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 Ca2+ release from these organelles was unknown. Here we present evidence that Trypanosome brucei acidocalcisomes possess an inositol 1,4,5-trisphosphate receptor (TbIP3R) for Ca2+ release. Localization studies in cell lines expressing TbIP3R in its endogenous locus fused to an epitope tag revealed its partial colocalization with the vacuolar proton pyrophosphatase, a marker of acidocalcisomes. IP3 was able to stimulate Ca2+ release from a chicken B-lymphocyte cell line in which the genes for all three vertebrate IP(3)Rs have been stably ablated (DT40-3KO) and that were stably expressing TbIP3R, providing evidence of its function. IP3 was also able to release Ca2+ from permeabilized trypanosomes or isolated acidocalcisomes and photolytic release of IP3 in intact trypanosomes loaded with Fluo-4 elicited a transient Ca2+ increase in their cytosol. Ablation of TbIP3R by RNA interference caused a significant reduction of IP3-mediated Ca2+ 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 IP3R as a Ca2+ release channel in acidocalcisomes of trypanosomes and suggest that a Ca2+ signaling pathway that involves acidocalcisomes is required for growth and establishment of infection.