BIOPHYSICAL CHARACTERIZATION OF SmE16 PROTEIN FROM Schistosoma mansoni: ANALYSIS OF CALCIUM BINDING EFFECTS AND INVESTIGATION OF POTENTIAL FUNCTIONS AT THE HOST–PARASITE INTERFACE

Schistosomiasis is a neglected tropical disease that affects millions of people worldwide. Caused by parasites of the Schistosoma genus, it has a significant socioeconomic impact in developing countries due to its high morbidity and mortality rates. Currently, the only widely available drug for treating schistosomiasis is praziquantel, which disrupts calcium homeostasis in adult worms. Thus, calcium-binding proteins are considered promising targets for future therapeutic interventions. SmE16 is an immunoreactive protein from Schistosoma mansoni predicted to bind calcium due to the presence of EF-hand motifs. Recent studies have revealed its expression in multiple life stages and tissues of the parasite, including its regurgitated content, indicating that SmE16 is a secreted protein located at the host–parasite interface. This study aims to elucidate the function of SmE16 through biophysical characterization, identification of interaction partners and assessment of its potential involvement in various signaling pathways. ITC assays demonstrated that SmE16 binds calcium ions, whereas no interaction with magnesium ions was observed. Crosslinking and SEC experiments revealed dimer formation in the presence of calcium, suggesting a calcium-dependent functional role. Circular dichroism analysis indicated that calcium induces minimal changes in SmE16’s secondary structure but leads to more pronounced alterations in tertiary structure, which were also detected by fluorescence spectroscopy. Additionally, calcium significantly increased protein stability, as demonstrated by DSC. In situ hybridization experiments showed that SmE16 transcripts are enriched in the esophageal glands of the parasite. Pulldown assays using mouse serum revealed that SmE16 interacts with host proteins, and these interactions are enhanced in the presence of calcium ions. As a result, eight candidate host proteins were identified as potential SmE16 partners, including three involved in innate immunity and the complement system, suggesting a possible immunomodulatory role for SmE16 in host immune response regulation. Our biophysical analyses revealed that SmE16 undergoes significant structural changes in the presence of calcium, supporting its potential function as a calcium sensor. Its ability to bind calcium and interact with host proteins suggests that SmE16 may play an active role in calcium-mediated signaling pathways.