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Multidrug and toxic compound extrusion (MATE) transporters mediate excretion of xenobiotics and toxic metabolites, thereby conferring multidrug resistance in bacterial pathogens and cancer cells. Structural information on the alternate conformational states and knowledge of the detailed mechanism of MATE transport are of great importance for drug development. However, the structures of MATE transporters are only known in V-shaped outward-facing conformations. Here, we present the crystal structure of a MATE transporter from Pyrococcus furiosus (PfMATE) in the long-soughtafter inward-facing state, which was obtained after crystallization in the presence of native lipids. Transition from the outward-facing state to the inward-facing state involves rigid body movements of transmembrane helices (TMs) 2-6 and 8-12 to form an inverted V, facilitated by a loose binding of TM1 and TM7 to their respective bundles and their conformational flexibility. The inward-facing structure of PfMATE in combination with the outward-facing one supports an alternating access mechanism for the MATE family transporters. multidrug resistance | membrane protein structure | MATE transporter | inward-facing conformation | lipids

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ZIP9 is a Zn transporter, testosterone receptor, and mediator of signaling events through G-proteins. Despite these pivotal properties, however, its physiological and pathophysiological significance has not yet been comprehensively addressed. Using a cell line that lacks the classical androgen receptor we show that ZIP9-mediated phosphorylation of Erk1/2, CREB, or ATF-1 and expression of claudin-5 and zonula occludens-1 by testosterone can be completely antagonized by bicalutamide (Casodex), an anti-androgen of significant clinical impact. Computational modeling and docking experiments with ZIP9 reveal typical characteristics of ZIP transporters and an extracellular binding site for testosterone capable of accommodating bicalutamide. The presence of this site is verified by our demonstration that the membrane-impermeable testosterone analogue T-BSA-FITC labels the membrane only when ZIP9 is expressed and that this labeling is completely prevented by bicalutamide. The study connects structural features of ZIP9 to its functions and indicates a possible relevance of ZIP9 as a pharmacological target.

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Background: LysP plays an important role in transport of lysine in bacteria. Results: Binding of lysine to purified STM2200 was investigated using ITC; key residues involved in binding are predicted by structure modeling and verified experimentally. Conclusion: L-Lysine binds to STM2200 selectively, and important residues were identified. Significance: Our findings provide a functional characterization of a lysine transporter.

Multidrug and toxic compound extrusion (MATE) transportThe ability of all living organisms to protect themselves against toxic substances is essential for their survival. Organisms acquire this ability through evolutionary force (1). Due to the development of scientific knowledge, more and more medical drugs are being developed and applied. As a consequence, general defense mechanisms to resist antibiotics, drugs, and other toxic compounds have been evolved in bacteria (2). Being one of those defense mechanisms, the active extrusion of the toxic compounds from the cells plays an important role (3). In bacteria, the extrusion is often carried out by multidrug transporters, which are integral membrane proteins. They are classified into five distinct families: ATP-binding cassette, multidrug and toxic compound extrusion (MATE), 4 major facilitator superfamily, resistance nodulation division, and small multidrug resistance transporters (4).In 1999, the MATE family proteins were first recognized as a new group of the multidrug transporters, existing in all three domains of life (5). Most members of this family consist of a single chain of 450 -550 amino acid residues and exhibit 12 putative transmembrane helices (TMHs). Up to now, about 900 proteins have been annotated as MATE transporters on the basis of amino acid sequence similarities. Moreover, a subclassification of the MATE family has been suggested, NorM-, DinF-(DNA damage-inducible protein F), and eukaryotic subfamily (4, 6, 7). Various compounds can be recognized by NorM proteins, including dyes, fluoroquinolones, and aminoglycosides (7). As secondary active transporters, MATE proteins utilize transmembrane Na ϩ or H ϩ electrochemical gradients to extrude toxic compounds.So far atomic structures of four MATE transporters have been reported in their putative outward-facing states, in both drug-bound and/or drug-free states. NorM_VC from Vibrio cholerae and NorM_NG from Neisseria gonorrhoeae, are both Na ϩ driven, whereas PfMATE from Pyrococcus furiosus and DinF-BH from Bacillus halodurans are published to be H ϩ -dependent antiporters (8 -11). All these four proteins contain 12 TMHs with intracellular N and C termini, and possess a hydrophobic internal cavity, formed by two symmetric bundles with 6 TMHs each. The two bundles are linked by a cytoplasmic loop between the 6th and 7th TMH. All MATE proteins share about 40% sequence similarity, and they employ the same rockerswitch mechanism for substrate extrusion (12). In addition, the crystal structures have also revealed different locations of cationand substrate-binding sites, suggesting a mechanistic diversity among MATE transporters. In the structure of drug-bound NorM_NG, the binding site is located close to the membraneperiplasm interface, whereas in the structure of PfMATE, the

X-ray crystallography is still the most prominent technique in use to decipher the 3D structures of membrane proteins. For successful crystallization, sample quality is the most important parameter that should be addressed. In almost every case, highly pure, monodisperse, and stable protein sample is a prerequisite. Vapor diffusion is in general the method of choice for obtaining crystals. Here, we discuss a detailed protocol for overproduction and purification of the inner-membrane multidrug transporter AcrB and of DARPins, which are used for crystallization of the AcrB/DARPin complex, resulting in high-resolution diffraction and subsequent structure determination.