Characterization of Two EF-hand Domain-containing Proteins from Toxoplasma gondii

The universal role of calcium (Ca2+) as a second messenger in cells depends on a large number of Ca2+-binding proteins (CBP), which are able to bind Ca2+ through specific domains. Many CBPs share a type of Ca2+-binding domain known as the EF-hand. The EF-hand motif has been well studied and consists of a helix-loop-helix structural domain with specific amino acids in the loop region that interact with Ca2+. In Toxoplasma gondii a large number of genes (approximately 68) are predicted to have at least one EF-hand motif. The majority of these genes have not been characterized. We report the characterization of two EF-hand motif-containing proteins, TgGT1_216620 and TgGT1_280480, which localize to the plasma membrane and to the rhoptry bulb, respectively. Genetic disruption of these genes by CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9) resulted in mutant parasite clones (Delta tg216620 and Delta tg280480) that grew at a slower rate than control cells. Ca2+ measurements showed that Delta tg216620 cells did not respond to extracellular Ca2+ as the parental controls while Delta tg280480 cells appeared to respond as the parental cells. Our hypothesis is that TgGT1_216620 is important for Ca2+ influx while TgGT1_280480 may be playing a different role in the rhoptries.