Reversible compartmentalization of de novo purine biosynthetic complexes in living cells

Although the biosynthetic pathway for purines is well established, most studies have focused on the properties of the individual enzymes catalyzing the different steps of the pathway. The interactions of these enzymes have not been fully understood. In this study, fluorescence microscopy techniques were used to monitor enzyme interaction in both purine-rich and purine-depleted environments. GFP-tagged human TrifGART (hTrifGART), which catalyzes steps 2, 3, and 5 of the pathway shown in Figure 1, and human formylglycinamidine ribonucleotide synthase (hFGAMS), which catalyzes step 4, expressed in HeLa cells showed cytoplasmic clustering and exhibited a high degree of colocalization when coexpressed in a purine-depleted environment. In contrast, clustering and colocalization were not observed in a purine-rich environment. Endogenous and GFP-tagged hTrifGART were also seen to colocalize. The six other enzymes in the pathway [PRPP amidotransferase (hPPAT) (step 1), hTrifGART, hFGAMS, hPAICS, adenylosuccinate lyase (hASL) (step 8), and hATIC (steps 9 and 10)] showed the same clustering activity with hFGAMS in purine-depleted media. The presence of exogenous pathway disrupters such as hypoxanthine resulted in the retention of the clusters. Collectively, these findings suggest that enzyme complex association and dissociation occurs in response to changing purine levels in the cell.