The contribution of the Golgi and the endoplasmic reticulum to calcium and pH homeostasis in Toxoplasma gondii

The cytosolic Ca²⁺ concentration of all cells is highly regulated demanding the coordinated operation of Ca²⁺ pumps, channels, exchangers, and binding proteins. In the protozoan parasite Toxoplasma gondii, calcium homeostasis, essential for signaling, governs critical virulence traits. However, the identity of most molecular players involved in signaling and homeostasis in T. gondii is unknown or poorly characterized. In this work, we studied a putative calcium proton exchanger, TgGT1_319550 (TgCAXL1), which belongs to a family of Ca²⁺/proton exchangers that localize to the Golgi apparatus. We localized TgCAXL1 to the Golgi and the endoplasmic reticulum (ER) of T. gondii and validated its role as a Ca²⁺/proton exchanger by yeast complementation. Characterization of a knock-out mutant for TgCAXL1 (Δcaxl) underscored the role of TgCAXL1 in Ca²⁺ storage by the ER and acidic stores, most likely the Golgi. Most interestingly, TgCAXL1 function is linked to the Ca²⁺ pumping activity of the Sarcoendoplasmic Reticulum Ca²⁺-ATPase (TgSERCA). TgCAXL1 functions in cytosolic pH regulation and recovery from acidic stress. Our data showed for the first time the role of the Golgi in storing and modulating Ca²⁺ signaling in T. gondii and the potential link between pH regulation and TgSERCA activity, which is essential for filling intracellular stores with Ca²⁺.