Activation of a zinc metallochaperone by the alarmone ZTP

Bacteria tightly regulate intracellular zinc levels to ensure sufficient zinc to support essential functions, while preventing toxicity. The bacterial response to zinc limitation includes the expression of putative zinc metallochaperones belonging to subfamily 1 of the COG0523 family of G3E GTPases. However, the client proteins and the metabolic processes served by these chaperones are unclear. Here, we demonstrate that the Bacillus subtilis YciC zinc metallochaperone (here renamed ZagA for ZTP activated GTPase A) supports de novo folate biosynthesis under conditions of zinc limitation through direct interaction with the zinc dependent GTP cyclohydrolase, FolE. Furthermore, we identify a role for the alarmone ZTP, a modified purine biosynthesis intermediate, in the response to zinc limitation. ZTP, a signal of 10-formyl-tetrahydrofolate deficiency (10f-THF) in bacteria, transiently accumulates as the Zn dependent GTP cyclohydrolase FolE begins to fail and stimulates the interaction between ZagA and FolE to sustain folate synthesis despite declining zinc availability.\n\nImportanceMetallochaperones provide a mechanism for cells to regulate the delivery of metals to newly synthesized apoproteins. By selectively targeting specific proteins for metallation, cells can ensure that key pathways remain functional even as metals become limiting for growth. The COG0523 family of proteins contain a subgroup of candidate metallochaperones (the YciC subfamily) induced under conditions of zinc limitation. Although YciC family proteins have been suggested to be GTP-dependent metallochaperones, specific interactions with client proteins have not been demonstrated. Here, we show that the Bacillus subtilis YciC (renamed ZagA) protein responds to ZTP as an activating ligand rather than GTP, and interacts specifically with a Zn-dependent enzyme critical for folate synthesis (FolE). Thus, under conditions of Zn limitation ZagA is synthesized, and as folate synthesis fails, it selectively delivers Zn to FolE to sustain folate synthesis.