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Metal(loid)-Microbe Interactions
Published in Vineet Kumar, Vinod Kumar Garg, Sunil Kumar, Jayanta Kumar Biswas, Omics for Environmental Engineering and Microbiology Systems, 2023
Rubina Khanam, Pedda Ghouse Peera, Sheikh Kulsum, Jayanta Kumar Biswas
Intracellular metal ions are sequestrated with the help of metal ion-binding proteins such as metallochaperones, metallothioneins (MTs), and glutathione (GSH). For example, in Pseudomonas and Anabaena genera, non-essential metal(loid)s such as Cd, Pb, and Hg are captured on cysteine-rich MT-polypeptides. GSH is used by certain bacteria as an alternative chelator to sequester metal ions. Through its R-SH group, GSH scavenges and detoxifies metals. For example, Rhizobium spp. utilizes GSH in regulating Cd tolerance. MTs are popular transporters/pumps for essential metal(loid)s; for example, Cu plays a vital role in taking Cu to metalloenzymes, thereby protecting cytoplasmic organelles.
Mechanisms of Bacterial Heavy Metal Resistance and Homeostasis
Published in Edgardo R. Donati, Heavy Metals in the Environment, 2018
Pallavee Srivastava, Meenal Kowshik
Bacteria are exposed to varying concentrations of metals in their surroundings. In an event of depletion or accumulation of metals to toxic levels, bacteria activate cellular responses to maintain metal homeostasis. This is achieved primarily through metal-sensing/metalloregulatory proteins that control transport and storage of target metal ion(s). The allosteric binding of target metal ions to these proteins brings about a conformational change in the DNA-binding domains that eventually results in transcriptional repression/derepression/activation of the downstream genes. These regulatory metal sensing proteins control the expression of various transporters, metallochaperones, and intermediary protein complexes involved in influx or efflux of the metal(s) for maintaining homeostasis (Ma et al., 2009). Recent studies have also implicated the role of RNA-mediated regulatory response involving riboswitches in metal ion homeostasis (Furukawa et al., 2015; Ramesh and Winkler, 2010). The metal ion(s) present in the vicinity of the bacterium is transported across the impermeable membrane through the metal transporters in a directional fashion. These transporters are integral membrane proteins embedded in the inner or plasma membrane of the organism and include ATP binding cassette (ABC) and Nramp transporters. ABC transporters have been identified and characterized for nearly all biologically relevant metal ions, while Nramps have been identified only as Mn and Fe transporters (Ma et al., 2009). Once transported across the membrane, specialized proteins designated as ‘metallochaperones’ associate with the metal in such a way that it can be readily transferred to an appropriate acceptor protein within a cellular compartment such as the periplasm and cytosol. The transfer of the metal to the acceptor proteins is essentially an intermolecular metal-ligand exchange brought about by the formation of transient protein-protein complexes (Tottey et al., 2005). In an event of toxic build-up of metal within the bacterial cell, the organism exports the toxicant out of the cell through efflux pumps that include cation diffusion facilitators (CDFs), P-type ATPases, and tripartite RND (resistance-nodulation-cell division) transporters (Kolaj-Robin et al., 2015; Argüello et al., 2007). Figure 1 illustrates the common mechanisms involved in maintaining essential metal ion homeostasis in bacteria.
Effects of cadmium stress on the morphology, physiology, cellular ultrastructure, and BvHIPP24 gene expression of sugar beet (Beta vulgaris L.)
Published in International Journal of Phytoremediation, 2023
Dali Liu, Zhuo Gao, Jiajia Li, Qi Yao, Wenbo Tan, Wang Xing, Zhenqiang Lu
In addition, when plants are subjected to heavy metal stress and metal toxicity, ions can be chelated with ion-trafficking proteins or small-molecule ligands that either guide and insert ion cofactors into the target enzyme or catalyze electron transport and redox transformations after entering cells. The chelate complexes can be transported to intracellular compartments by metallochaperones for metal sequestration and homeostasis in cells (Huffman and O'Halloran 2001; Dupont et al.2010). Metallochaperones are generally soluble intracellular proteins that tightly bind to metal ions to prevent them from causing damage to other cellular components (de Abreu-Neto et al.2013). Most metallochaperones contain a highly conserved MXCXXC motif with a ferredoxin-like structural fold (βαββαβ), which can bind the heavy metals Cd2+, Cu2+, or Zn2+ (Tehseen et al. 2010). Heavy metal-associated isoprenylated plant proteins (HIPPs) are important metallochaperones that contain both heavy metal-associated domains and prenylation motifs, and these characteristics enable HIPPs to be used for metal detoxification (Barth et al.2009; de Abreu-Neto et al.2013). The putative metal-binding protein CdI19 contains two heavy metal-binding motifs and a conserved prenylation site, which plays an important role in the maintenance of heavy metal homeostasis. Cd2+ can induce the expression of AtCdI19, and overexpression of CdI19 in Arabidopsis can enhance tolerance to Cd (Suzuki et al.2002).