Structural Role of the First Four Transmembrane Helices in ZntA, a P1B-Type ATPase from Escherichia coli

A novel Zn (II)/Cd (II)/Pb (II)-translocating PIB-type ATPase mediates metal resistance in Chryseobacterium sp. strain PMSZPI in metal-enriched soil of uranium ore deposit

Transition metals at higher concentrations are toxic to the cells. Membrane bound, ATP-driven efflux pumps belonging to the P-type ATPase superfamily maintain metal homeostasis by transporting metals/ions across the biological membranes. A soil bacterium, Chryseobacterium sp. strain PMSZPI, residing in metal enriched environment of uranium ore deposit exhibited high tolerance to multiple heavy metals. In an attempt to unveil one of the molecular determinants of metal resistance in PMSZPI, we characterized an unannotated, novel metal exporting PIB-2-ATPase that was categorized as Zn (II)/Cd (II)/Pb(II) PIB-2-ATPase based on amino-acid sequence alignment and the substrate specificities. The heterologously expressed and purified PIB-2-ATPase exhibited zinc/cadmium/lead dependent ATP hydrolysis activity, ATP dependent phosphorylation and activity inhibition in the presence of vanadate. In-vivo metal tolerance assays and analysis of intracellular metal contents indicated involvement of PIB-2-ATPase in metal efflux. The disordered N-terminal metal binding domain of PIB-2-ATPase was found to be inconsequential for its function. Mutagenesis studies revealed the role of the conserved transmembrane (TM) residues (cysteine, aspartate and lysine) in metal translocation. Overall, our data establishes the vital role of Zn(II)/Cd(II)/Pb(II) PIB-2-ATPase in conferring metal resistance in PMSZPI.

Characterization of three predicted zinc exporters in Brucella ovis identifies ZntR-ZntA as a powerful zinc and cadmium efflux system not required for virulence and unveils pathogenic Brucellae heterogeneity in zinc homeostasis

Brucella ovis causes non-zoonotic ovine brucellosis of worldwide distribution and is responsible for important economic losses mainly derived from male genital lesions and reproductive fails. Studies about the virulence mechanisms of this rough species (lacking lipopolysaccharide O-chains) are underrepresented when compared to the main zoonotic Brucella species that are smooth (with O-chains). Zinc intoxication constitutes a defense mechanism of the host against bacterial pathogens, which have developed efflux systems to counterbalance toxicity. In this study, we have characterized three potential B. ovis zinc exporters, including the ZntA ortholog previously studied in B. abortus. Despite an in-frame deletion removing 100 amino acids from B. ovis ZntA, the protein retained strong zinc efflux properties. Only indirect evidence suggested a higher exporter activity for B. abortus ZntA, which, together with differences in ZntR-mediated regulation of zntA expression between B. ovis and B. abortus, could contribute to explaining why the ΔzntR mutant of B. abortus is attenuated while that of B. ovis is virulent. Additionally, B. ovis ZntA was revealed as a powerful cadmium exporter contributing to cobalt, copper, and nickel detoxification, properties not previously described for the B. abortus ortholog. Deletion mutants for BOV_0501 and BOV_A1100, also identified as potential zinc exporters and pseudogenes in B. abortus, behaved as the B. ovis parental strain in all tests performed. However, their overexpression in the ΔzntA mutant allowed the detection of discrete zinc and cobalt efflux activity for BOV_0501 and BOV_A1100, respectively. Nevertheless, considering their low expression levels and the stronger activity of ZntA as a zinc and cobalt exporter, the biological role of BOV_0501 and BOV_A1100 is questionable. Results presented in this study evidence heterogeneity among pathogenic Brucellae regarding zinc export and, considering the virulence of B. ovis ΔzntA, suggest that host-mediated zinc intoxication is not a relevant mechanism to control B. ovis infection.

Plastic recognition and electrogenic uniport translocation of 1st-, 2nd-, and 3rd-row transition and post-transition metals by primary-active transmembrane P1B-2-type ATPase pumps

Transmembrane P1B-type ATPase pumps catalyze the extrusion of transition metal ions across cellular lipid membranes to maintain essential cellular metal homeostasis and detoxify toxic metals. Zn(ii)-pumps of the P1B-2-type subclass, in addition to Zn2+, select diverse metals (Pb2+, Cd2+ and Hg2+) at their transmembrane binding site and feature promiscuous metal-dependent ATP hydrolysis in the presence of these metals. Yet, a comprehensive understanding of the transport of these metals, their relative translocation rates, and transport mechanism remain elusive. We developed a platform for the characterization of primary-active Zn(ii)-pumps in proteoliposomes to study metal selectivity, translocation events and transport mechanism in real-time, employing a "multi-probe" approach with fluorescent sensors responsive to diverse stimuli (metals, pH and membrane potential). Together with atomic-resolution investigation of cargo selection by X-ray absorption spectroscopy (XAS), we demonstrate that Zn(ii)-pumps are electrogenic uniporters that preserve the transport mechanism with 1st-, 2nd- and 3rd-row transition metal substrates. Promiscuous coordination plasticity, guarantees diverse, yet defined, cargo selectivity coupled to their translocation.