Intracellular compartmentation is a key strategy for the functioning of a cell.In 2010,several studies revealed that the metabolic enzyme CTP synthase (CTPS) can form filamentous structures termed cytoophidia in prokaryotic and eukaryotic cells.However,recent structural studies showed that CTPS only forms inactive product-bound filaments in bacteria while forming active substrate-bound filaments in eukaryotic cells.In this study,using negative staining and cryo-electron microscopy,we demonstrate that Drosophila CTPS,whether in substrate-bound or product-bound form,can form filaments.Our results challenge the previous model and indicate that substrate-bound and product-bound filaments can coexist in the same species.We speculate that the ability to switch between active and inactive cytoophidia in the same cells provides an additional layer of metabolic regulation.