Coordinated action of multiple transporters in the acquisition of essential cationic amino acids by the intracellular parasite Toxoplasma gondii.

Stephen J Fairweather,Malcolm J Mcconville,Martin Blume,Esther Rajendran,Birte Steinhöfel,Stefan Bröer,Kiran Javed,Giel G Van Dooren,Kiaran Kirk,Dominique Soldati-Favre

doi:10.1371/journal.ppat.1009835

Abstract

Intracellular parasites of the phylum Apicomplexa are dependent on the scavenging of essential amino acids from their hosts. We previously identified a large family of apicomplexan-specific plasma membrane-localized amino acid transporters, the ApiATs, and showed that the Toxoplasma gondii transporter TgApiAT1 functions in the selective uptake of arginine. TgApiAT1 is essential for parasite virulence, but dispensable for parasite growth in medium containing high concentrations of arginine, indicating the presence of at least one other arginine transporter. Here we identify TgApiAT6-1 as the second arginine transporter. Using a combination of parasite assays and heterologous characterisation of TgApiAT6-1 in Xenopus laevis oocytes, we demonstrate that TgApiAT6-1 is a general cationic amino acid transporter that mediates both the high-affinity uptake of lysine and the low-affinity uptake of arginine. TgApiAT6-1 is the primary lysine transporter in the disease-causing tachyzoite stage of T. gondii and is essential for parasite proliferation. We demonstrate that the uptake of cationic amino acids by TgApiAT6-1 is 'trans-stimulated' by cationic and neutral amino acids and is likely promoted by an inwardly negative membrane potential. These findings demonstrate that T. gondii has evolved overlapping transport mechanisms for the uptake of essential cationic amino acids, and we draw together our findings into a comprehensive model that highlights the finely-tuned, regulated processes that mediate cationic amino acid scavenging by these intracellular parasites.

Highlights

Intracellular parasites of the phylum Apicomplexa are the causative agents of a diverse range of diseases in humans and domestic livestock, imposing major health and economic burdens in many countries
The causative agent of toxoplasmosis, Toxoplasma gondii, is a versatile intracellular parasite that can proliferate within nucleated cells of warm-blooded organisms
Essential cationic amino acid transporters in Toxoplasma parasites spectrometry-based metabolomics data have been uploaded to the Mendeley Data repository, with the GC-MS data from Fig 2A accessible at https://data. mendeley.com/datasets/64tdwbccx9/1 and the LCMS data in Fig 6 accessible at https://data. mendeley.com/datasets/893nsf2bnc/1, https://data.mendeley.com/datasets/xsb56m9gs8/ 1, and https://data.mendeley.com/ datasets/4xy7zppt2s/1

Summary

Introduction

Intracellular parasites of the phylum Apicomplexa are the causative agents of a diverse range of diseases in humans and domestic livestock, imposing major health and economic burdens in many countries. To allow for the loss of these pathways, parasites have co-evolved new mechanisms to acquire nutrients from their hosts [4,5]. As they proceed through their multi-stage life-cycles [6,7,8], apicomplexans adapt to a range of physiological and biochemical environments. Despite initial indications that T. gondii could synthesise aromatic amino acids utilising the shikimate pathway [11,19], the disease-causing tachyzoite stage of the parasite has been shown to be auxotrophic for tyrosine (Tyr), tryptophan (Trp), and phenylalanine (Phe) [20,21,22,23]. As a result of these auxotrophies, T. gondii is reliant primarily on multiple plasma membrane-localized transporters to salvage amino acids from their hosts [9], the endocytosis and lysosomal degradation of proteins may play a role [25]

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