Orthologs of <i>Plasmodium</i> ICM1 are dispensable for Ca ²⁺ mobilization in <i>Toxoplasma gondii</i>

Apicomplexan parasites mobilize ionic calcium (Ca²⁺) from intracellular stores to promote microneme secretion and facilitate motile processes including gliding motility, invasion, and egress. Recently, a multipass transmembrane protein, ICM1, was found to be <u>i</u>mportant for <u>c</u>alcium <u>m</u>obilization in <i>Plasmodium falciparum</i> and <i>P. berghei</i>. Comparative genomics and phylogenetics have revealed putative ICM orthologs in <i>Toxoplasma gondii</i> and other apicomplexans. <i>T. gondii</i> possesses two ICM-like proteins, which we have named TgICM1-L (TGGT1_305470) and TgICM2-L (TGGT1_309910). TgICM1-L and TgICM2-L localized to undefined puncta within the parasite cytosol. TgICM1-L and TgICM2-L are individually dispensable in tachyzoites, suggesting a potential compensatory relationship between the two proteins may exist. Surprisingly, mutants lacking both TgICM1-L and TgICM2-L are fully viable, exhibiting no obvious defects in growth, microneme secretion, invasion, or egress. Furthermore, loss of TgICM1-L, TgICM2-L, or both does not impair the parasite's ability to mobilize Ca²⁺. These findings suggest that additional proteins may participate in Ca²⁺ mobilization or import in Apicomplexa, reducing the dependence on ICM-like proteins in <i>T. gondii</i>. Collectively, these results highlight similar yet distinct mechanisms of Ca²⁺ mobilization between <i>T. gondii</i> and <i>Plasmodium</i>.IMPORTANCECa²⁺ signaling plays a crucial role in governing apicomplexan motility; yet, the mechanisms underlying Ca²⁺ mobilization from intracellular stores in these parasites remain unclear. In <i>Plasmodium</i>, the necessity of ICM1 for Ca²⁺ mobilization raises the question of whether this mechanism is conserved in other apicomplexans. Investigation into the orthologs of <i>Plasmodium</i> ICM1 in <i>T. gondii</i> revealed a differing requirement for ICM proteins between the two parasites. This study suggests that <i>T. gondii</i> employs ICM-independent mechanisms to regulate Ca²⁺ homeostasis and mobilization. Proteins involved in Ca²⁺ signaling in apicomplexans represent promising targets for therapeutic development.