BIOPHYSICAL INSIGHTS INTO THE CALCIUM-DEPENDENT ASSEMBLY OF S. MANSONI CENTRIN

Schistosomiasis is a neglected tropical disease caused predominantly in the Americas by Schistosoma mansoni. Although national positivity rates remain low, certain regions report prevalence rates exceeding 20% and high annual mortality.The parasite relies on tightly regulated calcium homeostasis for its survival, and praziquantel, the main drug currently used to treat schistosomiasis, has been shown to disrupt calcium homeostasis, which contributes to its antiparasitic activity. However, its broad pharmacological profile and limited efficacy against juvenile parasite stages highlight the need for new targets. Centrins are EF-hand family proteins containing calcium-binding motifs that are associated with centrioles and participate in processes related to cell division, making them potential molecular targets against the parasite. S. mansoni centrin (SmCentrin) shares 64% sequence similarity with human centrins, which can form higher-order assemblies in the presence of calcium. Similar to human centrins, SmCentrin has demonstrated the ability to oligomerize, a process dependent on protein concentration and favored under low ionic strength conditions. Polyethylene glycol (PEG), through its molecular crowding effect, can also influence protein association and oligomerization. In this study, we investigated the factors influencing SmCentrin oligomerization, focusing on the effects of Ca²⁺ and PEG. Negative-stain transmission electron microscopy  demonstrated that SmCentrin (40 μM, HEPES 50 mM, pH 7.0, KCl 40 mM) exhibits increased structural organization upon addition of 1–2 mM CaCl₂, even in the absence of partner proteins. Furthermore, Ca²⁺ promoted the appearance of ordered structures that could bend and associate, in some cases forming circular arrangements.. DLS experiments further showed that exposure of SmCentrin (50 μM) to  PEG (30-40%), resulted in a significant increase in particle size, consistent with the formation of larger protein assemblies under molecular crowing conditions.. These results indicate that Ca²⁺ and molecular crowding influence SmCentrin oligomerization, encouraging further studies to evaluate its potential as a molecular target in S. mansoni.