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 Tg ApiAT1 functions in the selective uptake of arginine. Tg ApiAT1 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 Tg ApiAT6-1 as the second arginine transporter. Using a combination of parasite assays and heterologous characterisation of Tg ApiAT6-1 in Xenopus laevis oocytes, we demonstrate that Tg ApiAT6-1 is a general cationic amino acid transporter that mediates both the high-affinity uptake of lysine and the low-affinity uptake of arginine. Tg ApiAT6-1 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 Tg ApiAT6-1 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. Author summary The causative agent of toxoplasmosis, Toxoplasma gondii , is a versatile intracellular parasite that can proliferate within nucleated cells of virtually all warm-blooded organisms. In order to survive, T. gondii must scavenge the cationic amino acids lysine and arginine from their hosts. In a previous study, we demonstrated that a plasma membrane-localized protein called Tg ApiAT1 facilitates the uptake of arginine into the parasite. We found that parasites lacking Tg ApiAT1 could proliferate when cultured in medium containing high concentrations of arginine, suggesting the existence of an additional uptake pathway for arginine. In the present study, we demonstrate that this second uptake pathway is mediated by Tg ApiAT6-1, a protein belonging to the same solute transporter family as Tg ApiAT1. We show that Tg ApiAT6-1 is the major lysine transporter of the parasite, and that it is critical for parasite proliferation. Furthermore, we demonstrate that Tg ApiAT6-1 can transport arginine into parasites in conditions where arginine concentrations are high and lysine concentrations are comparatively lower. These data support a model for the finely-tuned acquisition of essential cationic amino acids that involves multiple transporters, and which likely contributes to these parasites being able to survive and proliferate within a wide variety of host cell types.

Coordinated Action of Multiple Transporters in the Acquisition of Essential Cationic Amino Acids by the Intracellular Parasite <i>Toxoplasma gondii</i>

Coordinated Action of Multiple Transporters in the Acquisition of Essential Cationic Amino Acids by the Intracellular Parasite Toxoplasma gondii