Discovery of a tyrosine-rich sporocyst wall protein in Eimeria tenella
© Walker et al. 2016
Received: 4 January 2016
Accepted: 24 February 2016
Published: 2 March 2016
Eimeria is an important genus of apicomplexan parasites. A defining feature of these parasites is the oocyst, which is transmitted into the environment via the faeces of definitive hosts. The oocyst wall contains cross-linked, tyrosine-rich proteins and protects eight infectious sporozoites, housed in pairs within a second walled structure, the sporocyst. The biochemical basis for sporocyst wall formation is not known.
Here, we report the discovery of a novel tyrosine-rich protein, EtSWP1, in Eimeria tenella. Like the tyrosine-rich proteins of the oocyst wall, EtSWP1 is an intrinsically disordered protein with the tyrosine residues concentrated in a specific region of the protein, located immediately following the region of intrinsic disorder. We engineered E. tenella to express mCherry-tagged EtSWP1 and showed that the tagged protein localises specifically to sporocyst walls, indicating that the biochemistry of sporocyst wall assembly is analagous to that of oocyst walls.
Tyrosine-rich proteins are known to be key components of the oocyst wall and we now demonstrate, using gene and protein analyses combined with genetic manipulation, that a novel tyrosine-rich protein is specific for the sporocyst wall. This finding is important because it shows that the biochemistry of these two distinct walls is similar and, hence, brings targeted disruption of sporulation and, therefore, potential neutralisation of oocysts in the environment, a step closer.
Apicomplexan parasites of the genus, Eimeria, cause coccidiosis in a variety of livestock and poultry; in the case of the latter, the industry loses in excess of US$2 billion dollars per year due to coccidiosis . These obligate intracellular parasites are highly contagious, due primarily to the resilient nature of the oocyst, an important developmental stage that is transmitted into the environment via the faeces of definitive hosts. The oocyst wall is the primary barrier between the harsh external environment and the infectious cargo, eight dormant sporozoites. However, these sporozoites are further housed in pairs within four sporocysts, a structure that is itself characterised by a protective barrier, the sporocyst wall.
Several oocyst wall proteins have been identified and characterised to different degrees  including: a cysteine-rich protein, EtOWP6, which is found in the outer layer of the oocyst wall and is related to a family of proteins found in other apicomplexan parasites, namely Cryptosporidium parvum and Toxoplasma gondii; a histidine-rich protein, which localises to the inner oocyst wall; and, GAM56 and GAM82, proteins that are found in wall forming bodies type 2 of macrogametocytes, contain tyrosine-rich regions, and are processed into smaller, tyrosine-rich proteins, which are incorporated into the inner layer of the oocyst wall. GAM56 and GAM82 are also the major components of a transmission blocking vaccine to prevent coccidiosis in poultry . In contrast to oocysts, little is known about the biochemical composition of the sporocyst wall of coccidian parasites  despite recent transcriptome profiling of different E. tenella developmental stages [5, 6].
Amongst the 17 genes listed in Fig. 1a, ETH_00000115 stands out; the gene model is predicted to encode a protein that has 7.1 % tyrosine residues (16 of 224 amino acids), which is more than twice that of the next highest (encoded by ETH_00025345). This is potentially significant because, like the oocyst wall, the sporocyst wall of E. tenella fluoresces blue under UV excitation (Fig. 1b,c), a feature that indicates the possible presence of cross-linked tyrosine-rich proteins . Moreover, the predicted amino acid sequence (Fig. 1d) of ETH_00000115 shows that most of these tyrosines are confined to a specific region of the protein, which is analogous to the amino acid sequences of GAM56 and GAM82.
We searched for homologues of ETH_00000115 in other Coccidia but, with the exception of a highly conserved protein in Eimeria necatrix (Fig. 1d), conventional BLASTP analysis against non-redundant protein databases failed to identify homologues of ETH_00000115, including in any other Apicomplexa. Furthermore, ETH_00000115 showed no homology to any previously described protein domains, except for a predicted N-terminal signal peptide, identified using the SignalP 4.1 Server at www.cbs.dtu.dk/services/SignalP . However, by searching for the KY-rich sequence, YKCKKAKGKGKYYKK, we were able to identify a putative homologue in Eimeria brunetti, the otherwise ED-rich region and C-terminal sequence of which are dominated by numerous poly-alanine stretches (Fig. 1d). Like ETH_00000115, the E. necatrix and E. brunetti proteins contain a distinct KY-rich region. All three proteins contain only three cysteine residues but these appear to be strategically placed, with the first near the cleavage site of the signal peptides and the other two at the start of the KY-rich regions, separated by a conserved HGYK sequence.
In both GAM56 and GAM82, the tyrosine-rich regions are also rich in serine residues whereas in ETH_00000115, the tyrosine residues are accompanied by lysine residues. Upstream of, and almost directly adjacent to the lysine and tyrosine-rich (or KY-rich) domain is a 22-residue acidic domain or “ED-rich” domain, which is composed of ten glutamic acid (E) and five aspartic acid (D) residues. Directly downstream of the KY-rich domain are two copies of an imperfect repeat, we have called Repeat 1 (Fig. 1e).
Quantitative reverse-transcriptase PCR (qRT-PCR) was used to assess stage-specific expression of ETH_00000115. The relative transcript abundance of ETH-00000115 was determined relative to the et18s small subunit ribosomal RNA for each developmental stage using chicken infection regimens and PCR protocols exactly as described previously . The forward primer was 5′-CGCTGAGGAAGAAATGGAAG-3′ and the reverse primer was 5′-TAAGTGCAAAAAGGCCAAGG-3′. Analysis by qRT-PCR confirms that ETH_00000115 transcript is highly abundant in sporulated oocysts but essentially absent in other stages, such as merozoites, gametocytes and unsporulated oocysts (Fig. 1f).
The tyrosine-rich oocyst wall protein, GAM56 is known to possess significant regions of intrinsic disorder within its structure, which is dominated by random coils, with some helices but few sheets/strands . Two repercussions of this are predicted : first, intrinsically disordered proteins are susceptible to proteolytic cleavage, as observed for GAM56; and, second, the inherent flexibility of disordered proteins may increase the possibility of tyrosine residues from individual tyrosine-rich proteins coming into the proximity of each other more readily, thereby facilitating crosslinking. We used DISOPRED3  to analyse the amino acid sequences of EtSWP1, EnSWP1 and EbSWP1 to determine if these proteins possess similar secondary structures to GAM56; DISOPRED3 has been used recently, with significant success, to identify regions of intrinsic disorder in the structure of apicomplexan proteins . We found that all three SWP1s were dominated by coil structures, with some helices and very little sheet/strand structures (Additional file 1: Figure S1). Analyses using I-Tasser [16–19] generated similar predictions (Additional file 2: Figure S2). DISOPRED3 also predicted significant regions of intrinsic disorder within all the SWP1s, most notably in the ED-rich region preceding the KY-rich sequence (Additional file 3: Figure S3); GAM56 is, likewise, intrinsically disordered just prior to the tyrosine-rich region. We therefore predict that SWP1 behaves similarly to GAM56 in wall assembly.
BLASTP searches also failed to identify any homologues of EtSWP1 in the cyst forming coccidians, Toxoplasma gondii, Neospora caninum or Hammondia hammondi, but, by searching for proteins with similar percentages of lysine, tyrosine, glutamic acid and aspartic acid, three sequences (TGME49_037080, NCLIV_050960 and HHA_237080) that are highly conserved across these three parasites were discovered (each with ~21 % identity with EtSWP1). However, TGME49_037080 has already been identified as more likely to be a component of the oocyst wall and not the sporocyst wall of T. gondii by transcriptomic  and proteomic  analyses.
There are other tyrosine-rich proteins indicated by transcriptomic analysis of T. gondii oocysts as being potential components of the sporocyst wall  but the localisation of these proteins specifically to the sporocyst wall has not been confirmed. The discovery that EtSWP1 is targeted specifically to the sporocyst wall is important because, in combination with the recent finding that the sporocyst wall contains cysteine-rich proteins , it shows that the construction of the sporocyst wall is biochemically similar to the oocyst wall despite being assembled via a different cell biology mechanism; in contrast to oocyst wall formation, which occurs via sequential, coordinated migration and disaggregation of veil forming bodies, wall forming bodies type 1 and wall forming bodies type 2 , the wall of the sporocyst forms via condensation of cytoplasmic material not contained in defined organelles or vesicles [22, 24]. Our discovery of a tyrosine-rich protein in the sporocyst wall brings targeted disruption of sporulation and, therefore, potential neutralisation of oocysts in the poultry house, a step closer.
Animal experimentation was conducted under protocols approved by the University of Technology, Sydney Animal Ethics Committee (protocol numbers 2008–96 and 2008–188) under a protocol (registration number 2012-11-9) and/or by the ethics committee, CEEA VdL, and according to French legislation (French Government Decree 2001–464) and EEC regulations (86/609/CEE).
We are grateful to the staff of the Ernst Facility at the University of Technology, Sydney, for their care of the chickens used in this study. NCS gratefully acknowledges funding provided by the Australian Research Council for Discovery Project DP0664013. NCS and ABH are thankful for funding from Bellberry Limited. NCS and FB were recipients of an Australian Government Department of Innovation, Industry, Science and Research International Science Linkage grant (FR110089). RAW was supported by a Swiss Government Excellence Postdoctoral Scholarship from the Swiss Confederation. None of the funding bodies played any role in: the design, collection, analysis, or interpretation of data; the writing of the manuscript; or the decision to submit the manuscript for publication.
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