AztD, a Periplasmic Zinc Metallochaperone to an ATP Binding Cassette (ABC) Transporter System in Paracoccus denitrificans [Molecular Biophysics]
October 14th, 2015 by Handali, M., Roychowdhury, H., Neupane, D. P., Yukl, E. T.
Bacterial ATP binding cassette (ABC) transporters of transition metals are essential for acquisition of necessary elements from the environment. A large number of gram-negative bacteria including human pathogens have a fourth conserved gene of unknown function adjacent to the canonical permease, ATPase and solute binding protein (SBP) genes of the AztABC Zn transporter system. To assess the function of this putative accessory factor (AztD) from Paracoccus denitrificans, we have analyzed its transcriptional regulation, metal binding properties and interaction with the SBP (AztC). Transcription of the aztD gene is significantly upregulated under conditions of Zn starvation. Recombinantly expressed AztD purifies with slightly substoichiometric Zn from the periplasm of E. coli and is capable of binding up to 3 Zn ions with high affinity. Size exclusion chromatography and a simple intrinsic fluorescence assay were used to determine that AztD as isolated is able to transfer bound Zn nearly quantitatively to apo-AztC. Transfer occurs through a direct, associative mechanism that prevents loss of metal to the solvent. These results indicate that AztD is a Zn chaperone to AztC and likely functions to maintain Zn homeostasis through interaction with the AztABC system. This work extends our understanding of periplasmic Zn trafficking and the function of chaperones in this process.