Protein Synthesis in Isolated Symbionts from the Flagellate ProtozoonCrithidia deanei1

Quantitative Proteomic Map of the Trypanosomatid Strigomonas culicis: The Biological Contribution of its Endosymbiotic Bacterium

Strigomonas culicis is a kinetoplastid parasite of insects that maintains a mutualistic association with an intracellular symbiotic bacterium, which is highly integrated into the protist metabolism: it furnishes essential compounds and divides in synchrony with the eukaryotic nucleus. The protist, conversely, can be cured of the endosymbiont, producing an aposymbiotic cell line, which presents a diminished ability to colonize the insect host. This obligatory association can represent an intermediate step of the evolution towards the formation of an organelle, therefore representing an interesting model to understand the symbiogenesis theory. Here, we used shotgun proteomics to compare the S. culicis endosymbiont-containing and aposymbiotic strains, revealing a total of 11,305 peptides, and up to 2,213 proteins (2,029 and 1,452 for wild type and aposymbiotic, respectively). Gene ontology associated to comparative analysis between both strains revealed that the biological processes most affected by the elimination of the symbiont were the amino acid synthesis, as well as protein synthesis and folding. This large-scale comparison of the protein expression in S. culicis marks a step forward in the comprehension of the role of endosymbiotic bacteria in monoxenous trypanosomatid biology, particularly because trypanosomatids expression is mostly post-transcriptionally regulated.

Development of a toolbox to dissect host-endosymbiont interactions and protein trafficking in the trypanosomatid Angomonas deanei

Background

Bacterial endosymbionts are found across the eukaryotic kingdom and profoundly impacted eukaryote evolution. In many endosymbiotic associations with vertically inherited symbionts, highly complementary metabolic functions encoded by host and endosymbiont genomes indicate integration of metabolic processes between the partner organisms. While endosymbionts were initially expected to exchange only metabolites with their hosts, recent evidence has demonstrated that also host-encoded proteins can be targeted to the bacterial symbionts in various endosymbiotic systems. These proteins seem to participate in regulating symbiont growth and physiology. However, mechanisms required for protein targeting and the specific endosymbiont targets of these trafficked proteins are currently unexplored owing to a lack of molecular tools that enable functional studies of endosymbiotic systems.

Results

Here we show that the trypanosomatid Angomonas deanei, which harbors a β-proteobacterial endosymbiont, is readily amenable to genetic manipulation. Its rapid growth, availability of full genome and transcriptome sequences, ease of transfection, and high frequency of homologous recombination have allowed us to stably integrate transgenes into the A. deanei nuclear genome, efficiently generate null mutants, and elucidate protein localization by heterologous expression of a fluorescent protein fused to various putative targeting signals. Combining these novel tools with proteomic analysis was key for demonstrating the routing of a host-encoded protein to the endosymbiont, suggesting the existence of a specific endosymbiont-sorting machinery in A. deanei.

Conclusions

After previous reports from plants, insects, and a cercozoan amoeba we found here that also in A. deanei, i.e. a member of a fourth eukaryotic supergroup, host-encoded proteins can be routed to the bacterial endosymbiont. This finding adds further evidence to our view that the targeting of host proteins is a general strategy of eukaryotes to gain control over and interact with a bacterial endosymbiont. The molecular resources reported here establish A. deanei as a time and cost efficient reference system that allows for a rigorous dissection of host-symbiont interactions that have been, and are still being shaped over evolutionary time. We expect this system to greatly enhance our understanding of the biology of endosymbiosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0820-z) contains supplementary material, which is available to authorized users.

Characterization of a porin channel in the endosymbiont of the trypanosomatid protozoan Crithidia deanei

Crithidia deanei is a trypanosomatid protozoan that harbours a symbiotic bacterium. The partners maintain a mutualistic relationship, thus constituting an excellent model for studying metabolic exchanges between the host and the symbiont, the origin of organelles and cellular evolution. According to molecular analysis, symbionts of different trypanosomatid species share high identity and descend from a common ancestor, a β-proteobacterium of the genus Bordetella. The endosymbiont is surrounded by two membranes, like Gram-negative bacteria, but its envelope presents special features, since phosphatidylcholine is a major membrane component and the peptidoglycan layer is highly reduced, as described in other obligate intracellular bacteria. Like the process that generated mitochondria and plastids, the endosymbiosis in trypanosomatids depends on pathways that facilitate the intensive metabolic exchanges between the bacterium and the host protozoan. A search of the annotated symbiont genome database identified one sequence with identity to porin-encoding genes of the genus Bordetella. Considering that the symbiont outer membrane has a great accessibility to cytoplasm host factors, it was important to characterize this single porin-like protein using biochemical, molecular, computational and ultrastructural approaches. Antiserum against the recombinant porin-like molecule revealed that it is mainly located in the symbiont envelope. Secondary structure analysis and comparative modelling predicted the protein 3D structure as an 18-domain β-barrel, which is consistent with porin channels. Electrophysiological measurements showed that the porin displays a slight preference for cations over anions. Taken together, the data presented herein suggest that the C. deanei endosymbiont porin is phylogenetically and structurally similar to those described in Gram-negative bacteria, representing a diffusion channel that might contribute to the exchange of nutrients and metabolic precursors between the symbiont and its host cell.

Cryptic paraflagellar rod in endosymbiont-containing kinetoplastid protozoa

Cilia and flagella are central to many biological processes in a diverse range of organisms. The kinetoplastid protozoa are very appealing models for the study of flagellar function, particularly in the light of the availability of extensive trypanosomatid genome information. In addition to the highly conserved 9 + 2 axoneme, the kinetoplastid flagellum contains a characteristic paraflagellar rod structure (PFR). The PFR is necessary for full motility and provides support for metabolic regulators that may influence flagellar beating. However, there is an intriguing puzzle: one clade of endosymbiont-containing kinetoplastids apparently lack a PFR yet are as motile as species that possess a PFR and are able to attach to the invertebrate host epithelia. We investigated how these organisms are able to locomote despite the apparent lack of PFR. Here we have identified a PFR1 gene in the endosymbiont-bearing trypanosome Crithidia deanei. This gene is expressed in C. deanei and is able to partially complement a pfr1 null mutation in Leishmania mexicana cells, demonstrating that the encoded protein is functional. Careful reexamination of C. deanei flagellar ultrastructure revealed a greatly reduced PFR missed by many previous analyses. This affirms the PFR as a canonical organelle of kinetoplastids. Moreover, although PFR proteins have been conserved in evolution, primary sequence differences contribute to particular PFR morphotypes characteristic of different kinetoplastid species.

Endosymbiosis in protozoa of the Trypanosomatidae family

A small number of trypanosomatids present bacterium endosymbionts in the cytoplasm, which divide synchronously with the host cell. Crithidia oncopleti, Crithidia deanei. Crithidia desouzai, Blastocrithidia culicis and Herpetomonas roitmani are the best characterized species. The endosymbiont is surrounded by two membranes separated from each other by an electron-lucent space. The presence of the endosymbiont led to the appearance of morphological changes which include the lack of the paraflagellar rod associated to the axoneme, the morphology of the kinetoplast and the association of the sub-pellicular microtubules with portions of the protozoan plasma membrane. Aposymbiotic strains could be obtained by antibiotic treatment, opening the possibility to make comparative analysis of endosymbiont-containing an endosymbiont-free populations of the same species. It is clear that metabolic cycles are established between the prokaryiont and the host cell. The results obtained show that endosymbiont-containing species of trypanosomatids constitute an excellent model to study basic processes on the endosymbiont-host cell relationship and the origin of new organelles.