Cnidaria small cysteine-rich protein (SCRiP) family

General SCRiP proteins were originally identified as genes unique to reefbuilding corals (Scleractinia) that are downregulated during heat stress (Sunagawa et al., 2009). Given the similarity in the temporal expression pattern they share with galaxin, a key protein involved in the biomineralization process (Fukuda et al., 2003), SCRiPs were claimed to be implied in calcification of the coral skeleton (Sunagawa et al., 2009).

Several reasons drove Jouiaei and colleagues (2015) to search toxic function to this family. The high sequence diversity in SCRiPs is uncharacteristic of homeostatically important proteins such as those involved in the biomineralization process the domain. One domain characteristic of proteins found in rattelsnake venoms, the myotoxin domain is found in Mcap-SCRiP1. Moreover, like SCRiPs, several toxins in sea anemones have also been shown to exhibit downregulation in expression as a result of thermal stress (Richier et al. 2008). In addition, BLAST searches surprisingly retrieved SCRiP homologs in sea anemones, Anemonia viridis (Avir-SCRiP) and Metridium senile (Msen-SCRiP) refuting the claim that these peptides are specific to scleractinian corals.

To test toxic function of this family, Amil-SCRiP2 and Amil-SCRiP3 from Acropora millepora were expressed and tested on insect and fish. Interestingly, the injection of these SCRiPs in blowfly larvae (Sarcophaga falculata) did not result in toxicity, while the incubation of zebrafish (Danio rerio) larvae at a concentration of 230 mg/ml resulted in severe neurotoxicity. These results support the notion that SCRiPs may serve as neurotoxins, making them the first peptide neurotoxin family described from scleractinian corals (Jouiaei et al., 2015).