De novo pyrimidine pathway utilizes CTP synthase (CTPsyn) as a rate limiting enzyme for the biosynthesis of cytidine triphosphate (CTP). In nutrient depletion conditions, CTPsyn assembles into filamentary structures. This self- assembly, higher ordered structure suggests a key regulation of metabolism in the cell. The question that allures us the most is why this metabolic enzyme needs to form filaments? Is this an easy way to tune in the production of CTP whenever required? There has been already an ongoing debate on whether this specialized structure is catalytically active or just a protein aggregate. Our lab has demonstrated that CTPsyn filaments are regulated by ubiquitination and support S phase during endocycles in follicle cells. Recently it has been reported that IMPHD2 co-localizes with the CTPsyn filaments in human cell lines. Moreover, in Drosophila, the DAck was also found to co-localize with the CTPsyn filaments in the germ cells where the structure maintains the integrity of cell membrane through phospholipid production. It is possible that the structure has different associated proteins at different physiological conditions which can regulate their enzymatic activity. Therefore, it would be very important to identify the structural components of these filaments and elucidate their activities. Here we use a system in which CTPsyn is tagged to a genetically engineered peroxidases, APEX2. APEX2 can oxidise the biotin-phenol to generate a very short-lived biotin-phenoxyl radical which covalently tags the endogenous proteins adjacent to the APEX2 protein. Using the above approach we have successfully labelled the CTP synthase filaments in the live Drosophila ovary tissues with biotin and have successfully identified the biotinylated “targets” of the filament structure in germline cells, and in the follicle cells using mass spectrometry. Currently we are examining the involvement of the candidates in CTPsyn filament formation using co-localization or knockdown approaches.