyr394 residue of SPAO2 may play a similar role. Within the C-terminal halves of the two putative Schiz. pombe CAOs, three His residues (His458, His460 and His627 for SPAO1; His445, His447 and His604 for SPAO2) may act as a potential copper ligand. In addition to the above-mentioned residues, SPAO1 contains within its N-terminal region two amino acids (Tyr307 and Asp321) that may promote the active conformation of TPQ during the enzymic reaction. To ascertain the potential role of spao1+ and spao2+ in nitrogen and carbon metabolism, we expressed these genes in S. cerevisiae, an organism that does not possess endogenous CAOs (Large, 1986), thereby providing an excellent background for their characterization. The spao1+ and spao2+ genes were placed under the control of the ADH or GPD gene promoter. We tested the ability of SPAO1 or SPAO2 expressed in S. cerevisiae cells to produce amine oxidase activity, using a peroxidase-catalysed chemiluminescent assay to detect the production of H2O2 (Bruun & Houen, 1996). Expression of SPAO1 under the control of the GPD promoter on a centromeric plasmid produced a stronger chemiluminescent signal than that observed in cells expressing spao1+ under the control of the ADH promoter (Fig. 2A). The CAO activity levels were consistent with the relative strength of the promoters, ADH being weaker than GPD (Mumberg et al., 1995). Surprisingly, expression of ADH–spao2+ or GPD–spao2+ on a centromeric plasmid failed to produce detectable CAO activity. Expression of SPAO2 at higher levels using a multicopy plasmid still failed to generate CAO activity (J. Laliberté and S. Labbé, unpublished data). We verified the expression of spao1+ and spao2+ genes in the host cells by the RNase protection assay (Fig. 2B). These analyses clearly showed that readily detectable levels of both spao1+ and spao2+ mRNA were present in the host cells, indicating that the lack of CAO activity by SPAO2 was not due to lack of expression. Taken together, these data reveal that the orthologous Schiz. pombe SPAO1 protein, but not SPAO2, can produce an active enzyme when ectopically expressed in S. cerevisiae.
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show that the SPAO1–GFP fusion protein was fully functional, demonstrating a level of CAO activity consistent with the strength of the promoter, and comparable to the untagged SPAO1.
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). These were created by decreasing the concentration of (NH4)2SO4 (0.5 %, w/v) in SD medium by two orders of magnitude (0.005 %, w/v). Furthermore, when cells are grown in the presence of oleic acid, the size and abundance of peroxisomes increase, allowing visualization of peroxisomal proteins by microscopy (Veenhuis et al., 1987). Under these conditions, a GFP harbouring the peroxisome targeting signal, the tripeptide SKL at its C terminus, is efficiently targeted into the peroxisomal lumen and exhibits the characteristic punctate peroxisomal staining pattern (Fig. 4A, lower third panel). Similarly, fusion of peroxisome targeting signal type 2 (PTS2) (Petriv et al., 2004) to the N terminus of GFP specifically directed the protein to the peroxisomes (data not shown). Using these same conditions, we determined the localization of SPAO1–GFP, expressed under the control of the ADH promoter, by fluorescence microscopy. As shown in Fig. 4(A), the fusion protein accumulated in the cytosol of S. cerevisiae cells. Like GFP alone, SPAO1–GFP exhibited fluorescence throughout the cytoplasm and was excluded from the vacuole, which was detected as indentations by Nomarski optics (Fig. 4A). Importantly, under these conditions of growth, the vacuoles were morphologically predominant within the cells, most likely as a consequence of nutrient limitation. To ensure that the fluorescence observed was due to the SPAO–GFP fusion protein rather than products from proteolytic cleavage of the linker between SPAO1 and GFP, total protein extracts from cells transformed with plasmids expressing the indicated GFP and SPAO1–GFP molecules were analysed by immunoblotting (Fig. 4B). These results showed that the SPAO1–GFP protein was present exclusively as a chimeric fusion (Fig. 4B). When the expression of SPAO1–GFP was driven by the GPD promoter, SPAO1–GFP was also found in the cytoplasm (J. Laliberté and S. Labbé, unpublished data). Under the conditions of nitrogen limitation used in our studies, SPAO1–GFP did not localize to the peroxisomes. Thus, SPAO1 is primarily a cytosolic protein when expressed in S. cerevisiae. Moreover, S. cerevisiae cells expressing SPAO1–GFP, GFP alone or GFP–SKL were also examined during growth under nitrogen-replete conditions in oleate-containing medium. Although the vacuoles were not detected under these conditions, like GFP alone, the SPAO1–GFP protein was mainly seen throughout the cells, suggesting that it was localized to the cytosol. On the other hand, cells harbouring GFP–SKL exhibited the characteristic punctate peroxisomal pattern of small spots (see Supplementary Fig. S1).
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; Knight et al., 1996; Peña et al., 2000; Puig et al., 2002b). Although Ctr1 and Ctr3 are functionally redundant, these two proteins mediate copper uptake independently of each other. CTR1 and CTR3 genes are regulated at the level of gene transcription (Labbé et al., 1997). They are induced under conditions of copper starvation and are repressed under conditions of copper repletion. Furthermore, it has been demonstrated that expression and copper-dependent regulation of CTR1 and CTR3 genes require the presence of a functional MAC1 gene (Labbé et al., 1997). To determine if Mac1, Ctr1 or Ctr3 are required for providing copper to SPAO1, CAO activity was assayed in whole-cell extracts from wild-type, mac1 and ctr1 ctr3 cells transformed with a wild-type copy of the spao1+ gene expressed from a centromeric plasmid (Fig. 5). Under copper-limiting conditions, in the presence of 300 µM TTM in the growth medium, strains bearing the disrupted mac1 or ctr1 ctr3 alleles were devoid of detectable CAO activity (Fig. 5A, left panel). As expected, CAO activity could be restored by the addition of elevated copper concentrations (10 µM) to the growth medium (Fig. 5A, right panel). This is likely due to the fact that copper ions are taken up via the low-affinity copper-transport system, thereby bypassing any requirement for the high-affinity copper-transport pathway. To further investigate if the absence of CAO activity in the mac1 and ctr1 ctr3 mutant strains grown under conditions of copper deprivation was not due to a defect in SPAO1 expression, we transformed the spao1+–GFP fusion allele into S. cerevisiae wild-type, mac1 and ctr1 ctr3 cells. As shown in Fig. 5(B) (left panel), in the presence of 300 µM TTM, the mac1 and ctr1 ctr3 mutant strains failed to exhibit measurable CAO activity, whereas the wild-type strain bearing functional MAC1, CTR1 and CTR3 genes exhibited high levels of CAO activity. CAO activity was restored by the addition of 10 µM CuSO4 to the growth medium (Fig. 5B, right panel). To verify that the SPAO1–GFP fusion protein was expressed in the wild-type, mac1 and ctr1 ctr3 cells, total protein extracts from transformed cells were analysed by immunoblotting (Fig. 5B). These results showed that the SPAO1 protein was clearly produced in the mac1 and ctr1 ctr3 strains, indicating that the absence of activity in the mutant strains was not due to the lack of SPAO1 expression. Taken together, these results show that under conditions of copper starvation, the production of an active CAO in S. cerevisiae requires CTR-mediated copper transport, as well as the transcription factor Mac1, which is essential for the expression of the high-affinity copper uptake genes.
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, ccc2, cox17 or ccs1 deletion and performed peroxidase-catalysed chemiluminescent assays on lysates from the transformed cells. As shown in Fig. 6(A) (left panel), the atx1 strain transformed with spao1+ exhibited no CAO activity; however, CAO activity was restored by the addition of 10 µM CuSO4 to the growth medium (Fig. 6A, right panel). Furthermore, transformation of a wild-type ATX1 on a centromeric vector restored CAO activity in the atx1 mutant cells expressing SPAO1 (data not shown). Deletion of the copper chaperones Cox17 and CCS1 did not affect the CAO activity in the transformed cells (Fig. 6A). In contrast, deletion of CCC2 resulted in a partial loss of CAO activity that was corrected by the addition of copper to the growth medium (Fig. 6A). To ensure that the absence of CAO activity was not due to the lack of expression of the SPAO1 protein, we also transformed the mutant isogenic strains with a centromeric plasmid expressing the spao1+–GFP fusion gene. Under conditions of copper starvation, cells bearing a cox17 or ccs1 deletion displayed elevated levels of CAO activity (Fig. 6B, left panel) that were comparable to those found in the same mutant strains expressing the untagged SPAO1. In contrast, an atx1 mutant strain expressing the spao1+–GFP allele failed to activate SPAO1 under copper-limiting conditions (Fig. 6B, left panel). The lack of CAO activity did not reflect the absence of protein since SPAO1–GFP fusion protein was stably expressed and readily detectable by immunoblotting in these cells (Fig. 6B). Analysis by peroxidase-catalysed chemiluminescent assay showed that a ccc2 mutant strain harbouring the spao1+–GFP allele appeared to have less CAO activity than the wild-type strain, even though the protein levels were comparable (Fig. 6B). This indicates that the Ccc2 protein may also participate in conferring CAO activity on SPAO1. Taken together, these results reveal that optimal activity of the Schiz. pombe SPAO1 protein expressed in S. cerevisiae requires functional ATX1 and CCC2 genes, with ATX1 making a greater contribution than CCC2.
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and ccc2 null mutants), an observation that was recapitulated in the present study. As shown in Fig. 7, the wild-type strain expressing an empty vector failed to grow on minimal medium containing ethylamine as the sole nitrogen source. However, expression of the SPAO1 protein in the wild-type strain allowed the cells to grow on minimal medium prepared with ethylamine. SPAO1 catalysed the oxidative deamination of ethylamine to the corresponding aldehyde, with concomitant release of H2O2 and NH3 that was sufficient to provide the cells with a nitrogen source. We examined the requirement of Atx1, Ccc2, Cox17 and Ccs1 for SPAO1 activity by testing the ability of mutant cells transformed with spao1+ to grow in medium containing ethylamine as the sole source of nitrogen. atx1 cells were unable to grow on ethylamine medium containing 100 µM BCS, a specific copper chelator, while cells harbouring a ccc2 deletion showed very limited growth (Fig. 7, lane 4). The growth deficiency of both mutants was efficiently reversed by the addition of exogenous copper (100 µM) to the ethylamine medium (Fig. 7). In contrast, insertional inactivation of cox17 or ccs1 had no effect on the growth of these mutant strains expressing spao1+ on ethylamine medium in the presence of 100 µM CuSO4 or 100 µM BCS. Thus, the Cox17 and Ccs1 metallochaperones are not required for delivering copper to SPAO1. The results in this section clearly establish that Atx1, and to a lesser extent Ccc2, participate in the production of an active recombinant CAO in S. cerevisiae cells.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). To gain insight into their biological functions, we took advantage of the fact that the S. cerevisiae genome does not encode a CAO homologue. It has previously been demonstrated that an active enzyme can be produced by the heterologous gene expression of HPAO in S. cerevisiae (Cai & Klinman, 1994a), thus, we utilized this host to ascertain if primary amines can be oxidized through the expression of either SPAO1 or SPAO2. Although wild-type S. cerevisiae strains expressing spao1+ were competent in catalysing the oxidation of ethylamine to its corresponding aldehyde with the subsequent release of NH3 and H2O2, we were unable to demonstrate CAO activity for SPAO2. This lack of activity may be due to the fact that the SPAO2 protein harbours a Tyr295
Phe amino acid substitution. Based on the predicted 3D model of active CAO, a conserved peptidyl Tyr residue is required for proper orientation of its redox cofactor TPQ (Mure et al., 2005), thus, its substitution to Phe in SPAO2 might render the enzyme incapable of catalysing ethylamine oxidation. Another reason may be the nature of the substrates that we used. The primary amine substrates that we tested included monoamines (e.g. ethylamine), aromatic monoamines (e.g. benzylamine) and diamines (e.g. putrescine and 1,8-diaminooctane). None of these substrates was oxidized by SPAO2 (J. Laliberté and S. Labbé, unpublished data). In contrast, SPAO1 exhibited a broad range of substrate specificity. The best substrates for SPAO1 were ethylamine, putrescine and 1,8-diaminooctane, while the aromatic monoamine benzylamine was less efficiently oxidized (J. Laliberté and S. Labbé, unpublished data). Collectively, these data demonstrate that CAO activity was present in S. cerevisiae cells expressing spao1+, but not in cells expressing spao2+.
Fluorescence microscopy experiments revealed that a functional SPAO1–GFP fusion protein expressed in S. cerevisiae resides predominantly in the cytosol. Based on previous data showing that HPAO is imported into the peroxisomes (Faber et al., 1994), we examined the sequence of the SPAO1 protein for the presence of putative peroxisome targeting signals. The majority of peroxisomal matrix proteins are targeted to the peroxisome by a peroxisome targeting signal type 1 (PTS1) found at the extreme C terminus of the proteins (Neuberger et al., 2003a; Petriv et al., 2004). PTS1 is a tripeptide with the consensus sequence (S/C/A)(K/R/H)(L/M) (Petriv et al., 2004). Using a PTS1 predictor program (Neuberger et al., 2003b), we found no peroxisomal import motif within the C-terminal region of SPAO1, making it improbable that the C terminus is recognized by the soluble receptor molecule Pex5 that targets proteins to the peroxisome (Brocard et al., 1994). A small subset of peroxisomal matrix proteins (including HPAO) is targeted by PTS2, which is composed of a nanopeptide located at the N termini of the target proteins (Faber et al., 1994; Petriv et al., 2004). The accepted consensus sequence for PTS2 is (R/K)(L/V/I/Q)XX(L/V/I/H/Q)(L/S/G/A/K)X(H/Q)(L/A/F) (Petriv et al., 2004). Examination of the SPAO1 amino acid sequence revealed two potential N-terminal peroxisomal targeting signals, R21-L-S-D-P-L-D-P-L29 and R42-H-E-Y-P-S-K-H-F50. However, both candidate signals harbour two important mismatches (underlined residues) at highly conserved positions, making it highly improbable that they will interact with Pex7, the receptor for PTS2-containing proteins (Rehling et al., 1996). Consistent with these observations, analysis using the PSORT II program classified SPAO1 as a non-peroxisomal protein. Although we cannot exclude the possibility that signal peptides in Schiz. pombe proteins destined for the peroxisomal matrix differ from the currently accepted PTS1 and PTS2 consensus sequences, computational analyses have shown that these sequences are highly conserved from fungi to plants and mammals (Neuberger et al., 2003b; Reumann, 2004). Consistent with the absence of a canonical PTS1 or PTS2 targeting sequence, our experiments failed to localize the SPAO1 protein into S. cerevisiae peroxisomes, even upon induction of the peroxisome proliferation response under conditions of nitrogen starvation, as determined for HPAO in the methylotrophic yeast H. polymorpha (Faber et al., 1994). Instead, our data reveal that an active SPAO1 is primarily a cytosolic protein when expressed in S. cerevisiae.
Despite the fact that S. cerevisiae does not express a protein homologous to SPAO1 or to any of the members of the CAO family, copper is made available to the newly synthesized cytosolic SPAO1 protein when expressed in budding yeast. How does SPAO1 obtain copper? Under conditions of copper depletion and in the absence of the plasma-membrane-associated high-affinity copper transporters Ctr1 and Ctr3, mutant cells expressing SPAO1 did not exhibit CAO activity, even though the protein was efficiently expressed. Likewise, in the absence of Mac1, the transcription factor that activates high-affinity copper-transport genes under copper-limiting conditions, no CAO activity was observed. As expected, the requirement for Ctr1/3 or Mac1 for CAO activity was bypassed by addition of copper to the growth medium. Therefore, lack of CAO activity observed in these mutant strains is consistent with a failure of the high-affinity copper-transport machinery to assimilate sufficient copper to provide the SPAO1 protein with the metal cofactor.
To further investigate the process by which copper is supplied to SPAO1, we tested the effects of deletions of the ATX1, COX17, CCS1 and CCC2 genes, which are involved in intracellular copper delivery, on SPAO1 activity. We found that production of active SPAO1 was dependent on Atx1 and to a lesser extent, Ccc2. However, the precise mechanism by which this process occurs is unknown. Previous studies have shown that the S. cerevisiae Atx1 resides in the cytosol (Lin et al., 1997). Both SPAO1 and Atx1 co-localize in the cytosol, therefore, the possibility exists that SPAO1 and Atx1 physically interact with each other. Interestingly, X-ray crystal structures of Atx1 have revealed a region harbouring multiple Lys residues that may generate a positively charged patch on the protein surface (Banci et al., 2001; Rosenzweig et al., 1999). Furthermore, the Lys-rich face of Atx1 has been shown to be necessary for physical interaction with Ccc2 and subsequent delivery of copper to its target protein (Portnoy et al., 1999). Analysis of X-ray crystal structures of CAOs indicates that the CAO homodimer has two identical active sites arranged along a molecular twofold symmetrical axis (Duff et al., 2003; Li et al., 1998; Lunelli et al., 2005; Parsons et al., 1995). Each active site contains one copper and one TPQ, both at a very close distance. Interestingly, the cavity leading to the active site is characterized by the presence of an area with a negative electrostatic potential, making it a potential zone for electrostatic interactions. Atx1 encodes a very small polypeptide of only 8.2 kDa, and exhibits multiple Lys residues that form a positively charged surface on the protein; therefore, it is possible that Atx1 docks on the active site of SPAO1, which is predicted to be negatively charged to allow the direct transfer of copper ions from one protein to the other. It is also possible that Atx1 delivers copper to an intermediate soluble factor which is then responsible for inserting copper into SPAO1; however, no such factor has yet been identified. It should be noted that no obvious Atx1-like domain was found in SPAO1 (J. Laliberté and S. Labbé, unpublished data). It would be interesting to assess the importance of the basic residues in Atx1 for the activation of SPAO1. The results from this study also show that, even though equal amounts of SPAO1 protein are synthesized, a ccc2 mutation results in reduced CAO activity compared to the total activity observed in a wild-type strain. The copper-transporting ATPase Ccc2 is required for delivery of copper to the late secretory pathway, therefore, the significance of this result is difficult to reconcile, unless the cytoplasmic N terminus of Ccc2 can serve as a source of copper for Atx1, in order to provide copper to the cytosolic SPAO1. Previous studies using two-hybrid analysis have shown that Atx1 can interact with the N-terminal metal-binding domains of Ccc2 (Portnoy et al., 1999; Pufahl et al., 1997). Perhaps, under certain circumstances, Atx1 may acquire copper from Ccc2 for insertion into cytosolic proteins. It is noteworthy that previous data have suggested that Ccc2 can obtain copper in an Atx1-independent manner, possibly via copper-induced endocytosis of Ctr1 from the plasma membrane (Lin et al., 1997). Thus, Ccc2 may have an effect on intracellular copper distribution and utilization. Alternatively, Atx1 and Ccc2 could act in parallel, as redundant mechanisms to independently supply copper to SPAO1. However, our unpublished data do not support this mechanism because overexpression of CCC2 was incapable of suppressing the lack of SPAO1 activity in atx1 mutant cells.
The inability of S. cerevisiae to utilize amines is most likely correlated with the lack of endogenous CAOs in this organism. Thus, the wild-type S. cerevisiae strains used in this study were incapable of growing on minimal medium containing ethylamine as the sole nitrogen source, unless the SPAO1 protein from Schiz. pombe was ectopically expressed. Using this as a functional assay, we tested the ability of SPAO1 to allow yeast strains harbouring deletions in the ATX1, CCC2, COX17 and CCS1 genes to utilize ethylamine as a nitrogen source. Under conditions of copper deprivation, cells lacking ATX1 cannot grow on medium with ethylamine. This is due to the lack of CAO activity in SPAO1 protein expressed in these cells. Deletion of the CCC2 allele (ccc2) resulted in weak growth of cells on medium containing ethylamine and the copper chelator BCS. As expected, both mutant phenotypes were corrected by the addition of copper to the growth medium. In contrast to the atx1 and ccc2 mutants, strains lacking Cox17 and Ccs1 grew robustly in the presence of ethylamine and BCS, suggesting that these copper chaperones are not involved in supplying copper to SPAO1. Collectively, these observations support a role for Atx1 and, to a lesser extent, Ccc2, in the shuttling of copper to SPAO1. It remains to be established whether the Schiz. pombe SPBC1709.10c gene that encodes a putative chaperone orthologous to S. cerevisiae Atx1 is in fact required for delivering copper to SPAO1 in fission yeast. Further studies on SPAO1 and SPAO2 in fission yeast cells will elucidate the function of these proteins and the mechanisms by which copper is loaded into CAOs in this organism.
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ACKNOWLEDGEMENTS |
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ctr3 double mutant disruption strain, and to Raymund J. Wellinger and Sherif Abou Elela for the S. cerevisiae atx1, ccc2, ccs1 and cox17 mutant strains. We would like to thank Richard Rachubinski and Rick Poirier for plasmids used in this study. J. L. is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). This work was supported by the Canadian Institutes of Health Research (CIHR) Grant MOP-36450 to S. L and by the Fondation de la Recherche sur les Maladies Infantiles du Québec. S. L. is a New Investigator Scholar from the CIHR. |
REFERENCES |
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