TPPP-like proteins contain one or more p25alpha domains. They obtained their name after tubulin polymerization promoting protein (TPPP1), the first identified member of this protein family. Originally, it was named p25alpha protein, which became the eponym of the domain. Myzozoans are a monophyletic clade, and a sister clade to the Ciliata, within Alveolata.
It is known that TPPP binds to tubulin, promotes its polymerization into microtubules, and stabilizes them . This function is conserved in animals from sponges to mammals . Tubulin-based cytoskeletal elements of apicomplexans have distinct traits, which may be related to the unique biology of these parasites . In principle, TPPP-like proteins can interact with and stabilize any of these microtubular elements, which has been confirmed experimentally.
The apical complex can be divided into three components, namely, the apical cap, the conoid, and the secretory organelles, micronemes, and rhoptries . The conoid, which plays an important role in the host cell invasion, is best described from the apicomplexan families Eimeriidae and Sarcocystidae (Sarcocystis, Toxoplasma, Neospora, etc.). Apicortin seems to be in strict relation with this organelle. In T. gondii, apicortin was shown to be localized exclusively at the conoid and is essential for providing its correct structure and function . Tubulin disappeared or was present in a reduced amount in the apical complex of an apicortin-null mutant, while microtubules were normal . The deletion of apicortin resulted in shorter and misshaped conoids . These defects were reversed by the expression of the apicortin coding sequence. Apicortin possesses two microtubule-binding domains (a partial p25alpha and a DCX), which stabilize and bundle microtubules . Thus, apicortin is an ideal protein for stabilizing the tubulin-based conoid fibers. This stabilizing effect of apicortin was shown in a reconstituted in vitro system as well .
In P. falciparum, which has no conoid, apicortin is localized at the apical end and may be involved in the formation of the apical complex . In both T. gondii and P. falciparum, the downregulation of apicortin leads to impaired host cell invasion . Interestingly, in mammalian, but not in avian, Plasmodium parasites, the p25alpha domain is degenerated, i.e., some otherwise conservative amino acids are missing. At this moment, it is not clear yet whether the presence of a conoid-like structure in the ookinete stage of the avian parasite P. gallinaceum  is in connection with these sequence differences, or whether it happened accidentally, and a similar structure will also be found in mammalian parasites in the future. However, this altered sequence does not affect the ability of P. falciparum apicortin to bind to tubulin; in silico docking showed that the amino acids of the p25alpha domain are involved in the binding .
TPPP-like proteins occur predominantly in species that are flagellated. The eukaryotic flagellum is a microtubule-based organelle; thus, the connection is not surprising. Although the correlation between the incidence of flagellum and the p25alpha domain is strong , there is only some evidence for the functional relationship between TPPP-like proteins and the flagellum. They are related to short-type TPPPs. The connection was first shown in Chlamydomonas reinhardtii, a biflagellate green alga . Its TPPP ortholog, FAP265 protein, can be found in the flagella, and is indispensable in their formation, as proven by using FAP265 null mutants . Myzozoan species are flagellated; however, apicomplexans are in a special position in this respect . The flagella present in the common ancestor of the Myzozoa (and Alveolata) were lost during evolution. In many parasitic apicomplexans, flagella are only found in male microgametes with exceptions, such as Piroplasmida and the Cryptosporidium genus, where no flagellum occurs. Flagellum formation in apicomplexans does not require the so-called intraflagellar transport proteins, but all elements of the flagellar apparatus assemble directly within the cytoplasm in a process termed exflagellation .
There are examples for the flagellum-connected role of short-type TPPPs in Apicomplexa. In Plasmodium genus, P. falciparum short-type TPPP (PFL1770c; XP_001350760) was shown to be essential for gametocytogenesis, using piggyBac transposon-mediated insertional mutagenesis, but the mutant failed to form mature gametocytes . Recently, it has also been found that the TPPP (Py05543; EAA17578) is necessary for male gametocyte exflagellation in Plasmodium yoelli . Py05543− KO parasites, obtained by CRISPR/Cas9-mediated genome editing, were deficient in the exflagellation of male gametes by observing deficient exflagellation center formation .
In this respect, it is noteworthy that Cryptosporidium species, which lack flagella , do not have short-type TPPPs. This can be interpreted as meaning that the parasite does not need this protein. A similar loss was described in Cryptosporidium for δ- and ε-tubulin genes/proteins, which are marker proteins for the flagellum and basal bodies . Piroplasmids also have no flagella, not even in microgametes . Their short-type TPPPs are the shortest among all short-type TPPPs. The lengths of TPPP of Theileria annulata, B. microti, and B. bovis are 121, 123, and 124 amino acids, respectively. The length of most short-type TPPPs is approximately 140–150 amino acids. Furthermore, among all apicomplexan TPPPs, the piroplasmid TPPPs are the most divergent. With the loss of the flagellum, their function may have been lost, so they evolved faster, and perhaps acquired a new function.
Researchers find very different proteins in this group. Their length varies between 232 and 2627 amino acids. Interestingly, there are two short-type p25alpha domains in all of them. Sometimes, they do not have another domain; this type of protein is not found in apicomplexans and colpodellids, but is present in chromerids, perkinsids, and dinoflagellates. There is such a protein also in D. oweni; this species was previously classified as an apicomplexan, but according to the latest phylogenetic classification, it belongs to a separate group, the squirmids, which are sisters to (apicomplexans and chrompodellids) . The fact that D. oweni has this type of protein (two p25alpha domains and no other domains) fits into this picture; i.e., D. oweni is not an apicomplexan. The function of these proteins is unknown; it would be expected that, similar to short-type TPPPs and apicortins, they exert their effect by binding to a tubulin polymer.
Only in dinoflagellates can find larger proteins consisting of 1–2 thousand amino acids that contain more than three domains, including catalytic ones. For these proteins, the additional domains determine the function, e.g., adenylate cyclase or methyltransferase activity. The role of the p25alpha domains is unknown; however, there are cases when the importance of the tubulin-binding ability is likely, e.g., in the alpha-tubulin suppressor domain-containing protein CAI3986388 (Cladocopium goreaui). In the narrow sense, I do not consider these proteins to be TPPP-like ones.
Among the TPPP-like proteins, short-type TPPPs and apicortins occur in all myzozoan phyla. Both experimental results and phylogenetic considerations indicate that these tubulin-binding proteins play an important role in the structure and/or function of the flagellum and the apical complex/conoid, respectively. Moreover, an evolutionary connection between the flagellum and apical complex was proposed . Recent results suggest that the conoid complex evolved from flagellar components  or the flagellar root apparatus ; that is, ‘the apical complex is the most conspicuously retained element of the associated flagellar root structures’ .
In light of this, it is not surprising that short TPPPs are also common in Ciliata, the sister phylum of Myzozoa, since their surface is covered by hundreds of cilia; they usually contain several paralogs of short-type TPPPs. However, they do not have apicortin, and they do not possess conoid or any similar structural elements. The short-type TPPP is also present in other protists (Euglenozoa and Chlorophyta), which are also flagellated. Myzozoans are flagellated, but in apicomplexans, only the male gametes have flagellum/flagella in some species. If not, then the loss of the flagellum resulted in either the loss of TPPPs (Cryptosporidium) or a degenerate protein significantly diverging from other TPPPs (Piroplasmida). The conoid is also absent from some apicomplexans (Piroplasmida and Haemosporida), but despite this, apicortin is present in almost all of them. There can be several reasons for this. On the one hand, conoid-like structures are also found in some of these species , and on the other hand, the apical polar ring is also present in these cases; furthermore, since these species once possessed a conoid, the apicortin remained as a relic. The only exception is B. microti, which has an extremely small genome  and lacks the ‘redundant’ protein.
It cannot be ignored the fact that apicortin is also found in some Opisthokonta (flagellated fungi, the placozoan T. adhaerens) , which do not have a conoid or similar structure. These apicortins differ from apicomplexan orthologs in that, although they have a partial p25alpha domain and the entire DCX domain, the opisthokont apicortin lacks the long, disordered N-terminal region , which appears to be necessary for the formation of the proper conoid structure . Their function is probably to stabilize tubulin polymers/microtubules in these species as well; this is also indicated by the fact that apicortin is the only TPPP-like protein in T. adhaerens and some flagellated fungi . Since apicortin is present only in these few primitive opisthokonts apart from Myzozoa, it is possible that the opisthokonts acquired this protein from myzozoans by horizontal gene transfer. This is supported by the high degree of sequence identity and similarity between apicomplexan and chromerid apicortins on the one hand, and Trichoplax and fungal apicortins on the other, which can reach 53% and 67%, respectively, which is roughly the same value as the similarity of apicomplexan apicortins to each other. The similarity exists not only between the domain sequences, but also in the interdomain linker. Since the last common ancestor of myzozoans and opisthokonts could be close to the first eukaryote, it is extremely unlikely that the gene remained so conserved for that much time, and horizontal gene transfer is the best explanation.
A common feature of TPPP-like proteins is that they contain one or more p25alpha domains . They occur only in eukaryotes, and like their name indicates, they are tubulin-binding proteins . This trait is conserved in animals, as it is present in everything from sponges to humans . The phylogenetic distribution of TPPP-like proteins is widespread in protists as well, but not in plants . The role of TPPP-like proteins may be quite different in protists than in humans. Short-type TPPPs and apicortins are found in Myzozoa, a major monophyletic group of Alveolata. Although there are few experimental examples so far, short-type TPPPs have a role in the flagellar structures, interacting with tubulin polymers, and perhaps play a role in the exflagellation of microgametes . Apicortin may play a crucial role in the formation of the conoid/apical complex . Thus, TPPP-like proteins may be potential therapeutic targets for the treatment of diseases such as malaria and toxoplasmosis .