The Role of Acidocalcisomes in Parasitic Protists1

ABSTRACT. Acidocalcisomes are acidic organelles with a high concentration of phosphorus present as pyrophosphate (PP_i) and polyphosphate (poly P) complexed with calcium and other cations. The acidocalcisome membrane contains a number of pumps (Ca²⁺‐ATPase, V‐H⁺‐ATPase, H⁺‐PPase), exchangers (Na⁺/H⁺, Ca²⁺/H⁺), and channels (aquaporins), while its matrix contains enzymes related to PP_i and poly P metabolism. Acidocalcisomes have been observed in pathogenic, as well as non‐pathogenic prokaryotes and eukaryotes, e.g. Chlamydomonas reinhardtii, and Dictyostelium discoideum. Some of the potential functions of the acidocalcisome are the storage of cations and phosphorus, the participation of phosphorus in PP_i and poly P metabolism, calcium homeostasis, maintenance of intracellular pH homeostasis, and osmoregulation. In addition, acidocalcisomes resemble lysosome‐related organelles (LRO) from mammalian cells in many of their properties. For example, we found that platelet dense granules, which are LROs, are very similar to acidocalcisomes. They share a similar size, acidic properties, and both contain PP_i, poly P, and calcium. Recent work that indicates that they also share the system for targeting of their membrane proteins through adaptor protein 3 reinforces this concept. The fact that acidocalcisomes interact with other organelles in parasitic protists, e.g. the contractile vacuole in Trypanosoma cruzi, and other vacuoles observed in Toxoplasma gondii, suggests that these cellular compartments may be associated with the endosomal/lysosomal pathway.