The phylum Apicomplexa comprises a large group of early branching eukaryotes that includes a number of human and animal parasites. Calcium controls a number of vital processes in apicomplexans including protein secretion, motility, and differentiation. Despite the importance of calcium as a second messenger, very little is known about the systems that control homeostasis or that regulate calcium signaling in parasites. The recent completion of many apicomplexan genomes provides new opportunity to define calcium response pathways in this group of parasites in comparison to model organisms.

Whole-genome comparison between the apicomplexans Plasmodium spp., Cryptosporidium spp., and Toxoplasma gondii revealed the presence of several P-Type Ca²⁺ transporting ATPases including a single endoplasmic reticulum (ER)-type sarcoplasmic–endoplasmic reticulum Ca²⁺ ATPase, several Golgi-like Ca²⁺ ATPases, and a single Ca²⁺/H⁺ exchanger. Only T. gondii showed evidence of plasma membrane-type Ca²⁺ ATPases or voltage-gated calcium channels. Despite pharmacological evidence for IP₃ and ryanodine-mediated calcium release, animal-type calcium channels were not readily identified in parasites, indicating they are more similar to plants. Downstream of calcium release, a variety of EF-hand–containing proteins regulate calcium responses. Our analyses detected a single conserved calmodulin (CaM) homologue, 3 distinct centrin (CETN)-caltractin–like proteins, one of which is shared with ciliates, and a variety of deep-branching, CaM-CETN–like proteins. Apicomplexans were also found to contain a wide array of calcium-dependent protein kinases (CDPKs), which are commonly found in plants. Toxoplasma gondii contains more than 20 CDPK or CDPK-related kinases, which likely regulate a variety of responses including secretion, motility, and differentiation.

Genomic and phylogenetic comparisons revealed that apicomplexans contain a variety of unusual calcium response pathways that are distinct from those seen in vertebrates. Notably, plant-like pathways for calcium release channels and calcium-dependent kinases are found in apicomplexans. The experimental flexibility of T. gondii should allow direct experimental manipulation of these pathways to validate their biological roles. The central importance of calcium in signaling and development, and the novel characteristics of many of these systems, indicates that parasite calcium pathways may be exploited as new therapeutic targets for intervention.