Acidocalcisomes of Trypanosoma brucei have an inositol 1,4,5-trisphosphate receptor that is required for growth and infectivity

Acidocalcisomes are acidic calcium stores rich in polyphosphate and found in a diverse range of organisms. The mechanism of Ca ²⁺ release from these organelles was unknown. Here we present evidence that Trypanosoma brucei acidocalcisomes possess an inositol 1,4,5-trisphosphate receptor (TbIP ₃R) for Ca ²⁺ release. Localization studies in cell lines expressing TbIP ₃R in its endogenous locus fused to an epitope tag revealed its partial colocalization with the vacuolar proton pyrophosphatase, a marker of acidocalcisomes. IP ₃ was able to stimulate Ca ²⁺ release from a chicken B-lymphocyte cell line in which the genes for all three vertebrate IP ₃Rs have been stably ablated (DT40-3KO) and that were stably expressing TbIP ₃R , providing evidence of its function. IP ₃ was also able to release Ca ²⁺ from permeabilized trypanosomes or isolated acidocalcisomes and photolytic release of IP ₃ in intact trypanosomes loaded with Fluo-4 elicited a transient Ca ²⁺ increase in their cytosol. Ablation of TbIP ₃R by RNA interference caused a significant reduction of IP ₃-mediated Ca ²⁺ release in trypanosomes and resulted in defects in growth in culture and infectivity in mice. Taken together, the data provide evidence of the presence of a functional IP ₃R as a Ca ²⁺ release channel in acidocalcisomes of trypanosomes and suggest that a Ca ²⁺ signaling pathway that involves acidocalcisomes is required for growth and establishment of infection.