This is the C-terminal domain of ISP1 protein, which plays a role in asexual daughter cell formation, such as in Toxi. gondii. The domain consists of a seven-stranded antiparallel beta-sandwich bordered on one end by an inter-strand loop (open end) and capped at the other end by an amphipathic C-terminal helix (closed end). The domain adopts a pleckstrin homology (PH) fold, despite having negligible sequence similarity. PH domains are often found in proteins that support protein-lipid and play a role in mediating membrane localization through IP binding. However, the Phospholipid Binding Properties of PH domains is not conserved in the TgISP1. Unlike PH domains, ISP1 is cysteine rich. The cysteine-rich nature of the ISPs and the number of surface-exposed cysteines may result in redox instability and may also facilitate higher order multimerization. A disulfide bond between beta 2 and beta 3 is likely a structural feature of the ISP1, as both cysteines appear broadly conserved [1].

1: The inner membrane complex sub-compartment proteins critical for replication of the apicomplexan parasite Toxoplasma gondii adopt a pleckstrin homology fold. Tonkin ML, Beck JR, Bradley PJ, Boulanger MJ; J Biol Chem. 2014;289:13962-13973. PMID:24675080