An apicoplast localized GTPase is essential for Toxoplasma gondii survival

Abstract

The apicoplast is an essential organelle found in Apicomplexa, a large phylum of intracellular eukaryotic pathogens. The apicoplast produces metabolites that are utilized for membrane biogenesis and energy production. A majority of apicoplast resident proteins are encoded by the nuclear genome and are trafficked to the apicoplast, and are referred to as Nuclear Encoded and Apicoplast Targeted (NEAT) proteins. In this study, we characterized a NEAT protein named TgBipA, that is a homolog of the highly conserved prokaryotic translational GTPase BipA. BipA is essential for bacterial survival in stress conditions and functions through interactions with the prokaryotic ribosome, although its role is not fully understood. Through genetic knockouts of TgBipA and immunofluorescence imaging we show that loss of TgBipA results in apicoplast genome replication defects, disruption of NEAT trafficking, loss of the apicoplast, and subsequently parasite death. Furthermore, we show through comparative studies that this phenotype closely resembles the delayed death phenomenon observed when inhibiting apicoplast translation. Finally, we show that TgBipA is an active GTPase in vitro, and its GTP hydrolysis activity is critical for its cellular function. Our findings demonstrate TgBipA is a GTPase which has an essential role in apicoplast maintenance, providing new insight into the cellular processes of the organelle. Importance StatementToxoplasma gondii, and many other parasites in the phylum Apicomplexa, are pathogens with significant medical and veterinary importance. Most Apicomplexa contain a non-photosynthetic plastid organelle named the apicoplast. This organelle produces essential metabolites and perturbation of apicoplast function results in parasite death. The apicoplast contains bacterial-like pathways for apicoplast genome replication and expression. Thus, the discovery of the apicoplast lead to optimism that this organelle would provide a wealth of anti-parasitic drug targets. Therefore, the identification and characterization of new apicoplast proteins could provide new opportunities for therapeutic development. In this study we characterized the function of a protein called TgBipA, a homolog of a highly conserved bacterial GTPase bipA which has been implicated in maturation of the 50S ribosomal subunit and adaptation to cellular stress. We show that TgBipA is essential for apicoplast maintenance and parasite survival.