Frühe Schritte in der Evolution eines Organells – Wie einzellige Wirtszellen ihre bakteriellen Endosymbionten zähmen
Zellbiologie
Zusammenfassung der Projektergebnisse
The endosymbiotic acquisition of mitochondria and plastids profoundly impacted eukaryote evolution. However, the ancient origin of these organelles impairs a profound understanding of the order and timing of events in the utterly complex process of organellogenesis. Focus of this Emmy Noether project was thus, to explore the molecular mechanisms underlying host/endosymbiont interaction in more recently established endosymbiotic associations with tightly integrated bacterial endosymbionts. This one-year extension of the program, specifically focused on the trypanosomatid Angomonas deanei (subfamily Strigomonadinae) for which we had previously developed protocols allowing for transgene expression and targeted gene knock-outs (KOs) that render this system an efficient, genetically tractable endosymbiosis model. All members of the Strigomonadinae carry a reduced β-proteobacterial endosymbiont which supplies the host with diverse metabolites, divides at a specific stage of the host cell cycle, and is vertically transmitted from one host generation to the next. This one-year extension of the program enabled us to complete a detailed study on nucleus-encoded, endosymbiont-targeted proteins (ETPs) in A. deanei. By a combination of protein mass spectrometry and expression of fluorescent fusion constructs, we identified seven host proteins that are recruited to distinct sites in/at the endosymbiont. Two ETPs of unknown function traffic to the endosymbiont cytosol and are additionally found in the host cell Golgi which probably functions as a transport intermediate. Three ETPs form a ring around the endosymbiont division site that remarkably resembles in shape and predicted protein function organellar division machineries. KO cell lines for one of the proteins in this putative hostcontrolled endosymbiont division machinery, in deed showed a clear division phenotype. The implementation of different conditional genetic tools that would further facilitate functional studies of ETPs has been hindered so far, likely due to permeability issues of the chemical ligands used. An interesting side result was that an ornithin cyclodeaminase encoded by a nuclear gene resulting from endosymbiotic gene transfer did not co-purify with the endosymbiont, but found a new localization in the glycosome, likely enabling glycosomal proline production. We hypothesize that recalibration of the metabolic capacity of the glycosomes that are closely associates with the endosymbiont helps to supply the endosymbiont with metabolites it is auxotroph for and thus, supports the metabolic integration of the endosymbiont. In sum, the data strongly support the emerging pattern that protein import evolves early during endosymbiosis providing the host with control over an endosymbiont and not as a consequence of EGT to enable re-import of the products of transferred genes. To explore the evolution of the capacity of specific proteins to interact with the endosymbiont we studied ETP orthologs from other trypanosomatids for amino acid sequence conservation and their subcellular localization when heterologously expressed in A. deanei. We found that targeting capacity can be a pre-existing property of the proteins or newly evolve by sequence divergence. To scrutinize the requirements for targeting capacity of ETPs further, we developed protocols for stable genomic insertions also in Strigomonas culicis, another member of the Strigomonadinae.
Projektbezogene Publikationen (Auswahl)
- (2019) Mikrobielle Symbiosen und die Evolution neuer Organellen. BIOspektrum 25: 268-270
Oberleitner, L., Ehret, G., and Nowack, E.C.M.
(Siehe online unter https://doi.org/10.1007/s12268-019-1043-9) - (2019) Putative sponge biomarkers in unicellular Rhizaria question an early rise of animals. Nature Ecol Evol 3: 577-581
Nettersheim, B., Brocks, J., Schwelm, A., Hope, J., Not, F., Lomas, M., Schmidt, C., Schiebel, R., Nowack, E.C.M., De Deckker, P., Pawlowski, J., Bowser, S., Bobrovskiy, I., Zonneveld, K., Kucera, M., Stuhr, M., and Hallmann, C.
(Siehe online unter https://doi.org/10.1038/s41559-019-0806-5) - (2020) The puzzle of metabolite exchange and identification of putative octotrico peptide repeat expression regulators in the nascent photosynthetic organelles of Paulinella chromatophora. Front Microbiol 11, 607182
Oberleitner, L., Poschmann, G., Macorano, L., Schott-Verdugo, S., Gohlke, H., Stühler, K., and Nowack, E.C.M.
(Siehe online unter https://doi.org/10.3389/fmicb.2020.607182) - (2021) Amoeba genome reveals dominant host contribution to plastid endosymbiosis. Mol Biol Evol. 38: 344-357
Lhee, D., Lee, J-M., Ettahi, K., Cho, C.H., Ha, J.-S., Chan, Y.-F., Zelzion, U., Stephens, T.G., Price, D.C., Gabr, A., Nowack, E.C.M., Bhattacharya, D., and Yoon, H.S.
(Siehe online unter https://doi.org/10.1093/molbev/msaa206)
