Copine A is essential for calcium homeostasis in Dictyostelium

Copines are a family of calcium-dependent phospholipid-binding proteins found in most eukaryotic organisms. The expression of multiple copine genes is dysregulated in various types of human cancers. Yet, a common mechanistic function for copines remains enigmatic. We are studying copines in Dictyostelium, which has six copine genes (cpnA-cpnF). Cells lacking cpnA (cpnA-) exhibit many phenotypes including defects in development, chemotaxis, adhesion, and contractile vacuole (CV) function. In this study, we identified a novel link between CpnA and calcium homeostasis. We found that cpnA- cells have more phosphatidylserine (PS) exposed in the outer leaflet of the plasma membrane due to having an increased intracellular calcium concentration. The PS exposure defect and the enlarged CV defect in cpnA- cells were rescued by chelating calcium. We further investigated the role of PatA, a CV-localized Ca²⁺-ATPase responsible for pumping calcium into the CV. Although cpnA- cells expressed normal levels of patA, immunofluorescence revealed reduced PatA localization to the CV membrane. Notably, patA knockdown (patA^KD) cells phenocopied cpnA- cells, displaying enlarged CVs, elevated intracellular calcium, and increased PS exposure. Taken together, our findings suggest that CpnA promotes calcium sequestration into the CV, likely by regulating PatA localization or activity. This role in calcium homeostasis provides a mechanistic framework for understanding copine function and offers insight into how calcium dysregulation associated with copines may contribute to cancer progression.