Apicomplexan apicortins possess a long disordered N-terminal extension

p25alpha Domain-Containing Proteins of Apicomplexans and Related Taxa

TPPP (tubulin polymerization promoting protein)-like proteins contain one or more p25alpha (Pfam05517) domains. TPPP-like proteins occur in different types as determined by their length (e.g., long-, short-, truncated-, and fungal-type TPPP) and include the protein apicortin, which possesses another domain, doublecortin (DCX, Pfam 03607). These various TPPP-like proteins are found in various phylogenomic groups. In particular, short-type TPPPs and apicortin are well-represented in the Myzozoa, which include apicomplexans and related taxa, chrompodellids, dinoflagellates, and perkinsids. The long-, truncated-, and fungal-type TPPPs are not found in the myzozoans. Apicortins are found in all apicomplexans except one piroplasmid species, present in several other myzozoans, and seem to be correlated with the conoid and apical complex. Short-type TPPPs are predominantly found in myzozoans that have flagella, suggesting a role in flagellum assembly or structure.

Apicortin, a Constituent of Apicomplexan Conoid/Apical Complex and Its Tentative Role in Pathogen-Host Interaction

In 2009, apicortin was identified in silico as a characteristic protein of apicomplexans that also occurs in the placozoa, Trichoplax adhaerens. Since then, it has been found that apicortin also occurs in free-living cousins of apicomplexans (chromerids) and in flagellated fungi. It contains a partial p25-α domain and a doublecortin (DCX) domain, both of which have tubulin/microtubule binding properties. Apicortin has been studied experimentally in two very important apicomplexan pathogens, Toxoplasma gondii and Plasmodium falciparum. It is localized in the apical complex in both parasites. In T. gondii, apicortin plays a key role in shaping the structure of a special tubulin polymer, conoid. In both parasites, its absence or downregulation has been shown to impair pathogen–host interactions. Based on these facts, it has been suggested as a therapeutic target for treatment of malaria and toxoplasmosis.

Interaction of Plasmodium falciparum apicortin with α- and β-tubulin is critical for parasite growth and survival

Cytoskeletal structures of Apicomplexan parasites are important for parasite replication, motility, invasion to the host cell and survival. Apicortin, an Apicomplexan specific protein appears to be a crucial factor in maintaining stability of the parasite cytoskeletal assemblies. However, the function of apicortin, in terms of interaction with microtubules still remains elusive. Herein, we have attempted to elucidate the function of Plasmodium falciparum apicortin by monitoring its interaction with two main components of parasite microtubular structure, α-tubulin-I and β-tubulin through in silico and in vitro studies. Further, a p25 domain binding generic drug Tamoxifen (TMX), was used to disrupt PfApicortin-tubulin interactions which led to the inhibition in growth and progression of blood stage life cycle of P. falciparum.

On the TPPP-like proteins of flagellated fungi

TPPP-like proteins, exhibiting microtubule stabilizing function, constitute a eukaryotic superfamily, characterized by the presence of the p25alpha domain. TPPPs in the strict sense are present in animals except Trichoplax adhaerens, which instead contains apicortin where a part of the p25alpha domain is combined with a DCX domain. Apicortin is absent in other animals and occurs mostly in the protozoan phylum, Apicomplexa. A strong correlation between the occurrence of p25alpha domain and that of the eukaryotic cilium/flagellum was suggested. Species of the deeper branching clades of Fungi possess flagellum but others lost it thus investigation of fungal genomes can help testing of this suggestion. Indeed, these proteins are present in early branching Fungi. Both TPPP and apicortin are present in Rozellomycota (Cryptomycota) and Chytridiomycota, TPPP in Blastocladiomycota, apicortin in Neocallimastigomycota, Monoblepharomycota and the non-flagellated Mucoromycota. Beside the "normal" TPPP occurring in animals, a special, fungal-type TPPP is also present in Fungi, in which a part of the p25alpha domain is duplicated. Dikarya, the most developed subkingdom of Fungi, lacks both flagellum and TPPPs. Thus it is strengthened that each ciliated/flagellated organism contains p25alpha domain-containing proteins while there are very few non-flagellated ones where p25alpha domain can be found.

Targeted repression of Plasmodium apicortin by host microRNA impairs malaria parasite growth and invasion

Mature human erythrocytes contain a rich pool of microRNAs (miRNAs), which result from differentiation of the erythrocytes during the course of haematopoiesis. Recent studies have described the effect of erythrocytic miRNAs on the invasion and growth of the malaria parasite Plasmodium falciparum during the asexual blood stage of its life cycle. In this work, we have identified two erythrocytic miRNAs, miR-150-3p and miR-197-5p, that show favourable in silico hybridization with Plasmodium apicortin, a protein with putative microtubule-stabilizing properties. Co-expression of P. falciparum apicortin and these two miRNAs in a cell line model resulted in downregulation of apicortin at both the RNA and protein level. To create a disease model of erythrocytes containing miRNAs, chemically synthesized mimics of miR-150-3p and miR-197-5p were loaded into erythrocytes and subsequently used for invasion by the parasite. Growth of the parasite was hindered in miRNA-loaded erythrocytes, followed by impaired invasion; micronemal secretion was also reduced, especially in the case of miR-197-5p. Apicortin expression was found to be reduced in miRNA-loaded erythrocytes. To interpret the effect of downregulation of apicortin on parasite invasion to host erythrocytes, we investigated the secretion of the invasion-related microneme protein apical membrane antigen 1 (AMA1). AMA1 secretion was found to be reduced in miRNA-treated parasites. Overall, this study identifies apicortin as a novel target within the malaria parasite and establishes miR-197-5p as its miRNA inhibitor. This miRNA represents an unconventional nucleotide-based therapeutic and provides a new host factor-inspired strategy for the design of antimalarial molecular medicine.

This article has an associated First Person interview with the first author of the paper.

Summary: The role of host erythrocyte microRNA in the downregulation of malaria parasite gene expression is investigated. Two microRNAs are identified, miR-197-5p and miR-150-3p, which affect parasite growth and invasion when enriched in erythrocytes.

The dense granule protein 8 (GRA8) is a component of the sub-pellicular cytoskeleton in Toxoplasma gondii

After host cell invasion, Toxoplasma secretes a variety of dense granule proteins (GRA proteins) from its secretory dense granules, which are involved in the biogenesis of the parasitophorous vacuole (PV). TgGRA8_I is predicted to contain proline-rich domains, which are structural features of some cytoskeleton-related proteins. In agreement with this observation, previous proteomic analyses revealed the presence of TgGRA8_I in the Toxoplasma sub-pellicular cytoskeleton. In the present study, we show (1) by docking analyses that TgGRA8_I may interact with both Toxoplasma β-tubulin and actin; (2) by immunoelectron microscopy, proteomic, biochemical, and cellular approaches that TgGRA8_I associates with sub-pellicular microtubules and actin at the parasite sub-pellicular cytoskeleton; (3) that type I parasites (RH strain) lacking the GRA8 gene (RHΔku80Δgra8) exhibit loss of conoid extrusion, diminished cell infection, and egress capabilities, and that these motility impairments were likely due to important alterations in their sub-pellicular cytoskeleton, in particular their sub-pellicular microtubules and meshwork. Parasites lacking the GRA4 gene (RHΔku80Δgra4) did not show modifications in the organization of the sub-pellicular cytoskeleton. Collectively, these results demonstrated that TgGRA8_I is a dense granule protein that, besides its role in the formation of the PV, contributes to the organization of the parasite sub-pellicular cytoskeleton and motility. This is the first proline-rich protein described in the Toxoplasma cytoskeleton, which is a key organelle for both the parasite motility and the invasion process. Knowledge about the function of cytoskeleton components in Toxoplasma is fundamental to understand the motility process and the host cell invasion mechanism. Refining this knowledge should lead to the design of novel pharmacological strategies for the treatment against toxoplasmosis.

Does apicortin, a characteristic protein of apicomplexan parasites and placozoa, occur in Eumetazoa?

Apicortin is a characteristic protein of apicomplexan parasites which has recently been identified in their free-living cousins, chromerids as well. The placozoan Trichoplax adhaerens is the only animal possessing this protein and apicortin is one of its most abundant proteins. The recently published transcriptome of the cnidarian Porites astreoides contains an apicortin-like sequence. Other cnidarians do not have it, thus it is its first occurrence not only in this phylum but also in Eumetazoa. However, its translated amino acid sequence is more similar to apicomplexan apicortins than to that of T. adhaerens, the GC ratio is much higher than either the genome-wide GC ratio of P. astreoides or that of the placozoan apicortin gene, and phylogenetic analyses suggest that this apicortin has an apicomplexan origin. Although these data might be indicative for a horizontal gene transfer event, we should be cautious to state it; it is more probable that it is a contamination from a gregarine, a marine Apicomplexa. Thus T. adhaerens remains the only animal where the presence of apicortin is proved.

Wider than Thought Phylogenetic Occurrence of Apicortin, A Characteristic Protein of Apicomplexan Parasites

Apicomplexan parasites cause serious illnesses, including malaria, in humans and domestic animals. The presence of apicortins is predominantly characteristic of this phylum. All the apicomplexan species sequenced contain an apicortin which unites two conserved domains: DCX and partial p25alpha. This paper identifies novel apicortin orthologs in silico and corrects in several cases the erroneous sequences of hypothetical apicortin proteins of Cryptosporidium, Eimeria, and Theileria genera published in databases. Plasmodium apicortins, except from Plasmodium gallinaceum, differ significantly from the other apicomplexan apicortins. The feature of this ortholog suggests that only orthologs of Plasmodiums hosted by mammals altered significantly. The free-living Chromerida, Chromera velia, and Vitrella brassicaformis, contain three paralogs. Their apicomplexan-type and nonapicomplexan-type apicortins might be "outparalogs." The fungal ortholog, Rozella allomycis, found at protein level, and the algal Nitella mirabilis, found as Transcriptome Shotgun Assembly (TSA), are similar to the known Opisthokont (Trichoplax adhaerens, Spizellomyces punctatus) and Viridiplantae (Nicotiana tabacum) ones, since they do not contain the long, unstructured N-terminal part present in apicomplexan apicortins. A few eumetazoan animals possess apicortin-like (partial) sequences at TSA level, which may be either contaminations or the result of horizontal gene transfer; in some cases the contamination has been proved.

A new protein superfamily: TPPP-like proteins

The introduction of the term ‘Tubulin Polymerization Promoting Protein (TPPP)-like proteins’ is suggested. They constitute a eukaryotic protein superfamily, characterized by the presence of the p25alpha domain (Pfam05517, IPR008907), and named after the first identified member, TPPP/p25, exhibiting microtubule stabilizing function. TPPP-like proteins can be grouped on the basis of two characteristics: the length of their p25alpha domain, which can be long, short, truncated or partial, and the presence or absence of additional domain(s). TPPPs, in the strict sense, contain no other domains but one long or short p25alpha one (long- and short-type TPPPs, respectively). Proteins possessing truncated p25alpha domain are first described in this paper. They evolved from the long-type TPPPs and can be considered as arthropod-specific paralogs of long-type TPPPs. Phylogenetic analysis shows that the two groups (long-type and truncated TPPPs) split in the common ancestor of arthropods. Incomplete p25alpha domains can be found in multidomain TPPP-like proteins as well. The various subfamilies occur with a characteristic phyletic distribution: e. g., animal genomes/proteomes contain almost without exception long-type TPPPs; the multidomain apicortins occur almost exclusively in apicomplexan parasites. There are no data about the physiological function of these proteins except two human long-type TPPP paralogs which are involved in developmental processes of the brain and the musculoskeletal system, respectively. I predict that the superfamily members containing long or partial p25alpha domain are often intrinsically disordered proteins, while those with short or truncated domain(s) are structurally ordered. Interestingly, members of this superfamily connected or maybe connected to diseases are intrinsically disordered proteins.