Sabina Rech

Bio: Sabina Rech is an academic researcher from University of California, Los Angeles. The author has contributed to research in topics: Molybdate & Anion binding. The author has an hindex of 1, co-authored 1 publications receiving 69 citations.

Crystal structure of the molybdate binding protein ModA.

Abstract: The specificity exhibited by the molybdate binding protein ModA for molybdate and tungstate reflects the size and ligands of the anion binding pocket.

Structure, Function, and Evolution of Bacterial ATP-Binding Cassette Systems

Abstract: Summary: ATP-binding cassette (ABC) systems are universally distributed among living organisms and function in many different aspects of bacterial physiology. ABC transporters are best known for their role in the import of essential nutrients and the export of toxic molecules, but they can also mediate the transport of many other physiological substrates. In a classical transport reaction, two highly conserved ATP-binding domains or subunits couple the binding/hydrolysis of ATP to the translocation of particular substrates across the membrane, through interactions with membrane-spanning domains of the transporter. Variations on this basic theme involve soluble ABC ATP-binding proteins that couple ATP hydrolysis to nontransport processes, such as DNA repair and gene expression regulation. Insights into the structure, function, and mechanism of action of bacterial ABC proteins are reported, based on phylogenetic comparisons as well as classic biochemical and genetic approaches. The availability of an increasing number of high-resolution structures has provided a valuable framework for interpretation of recent studies, and realistic models have been proposed to explain how these fascinating molecular machines use complex dynamic processes to fulfill their numerous biological functions. These advances are also important for elucidating the mechanism of action of eukaryotic ABC proteins, because functional defects in many of them are responsible for severe human inherited diseases.

Coordination chemistry of bacterial metal transport and sensing.

Abstract: The transition or d-block metal ions manganese, iron, cobalt, nickel, copper, zinc, and to a more specialized degree, molybdenum, tungsten and vanadium, have been shown to be important for biological systems. These metal ions are ubiquitously found in nature, nearly exclusively as constituents of proteins.1 The unique properties of metal ions have been exploited by nature to perform a wide range of tasks. These include roles as structural components of biomolecules, as signaling molecules, as catalytic cofactors in reversible oxidation-reduction and hydrolytic reactions, and in structural rearrangements of organic molecules and electron transfer chemistry.1 Indeed, metal ions play critical roles in the cell that cannot be performed by any other entity, and are therefore essential for all of life. However, an individual metal ion is capable of performing only one or a few of these functions, but certainly not all; as a result, nature has evolved mechanisms to effectively distinguish one metal from another. The coordination chemistry of metal ion-protein complexes is fundamental to this biological discrimination, and is largely the focus of this review. 1.1. Metal Ion Homeostasis Extensive regulatory and protein-coding machinery is devoted to maintaining the “homeostasis” of biologically required metal ions and underscores the essentiality of this process for cell viability. Homeostasis is defined as the maintenance of an optimal bioavailable concentration, mediated by the balancing of metal uptake and intracellular trafficking with efflux/storage processes so that the needs of the cell for that metal ion is met, i.e., the “right” metal is inserted into the “right” macromolecule at the appropriate time.2,3 Just as a scarcity of a particular metal ion induces a stress response that can lead to reprogramming of cellular metabolism to minimize the consequences of depletion of a particular metal ion, e.g., zinc in ribosome biogenesis4 or Cu vs. Fe in photosynthesis by Synechocystis,5 too much of the same metal ion can also be toxic to a cell or organism. Metal homeostasis is governed by the formation of specific protein-metal coordination complexes used to effect uptake, efflux, intracellular trafficking within compartments, and storage (Figure 1). How metal ions move to and from their target destinations in the active site of a metalloenzyme or as a structural component of biomolecules also contributes to intracellular metal homeostasis (Figure 1). Metal transporters move metal ions or small molecule-metal chelates across otherwise impermeable barriers in a directional fashion, and most of these are integral membrane proteins embedded in the inner or plasma membrane (Figure 1). Specialized protein chelators designated metallochaperones traffic metals within a particular cellular compartment, e.g., the periplasm or the cytosol, and function to “hold” the metal in such a way that it can be readily transferred to an appropriate acceptor protein. This intermolecular transfer is known or is projected to occur through transiently formed, specific protein-protein complexes that mediate coordinated intermolecular metal ligand exchange. Metallochaperones have been described for copper,6-9 nickel10 and iron-sulfur protein biogenesis,11 and recent work suggests that the periplasmic Zn(II) binding protein, YodA, has characteristics consistent with a role as a zinc chaperone in E. coli (Figure 1).12 Salient features of these chaperones are discussed in more detail in the context of acquisition and efflux of individual metal ions (Section 2). Finally, specialized transcriptional regulatory proteins, termed metalloregulatory or metal sensor proteins, control the expression of genes encoding these proteins that establish metal homeostasis in response to either metal deprivation or overload (Section 3). Figure 1 Schematic metal homeostasis models for iron, zinc and manganese, copper, nickel and cobalt, shown specifically in gram-negative bacteria. Homeostasis of molybdate and tungstate oxyanions are not shown, due primarily to a lack of knowledge of these systems, ... A hypothesis that emerges is that in order to effect the cellular homeostasis of a particular metal ion, each component of the homeostasis machinery (Figure 1) must be selective for that metal ion under the prevailing conditions, to the exclusion of all others.13 Furthermore, individual systems must be “tuned” such that the affinity or sensitivity of each component is well-matched, either to coordinate gene expression by pairs of metal sensor proteins that coordinately shut off uptake and up-regulate efflux or detoxification systems, or to facilitate vectorial transport from metal donor to metal acceptor target protein in a metal trafficking pathway in the cell (Figure 1).14-16

Structure of an ABC transporter in complex with its binding protein

Abstract: ATP-binding cassette (ABC) transporter proteins carry diverse substrates across cell membranes. Whereas clinically relevant ABC exporters are implicated in various diseases or cause multidrug resistance of cancer cells, bacterial ABC importers are essential for the uptake of nutrients, including rare elements such as molybdenum. A detailed understanding of their mechanisms requires direct visualization at high resolution and in distinct conformations. Our recent structure of the multidrug ABC exporter Sav1866 has revealed an outward-facing conformation of the transmembrane domains coupled to a closed conformation of the nucleotide-binding domains, reflecting the ATP-bound state. Here we present the 3.1 A crystal structure of a putative molybdate transporter (ModB2C2) from Archaeoglobus fulgidus in complex with its binding protein (ModA). Twelve transmembrane helices of the ModB subunits provide an inward-facing conformation, with a closed gate near the external membrane boundary. The ATP-hydrolysing ModC subunits reveal a nucleotide-free, open conformation, whereas the attached binding protein aligns the substrate-binding cleft with the entrance to the presumed translocation pathway. Structural comparison of ModB2C2A with Sav1866 suggests a common alternating access and release mechanism, with binding of ATP promoting an outward-facing conformation and dissociation of the hydrolysis products promoting an inward-facing conformation.