Résumé
A single beta-proteobacterial endosymbiont, Candidatus Kinetoplastibacterium crithidii, resides in the cytosol of the trypanosomatid Angomonas deanei and divides at a defined stage of its host's cell cycle. This endosymbiont has a highly reduced genome of 0.8 Mb and, notably, has lost most essential bacterial division genes, resulting in a loss of division autonomy. It has been previously demonstrated that a host-encoded dynamin-like protein, endosymbiont-targeted host protein (ETP)9, plays an indispensable role in the division of the endosymbiont. In this study, we identified a second nucleus-encoded component of the endosymbiont division machinery, termed ETP2, currently annotated as a "hypothetical protein." We observed that ETP2 arrives before ETP9 at the bacterial division site. ETP2 deletion or depletion results in division phenotypes with long, filamentous endosymbionts accompanied by severely distorted host cells or host daughter cells lacking endosymbionts. We found that ETP2 depletion results in mis-localization of ETP9, whereas ETP2 localization to the endosymbiont division site is independent of ETP9. In silico analyses revealed that ETP2 is found exclusively in endosymbiont-harboring trypanosomatids of the subfamily Strigomonadinae and is most likely an intrinsically disordered protein. Collectively, our data suggests that ETP2 is an integral component of the endosymbiont division machinery involved in recruiting ETP9 to the division site. This finding highlights the evolution of a complex host-derived molecular mechanism that exerts tight control over its endosymbiont without requiring gene transfers from the bacterium.IMPORTANCEThe ancient uptake and transformation of free-living bacteria into eukaryotic organelles involved extensive structural, physiological, and genetic changes. More recently established endosymbioses offer a unique opportunity to observe intermediate stages in the complex process by which a prokaryote becomes genetically integrated into a eukaryotic cell. Hence, studying the molecular mechanisms that govern host-endosymbiont interactions holds the potential for uncovering the scenarios and molecular processes behind organelle formation. The trypanosomatid Angomonas deanei has been recently reported to manifest nuclear control over its endosymbiont's division. In this study, we identified and characterized a new nucleus-encoded component of the endosymbiont division machinery. This study further supports that a novel intermediate between endosymbiont and organelle evolved in A. deanei and provides new leverage to entangle the evolution of its fascinating nucleus-controlled endosymbiont division machinery.