Biol. Bull. 211: 18-30. (August 2006)
© 2006 Marine Biological Laboratory
Two Atypical Carbonic Anhydrase Homologs From the Planula Larva of the Scleractinian Coral Fungia scutaria
Melissa L. de boer1,
Dave A. Krupp2 and
Virginia M. Weis1,*
1 Department of Zoology, Oregon State University, Corvallis, Oregon 97331
2 Department of Natural Sciences, Windward Community College, Kaneohe, Hawaii 96744
* To whom correspondence should be addressed. E-mail: weisv{at}science.oregonstate.edu
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Abstract
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In cnidarians, the enzyme carbonic anhydrase (CA) is important to inorganic carbon (Ci) flux in processes including calcification and dinoflagellate symbiont photosynthesis. Although CA is known to function in Ci delivery to symbionts in adults with mature symbioses, it is not known when CA becomes active in this capacity during the onset of symbiosis in developing hosts. We identified two CA cDNA sequences from the planula larvae of the Hawaiian scleractinian coral Fungia scutaria. Expression of these larval CAs did not differ between infected and uninfected larvae or vary over the course of infection. Bioinformatic analyses of the two homologs showed that the sequences are unusually short and are missing some residues that support active site structure in other CAs. This is the first description of a short form of CA. Phylogenetic analyses of the larval CAs grouped them with membrane-bound homologs from vertebrates. Studies in other calcifying cnidarians have identified membrane-associated CAs as functioning in calcification, and therefore the two larval CAs could play a role in the onset of calcification during metamorphosis. A longer CA isoform was amplified from adult F. scutaria cDNA but not from larvae, suggesting that the longer form is not expressed in larvae. The longer form grouped with cytosolic CAs including a symbiotic anemone homolog implicated in Ci delivery to dinoflagellate symbionts. The apparent absence of this "symbiosis" CA in larvae suggests that the Ci supply mechanism is not active during the initial onset of the association.
Abbreviations: CA, carbonic anhydrase • Ci, inorganic carbon • FCA, Fungia scutaria carbonic anhydrase
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Introduction
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Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible hydration of carbon dioxide: CO2 + H2O 
HCO3– + H+. In seawater (pH 8.2), most inorganic carbon (Ci) is in the form of bicarbonate (HCO3–), a form that diffuses slowly across membranes and is not readily converted to CO2 in the absence of enzymatic action (Kerby and Raven, 1985; Cook et al., 1986). Symbiotic photosynthetic dinoflagellates (zooxanthellae), enclosed within a vacuole inside the gastrodermal cells of a cnidarian host, are cut off by several layers of membrane from the Ci supply in the surrounding seawater. Although the symbionts are able to fix host metabolic CO2, their photosynthetic rates are comparable to those of free-living microalgae and often exceed the respiratory rates of the association (Muscatine, 1980; Raven, 1981). These symbionts must therefore gain access to the Ci supply in the surrounding seawater (Goreau, 1977; McCloskey and Muscatine, 1984; Muscatine et al., 1984) and do so with the help of host CA (Weis et al., 1989; Furla et al., 2000a, b).
CA plays a role in Ci acquisition in a broad array of marine symbioses. In cnidarians, several studies using sea anemones have examined the role of host CA in solving the symbiont’s Ci shortage. In one study (Weis, 1993), CA activity in the sea anemone Aiptasia pulchella was elevated in symbiotic versus nonsymbiotic (aposymbiotic) sea anemones and was localized, using colloidal gold immunocytochemistry, to the host-derived vacuolar membrane surrounding the symbiont. This CA did not appear to be intrinsically bound to the membrane. In another anemone study, Weis and Reynolds (1999) showed that the amount of host CA transcript is significantly elevated in symbiotic Anthopleura elegantissima versus aposymbiotic individuals. And in the anemone Anemonia viridis, Ci uptake for zooxanthellate photosynthesis is dependent upon an H+-ATPase and a membrane-bound CA in host ectodermal cells (Furla et al., 2000a). In addition, the presence and density of zooxanthellae in the upside-down jellyfish, Cassiopea xamachana, influences the level of CA activity: high densities of zooxanthellae correspond to high levels of CA activity in tissues of the host bell and oral arm (Estes et al., 2003). Finally, in the symbioses between the giant clam Tridacna spp. and zooxanthellae (Leggat et al., 2002) and between the deep-sea vestimentiferan worm Riftia pachyptila and chemoautotrophic bacteria (Cian et al., 2003a, b), host CA is expressed both in the respiratory organs, to concentrate Ci from the external seawater, and in host tissues lining the cavities that house the symbionts, to facilitate carbon transport to the autotrophic symbionts. In each of these associations, host CA appears to increase the availability of CO2 to the symbiont by catalytically releasing it from bicarbonate at or near the site of symbiont photosynthesis (or chemosynthesis in the case of chemoautotrophs). Thus, the symbionts are able to maintain high rates of carbon fixation within their intracellular environment (Weis et al., 1989; Weis, 1993; Weis and Reynolds, 1999).
To date, all studies examining the role of host "symbiosis" CA within the cnidarian-dinoflagellate associations have examined symbioses in adult animals. Nothing is known about when in the host’s life history this carbon supply mechanism is activated or how its activation relates to the onset of symbiosis. We were interested, therefore, in describing CA and its expression during the early life history stages of the solitary Hawaiian scleractinian coral Fungia scutaria, which has a tractable early life history that includes onset of symbiosis. Adults of this species release gametes into the water column where fertilization occurs. The free-swimming planula larvae that develop contain no symbionts. Acquisition of symbionts (infection) from the surrounding environment becomes possible at about 3 days after fertilization, upon development of a mouth, when the planulae begin to feed (Krupp, 1983; Schwarz et al., 1999). During feeding, the symbiont is drawn into the gastric cavity where phagocytosis by the host gastrodermal lining occurs (Schwarz et al., 1999). Each individual symbiont is then housed within a host gastrodermal cell, contained within a host-derived vacuolar membrane, or symbiosome (Wakefield and Kempf, 2001). The planulae swim in the water column for about a week before they settle and undergo metamorphosis.
Our initial focus on the role of CA during the onset of symbiosis in a scleractinian coral expanded when we identified two novel and unusually short CA sequences from the larvae. In addition to expression analysis, we also performed bioinformatic analyses on these sequences. These analyses are the first to be performed on any cnidarian CA sequence.
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Materials and Methods
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Collection
Several dozen adult specimens of Fungia scutaria were collected from reefs surrounding Coconut Island, Oahu, Hawaii, in July 1998 and August 2000. The corals were maintained and monitored as in Schwarz et al. (1999). When spawning occurred, eggs were collected in plastic bowls and diluted generously with 3–4 1 of 0.2-µm filtered seawater (FSW). Sperm from several males was mixed together in an approximate 1:10 dilution with FSW. Eggs were fertilized by using a turkey baster to gently squirt the dilute sperm mixture into the bowl. The bowls of fertilized eggs were transferred to covered running seawater tables and left overnight with occasional gentle mixing. About 12–18 h after fertilization and once each day thereafter, the water in each bowl of developing larvae was changed using 60 µm screen and fresh FSW (Schwarz et al., 1999).
Infection
Larval development was monitored daily using light microscopy. At about 3 days after fertilization, once the larvae had developed a mouth, they were exposed to freshly isolated symbionts (FIZ) from adult F. scutaria and to a brine shrimp preparation (Schwarz et al., 1999). To control for genetic differences between mothers, prior to infection, larvae from all bowls were combined into a single population and then redistributed into the original bowls. Larvae were then concentrated into a smaller volume (1–1.51) of FSW. Half of the bowls were exposed to FIZ + shrimp (15 ml of the FIZ preparation was gently poured into each bowl and mixed, followed by 5 ml of the brine shrimp preparation), and half of the bowls were exposed to shrimp only (5 ml shrimp preparation + 15 ml FSW). Larvae were allowed to incubate for 3 h before the water was changed and the larval concentration was reduced to previous levels. Infection rates were measured 24 h after the infection procedure. Percent infection was determined by taking a subsample of 100 larvae from each bowl and examining the larvae with light microscopy to quantify the proportion containing at least one zooxanthella. Three subsamples were counted from each bowl of larvae.
Sampling, RNA isolation and cDNA synthesis
Symbiotic and aposymbiotic larvae were sampled each day (in 2000) or every other day (in 1998) from infection until the onset of settlement. A 50-ml conical tube was filled with larvae suspended in FSW and centrifuged for 10 s at 8000 x g. The pelleted larvae were transferred to a 1.5-ml microfuge tube and centrifuged briefly at 12,000 x g in a tabletop microfuge. The seawater was quickly drawn off the larval pellet, and the tube dropped into a dry ice-methanol bath. The frozen samples were held at –50°C and shipped on dry ice to Oregon State University, where they were held at –80°C until processing. After host and algal tissue were separated via centrifugation, RNA was extracted from the larvae and quantities were determined spectrophotometrically (Weis and Reynolds, 1999). cDNA was synthesized from 1 µg total RNA using a first-strand cDNA synthesis kit (Gibco BRL).
Polymerase chain reaction amplification, cloning, and sequencing
Degenerate forward (dCA-F: 5'-CARTTYCAYTTYCAYTGGGG-3') and reverse (dCA-R: 5'-GGNGGNGTNGTNAGNGANCC-3' and dCA-R2: 5'-RCTGGNCKSYAGTTGTCCAC-3') primers designed from a consensus of 
-CA sequences were used in an attempt to amplify an internal piece of F. scutaria CA (FCA) sequence from 5-day-old aposymbiotic larval cDNA samples (Fig. 1). Annealing temperatures of 56°C and 58°C were used for the primer sets dCA-F/dCA-R and dCA-F/dCA-R2, respectively. Two microliters of the PCR reaction mixture was ligated into a plasmid vector (Invitrogen’s TA cloning kit). INVF' One Shot competent cells were transformed and plated on LB/ampicillin plates spread with Xgal and IPTG, then grown overnight at 37°C. White colonies were PCR screened for the correct insert size, using the vector primers M13F and M13R; those containing the correct size were sequenced. All sequencing reactions were performed on column-purified PCR products (Montage PCR centrifugal filter device) amplified using the vector primers M13F & M13R and plasmid DNA from overnight cultures of individual colonies purified with a standard small-scale protocol (Qiagen plasmid spin miniprep kit). Sequencing was performed using the Applied Biosystems Taq DyeDeoxy Terminator cycle sequencing kit, and the reaction product was analyzed on an Applied Biosystems model 373 DNA sequencer. A single contiguous sequence was generated using the Staden Package software, ver. 2003.0.
Rapid amplification of cDNA ends (RACE)
Primers specific to the F. scutaria cDNA piece, which was obtained using dCA-F and dCA-R, were designed and used in nested RACE reactions to obtain the 3' and 5' ends of the F. scutaria CA cDNA. The sense primer ("FCA-F" 5'-AGAATTTGCGCGGTTCTGAG-3') starts at nucleotide 497 and the antisense primers ("FCA-R" 5-CGTCGAGGCGTTCGGATACT-3' and "FCA-R2" 5'-CTCAGAACCGCGCAAATCT-3') start at nucleotides 597 and 516, respectively (Figs. 1, 2). The 3' end of larval F. scutaria CA was amplified in nested PCR reactions using the degenerate primer dCA-F, the specific primer FCA-F, and the reverse oligo (dT) primer 1-REV (Weis and Reynolds, 1999). The first reaction mixture contained 1.25 µl 1-REV, 1.25 µl dCA-F, and 1.25 µl cDNA from 5-day-old aposymbiotic F. scutaria larvae. An annealing temperature of 57°C was used in the first reaction. The nested PCR reaction mixture consisted of 0.5 µl 1-REV, 3 µl FCA-F, and 1.5 µl of a 1:100 dilution of the first reaction PCR product. An annealing temperature of 60°C was used in the nested reaction. The 5' end was amplified using FCA-R, FCA-R2 and the FirstChoice RLM-RACE kit (Ambion) according to the manufacturer’s instructions. Both ends were cloned and sequenced as described above.
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Figure 2. Nucleotide (nt) and predicted amino acid sequences of Fungia scutaria ‘CA’ obtained from aposymbiotic larvae. The start codon (ATG) is boxed and the putative promoter site, a TA rich region 36 bp upstream from the start, is underlined. Two different 3' ends, starting at nt 719, were obtained using RACE. The shorter sequence, FCA-a, is in bold. FCA-a (accession #DQ357229) is a 792-bp cDNA with a 603-bp open reading frame (ORF); FCA-b (accession #DQ357230) is an 849-bp cDNA with a 675-bp ORF. Both stop codons, TGA, are boxed, and both polyA signals are underlined. The FCA-a and FCA-b signals occur 14 and 18 bp upstream from the start of the polyA tails (not shown), respectively. Primer sites are shaded, with primer names over the sequence. Arrows indicate directionality.
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Sequence analysis
Deduced amino acid sequences were obtained using NCBI’s ORF Finder (www.ncbi.nlm.nih.gov/gorf/gorf.html) and were analyzed for conserved motifs using ScanProsite at ExPASy (Sigrist et al., 2002). A search for membrane-spanning domains was performed using TMpred at the ExPASy site as well (Hofmann and Stoffel, 1993). Related amino acid sequences were obtained from the GenBank database using the BLAST algorithm (Altschul et al., 1990). Sequences were aligned using ClustalX (Thompson et al., 1997) with a gap opening penalty of 35.00 and a gap extension penalty of 0.75 (pairwise alignment) and a gap opening penalty of 15.00 and a gap extension penalty of 0.30 (multiple alignments). Similarity matrix PAM250 was used for all identity/similarity calculations, and calculations included all insertion/deletions, although 3' end overhangs were masked using BioEdit (Hall, 1999). Phylogenic tree construction was performed using both parsimony (heuristic search) and distance methods in PHYLIP (default parameters except for randomizing the input order, restarted 10 times) (Felsenstein, 2004). Both methods produced similar trees. Treeview16 was used for drawing consensus trees (Page, 1996). The secondary and tertiary structures of the sequence were predicted and compared to known crystal structures by the Protein Structure Prediction Server (PSIPRED, Ver. 2.0) accessed from the University College London Website (Jones, 1999a, b; McGuffin et al., 2000; McGuffin and Jones, 2003). The highest hit from that search was used in the structural alignment/viewer program Cn3D at NCBI (Wang et al., 2002).
Real-time quantitative polymerase chain reaction
Real-time quantitative PCR (QPCR) was performed using the Prism 7700 sequence detector (Perkin Elmer/Applied Biosystems Division) and SYBR Green master mix. Specific primers FCA-F and FCA-R were used in all QPCR reactions. Identical reactions amplifying actin (forward: 5'-CTG ATG GAC AGG TCA TCA CCA T-3'; reverse: 5'-CTC GTG GAT ACC AGC AGA TTC C-3') were run concurrently and were used as a reference point for expression level. The default cycling parameters for the ABI PRISM 7700 sequence detection system were used in all reactions. CA expression level was normalized to actin expression and is reported as: 1/ (CA cycle# at mid-log phase amplification – actin cycle# at mid-log phase amplification). Replicate number is the number of PCR reactions performed on larvae of a specific age and infection state and collected in a specific year. All PCR products from preliminary runs were checked for the presence of primer dimers using agarose gel electrophoresis. Welch corrected unpaired student’s t tests were performed using GraphPad InStat version 3.05 for Windows 95/NT (GraphPad Software, San Diego, CA).
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Results
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Fungia scutaria carbonic anhydrase sequences
An internal piece of CA sequence was obtained from 5-day-old aposymbiotic larvae, using the degenerate primers dCA-F and dCA-R (Figs. 1, 2). Attempts to obtain CA sequence using the dCA-F and dCA-R2 primer pair were successful in adult corals but unsuccessful in larvae. The partial CA sequence obtained from adult coral using dCA-F and dCA-R2 is included in the multiple sequence alignment (Fig. 3). dCA-R2 was designed to the conserved amino acid sequence VDNyRPa present toward the C-terminal end of the alignment.
Two different F. scutaria CA 3' ends (FCA-a and FCA-b) were obtained from aposymbiotic larvae, using nested RACE reactions (Fig. 2). The complete FCA-a sequence consists of a 792-nucleotide cDNA with a 603-bp open reading frame (GenBank accession #DQ357229). The start codon is 39 nucleotides downstream from the 5' end. A likely promoter site is located 36 bp upstream from the start codon; the poly-A signal, AATAAA, occurs 28 nucleotides downstream from the in-frame stop codon and 14 bp upstream of the start of the poly-A tail. The deduced amino acid sequence is 201 amino acids in length.
FCA-b differs from FCA-a only at the 3' end of the cDNA (Fig. 2). The complete FCA-b sequence consists of an 849-nucleotide cDNA with a 675-bp open reading frame (GenBank accession #DQ357230). The sequence is identical to FCA-a from nucleotides 1–717; however, at nucleotide 718 (amino acid 194) there is a change in both nucleotide and deduced amino acid sequence. The in-frame stop codon occurs at nucleotide 814, and a polyadenylation signal, AATAA, occurs 8 nucleotides downstream from the stop and 18 nucleotides upstream from the start of the poly-A tail.
Sequence analysis and alignments
Both larval FCAs were analyzed independently; however, the results are reported only once if they were the same for both. Blast searches of GenBank using both FCA sequences revealed hundreds of significant alignments with -CAs, a family of related CA enzymes found mostly in animals.
Most hits from the top 150 significant alignments were to vertebrate CA isozyme group II; other CA groups included CA I, VII, XII, XIII, and XIV (all vertebrate). The only nonvertebrate CA sequences included in the top 150 hits were an -CA from the deep-sea vestimentiferan annelid R. pachyptila and an -CA sequence from the temperate sea anemone Anthopleura elegantissima. The top 10 BLAST hits for both FCA sequences are given in Table 1.
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Table 1 Top 10 hits from BLASTp searches of all non-redundant GenBank CDS translations, PDB, SwissProt, PIR, and PRF using predicted amino acid sequences of a and b forms of Fungia scutaria carbonic anhydrase (FCA)
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An alignment of the FCA deduced amino acid sequences to a subset of the top 150 BLAST hits (CA I, II, IV, and VII from mouse and human, as well as the sea anemone sequence) is shown in Figure 3. Both FCA-a and FCA-b are much shorter than the other sequences yet have retained three critical conserved histidine residues (direct-ligands) that bind directly to the zinc cofactor, as well as several amino acids (indirect-ligands) that function in forming a hydrogen bond network around the active site (Sheridan and Allen, 1981; Hewett-Emmett et al., 1984; Fukuzawa et al., 1990; Fukuzawa and Ishida, 1991). However, several other indirect-ligand sites are missing from the FCAs because of the truncated nature of the sequences. Conserved motifs present in the FCA deduced amino acid sequences include two N-glycosylation sites at amino acids 98–101 and 150–153; four protein kinase C phosphorylation sites at amino acids 28–30, 94–96, 187–189, and 207–209; two casein kinase II phosphorylation sites at amino acids 60–63 and 180–183; and one N-myristoylation site at amino acids 102–107. All aforementioned patterns have a high probability of occurrence. Additionally, two highly conserved motifs were found: an ATP/GTP-binding site motif A (P-loop) at amino acids 90–97 and a eukaryotic-type carbonic anhydrase signature spanning amino acids 125–141 (Figs. 3, 4). The p-loop motif was not found in the other CA sequences included in the alignment. A significant (score > 500) possibility of a 19 amino acid transmembrane helix (score = 1461) was found using the TMpred program (Fig. 3). The possible helix runs from amino acids 6–24 and displays a strong preference for the inside-to-outside orientation.
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Figure 4. Structural comparison of Fungia scutaria CA predicted amino acid sequences with the known crystal structure of human CAII (hCAII). (A) The carbon backbone of hCAII; it is dominated by a central, twisted 10-stranded beta sheet that is mostly antiparallel (yellow). A few short helices (red) are present on the outside of the enzyme; only one is visible from this orientation. The zinc cofactor is shown as a gray sphere. (B) Sequence conserved between FCA-b and hCAII is shown in red; insertions/deletions (indels) and the C-terminus residues absent from the FCA sequence are in yellow. (C) View of the carbon backbone minus the absent C-terminal residues. The yellow indicates a two amino acid indel (present in hCAII/absent in FCA) in the pairwise alignment. (D) Conserved domains and motifs present in the FCA sequences: the blue portion highlights the -CA domain and the red portion (containing the yellow indel) highlights the P-loop motif.
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Both FCA deduced amino acid sequences were analyzed using the PSIPRED program to compare the predicted secondary structures to known crystal structures. The results from those two searches were identical. The highest hit was to human CAII (confidence level = high / E- value = 0.001) and the second highest hit was to human CAIV (confidence level = high / E-value = 0.003). The human CAII structure was subsequently used in the structural alignment/viewer program Cn3D to perform structural comparisons with the FCAs (Fig. 4). Although the FCAs are much shorter than other CA sequences, they retain the catalytic domain and the three critical histidine residues that directly bind the zinc cofactor as well as several residues involved in the hydrogen bond network surrounding the active site. Figure 4 shows the location and folded structures of the highly conserved CA and P-loop motifs evident in the FCA sequences.
A more extensive multiple alignment was used to generate the phylogenetic tree (Fig. 5). The tree was constructed using parsimony analysis and rooted using an -CA sequence from the cyanobacterium Nostoc. Overall, all sequences fell into one of two clades with very strong support, a cytosolic and a membrane-bound clade. The FCA sequences grouped with the vertebrate membrane-bound forms, whereas the two other invertebrate CA sequences included (from the sea anemone Anthopleura elegantissima and the vestimentiferan Riftia pachyptila) grouped with the cytosolic vertebrate forms, far from the FCAs. A sequence identity matrix generated from this multiple alignment is shown in Table 2.
A second parsimony analysis shown in Figure 6 includes the CA fragment obtained from adult F. scutaria. This phylogeny generally mirrors that shown in Figure 5, and support for the cytosolic/membrane-bound branching remains very strong. There is, however, much less bootstrap support at the terminal nodes, most likely due to the masking of all N- and C-terminal ends overhanging the fragment. The F. scutaria adult CA fragment groups not with the FCAs from the larvae, but in the cytosolic clade with the other two invertebrate "symbiosis" CA sequences.
Real-time quantitative polymerase chain reaction
To monitor the amount of CA transcript in F. scutaria larvae during the onset of symbiosis, QPCR was performed on cDNA from symbiotic and aposymbiotic larvae, 4–7 days old, collected in 1998 and 2000 (Fig. 7). CA-specific primers used in the QPCR reactions did not differentiate between FCA-a and FCA-b. No significant difference in CA expression was found between symbiotic and aposymbiotic larvae (P > 0.05).
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Figure 7. Real-time quantitative PCR was used to monitor the expression level of Fungia scutaria CA in symbiotic (sym) and aposymbiotic (apo) larvae. Larvae collected in 1998 were used for the 4-day post-infection time point, and larvae collected in 2000 were used for the 1-day and 3-day post-infection time points. The 2-day post-infection time point is a mixture of samples from 1998 and 2000. Expression was evaluated relative to the expression level of the housekeeping gene actin. Relative expression level is equal to the inverse of the difference between the cycle # at mid-log phase amplification of FCA and the cycle # at mid-log phase amplification of actin. Number of replicates is given in parentheses. No significant difference in relative expression level was found between infection states at any time point tested (P > 0.05, unpaired student’s t test, Welch corrected).
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Discussion
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The two cDNA sequences isolated from aposymbiotic 5-day-old Fungia scutaria larvae are almost identical, differing only at their 3' ends. Both contain complete open reading frames including start and stop codons, poly-adenylation signals, and polyA tails consisting of 200+ adenine residues. The deduced amino acid sequences contain a conserved domain for eukaryotic carbonic anhydrase (CA) activity and show significant similarity to vertebrate -CA sequences. Percent residue identities garnered from sequence alignments are well above the similarity threshold t of 24.8% (>80 residues), implying structural homology with -CAs (Sander and Schneider, 1991).
Interestingly, both sequences of F. scutaria CA (FCA) are also short, lacking up to 80 amino acids from the C-terminus. And although the active site appears fully conserved and contains the three critical histidine residues that bind the zinc cofactor, amino acids important to the activity of the enzyme are missing. These missing residues are part of a network of hydrogen-bonding amino acids (indirect-ligands) that support the active site structure and are crucial for catalytic activity (McCall et al., 2000). Zinc affinity has been shown to decrease in CA molecules in which a portion of the indirect-ligands have been eliminated (Kiefer et al., 1995). We could find no other example of a similarly sized -CA in the sequence databases.
Conserved motifs present in the FCA sequences indicate that the enzyme may undergo glycosylation during the translational process as well as activation by phosphorylation, common post-translational modifications that occur in many carbonic anhydrases (Narumi and Miyamoto, 1974). The presence of a highly conserved ATP/GTP binding site (P-loop) is surprising as this is not a motif ordinarily found in CA; the consensus pattern, [AG]XXXXGK[TS], is not present in any of the CA sequences used to construct the phylogenetic tree. P-loops are found in many unrelated protein families including kinases, ras proteins, elongation factors, and ATP synthases (Saraste et al., 1990). A P-loop interacts with the phosphoryl group of a bound nucleotide and requires a divalent metal ion (like Mg2+) to function. The metal ion forms a complex with the nucleotide, which enhances the specificity of the enzyme. Of course, biochemical data are needed to determine whether the P-loop motif in this sequence is functional.
The FCA sequences do not clearly belong to any of the currently defined CA isoform groups. They show highest identity to the CA II isozyme in both primary (34% sequence identity with zebrafish) and secondary structure. However, the membrane-bound forms of CA come in a very close second (32% identity with mouse CA XIV), and in phylogenetic analyses, the FCA sequences consistently grouped with strong support with these isozymes. The mechanism of binding to the membrane varies among the membrane-associated forms of CA. Some forms, such as CA IV, utilize a GPI anchor; others, such as CA XIV, utilize a membrane-spanning domain (Stams et al., 1996; Mori et al., 1999). A GPI anchor motif is not present in the FCA deduced amino acid sequences although there is a likely transmembrane domain (Fig. 3).
While the FCA sequences do not clearly belong to any particular CA group, they do not group with the known "symbiosis" CA sequences from the vestimentiferan R. pachyptila or the fellow cnidarian A. elegantissima. Both of these CAs are thought to function in symbiosis as inorganic carbon (Ci) supply enzymes (Weis et al., 1989; Furla et al., 2000a; Cian et al., 2003a, b). The cytosolic CA sequences from these two animals have been linked to a mechanism that functions in supplying the symbiont with carbon. Although a probable match was isolated from adult F. scutaria, multiple attempts made to isolate the "symbiosis" isoform from F. scutaria larvae were unsuccessful. Furthermore, QPCR results showed no significant change in the levels of the FCA transcript (this study did not distinguish between FCA-a and FCA-b using QPCR) with the onset of symbiosis or with development.
These data do not rule out a role in symbiosis for the FCAs, however; and one study by Furla et al. (2000a) showed evidence of H+-ATPase (P-loop type) and membrane-bound CA involvement in Ci uptake for symbiont photosynthesis. That study suggests that bicarbonate absorption by ectodermal cells is carried out when H+ secretion, by the H+-ATPase, results in the formation of carbonic acid in the surrounding seawater. The carbonic acid is quickly dehydrated into CO2 by a membrane-bound CA; the CO2 then passively diffuses into the cell, where it is hydrated back into bicarbonate by a cytosolic CA. The fact that FCA shares similarities with other membrane-bound isozymes, including the capability to bind a membrane, and contains a potential P-loop site makes this sequence intriguing.
CA is also important in the process of calcification that results in the formation of the calcium carbonate skeleton formed by many nonsymbiotic and symbiotic cnidarians. For example, in the nonsymbiotic octocoral Leptogorgia virgulata, Lucas and Knapp (1997) show that CA plays a pivotal role in the formation of calcium carbonate spicules whether the carbon source is dissolved Ci from the environment or metabolically produced CO2. In this cnidarian, CA involved in the calcification process has been localized to the spicule vacuole membrane as well as to the inside of electron-dense Golgi-derived vesicles (Kingsley and Watabe, 1987; Lucas et al., 1996). Furla et al. (2000b) demonstrated that an intracellular CA plays a role in the absorption of metabolic Ci for calcification in the reef-building coral Stylophora pistillata, while Marshall and Clode (2003) found evidence for an extracellular CA involved in the calcification pathway in the coral Galaxea fascicularis. The larvae used in the present study are on the verge of settlement and metamorphosis, at which point they will begin deposition of their calcium carbonate skeleton; it is possible that the FCA sequences obtained from these larvae will function in this deposition process.
In conclusion, we present two -CA homologs isolated from pre-settlement scleractinian planula larvae. These sequences are atypical in length and seem most closely related to vertebrate membrane-bound forms of the enzyme. Expression of these homologs does not vary with larval symbiotic state or with development. Moreover, the deduced amino acid sequences show relatively low similarity to the other known cnidarian CA sequence, that from Anthopleura elegantissima, which functions in endosymbiont carbon supply.
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Acknowledgments
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We acknowledge field assistance during this project from Lea Hollingsworth, laboratory assistance from Wendy Phillips, and editorial assistance from the members of the Weis Lab. This research was funded by a grant from the National Science Foundation (IBN: 0342585) to V.M.W. This is contribution #1217 from the Hawaii Institute of Marine Biology.
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Footnotes
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Received 3 October 2005; accepted 8 February 2006.
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Abstract
Introduction
