Compartmentation of enzymes via filamentation has arisen as a mechanism for the regulation of metabolism.In 2010,three groups independently reported that CTP synthase (CTPS) can assemble into a filamentous structure termed the cytoophidium.In searching for CTPS-interacting proteins,here we perform a yeast two-hybrid screening of Drosophila proteins and identify a putative CTPS-interacting protein,Δ1-pyrroline-5-carboxylate synthase (P5CS).Using the Drosophila follicle cell as the in vivo model,we confirm that P5CS forms cytoophidia,which are associated with CTPS cytoophidia.Overexpression of P5CS increases the length of CTPS cytoophidia.Conversely,filamentation of CTPS affects the morphology of P5CS cytoophidia.Finally,in vitro analyses confirm the filament-forming property of P5CS.Our work links CTPS with P5CS,two enzymes involved in the rate-limiting steps in pyrimidine and proline biosynthesis,respectively.