P(1B)-type ATPases transport heavy metals (Cu+, Cu2+, Zn2+, Co2+, Cd2+, Pb2+) across membranes. Present in most organisms, they are key elements for metal homeostasis. P(1B)-type ATPases contain 6-8 transmembrane fragments carrying signature sequences in segments flanking the large ATP binding cytoplasmic loop. These sequences made possible the differentiation of at least four P(1B)-ATPase subgroups with distinct metal selectivity: P(1B-1): Cu+, P(1B-2): Zn2+, P(1B-3): Cu2+, P(1B-4): Co2+. Mutagenesis of the invariant transmembrane Cys in H6, Asn and Tyr in H7 and Met and Ser in H8 of the Archaeoglobus fulgidus Cu+-ATPase has revealed that their side chains likely coordinate the metals during transport and constitute a central unique component of these enzymes. The structure of various cytoplasmic domains has been solved. The overall structure of those involved in enzyme phosphorylation (P-domain), nucleotide binding (N-domain) and energy transduction (A-domain), appears similar to those described for the SERCA Ca2+-ATPase. However, they show different features likely associated with singular functions of these proteins. Many P(1B)-type ATPases, but not all of them, also contain a diverse arrangement of cytoplasmic metal binding domains (MBDs). In spite of their structural differences, all N- and C-terminal MBDs appear to control the enzyme turnover rate without affecting metal binding to transmembrane transport sites. In addition, eukaryotic Cu+-ATPases have multiple N-MBD regions that participate in the metal dependent targeting and localization of these proteins. The current knowledge of structure-function relationships among the different P(1B)-ATPases allows for a description of selectivity, regulation and transport mechanisms. Moreover, it provides a framework to understand mutations in human Cu+-ATPases (ATP7A and ATP7B) that lead to Menkes and Wilson diseases.

Characterization of a large family of outer membrane channels from gram-negative bacteria suggest how they can thrive in nutrient-poor environments and how channel inactivation can contribute to antibiotic resistance.

Zn 21 plays a critical role in plants as an essential component of key enzymes (Cu-Zn superoxide dismutase, alcohol dehydrogenase, RNA polymerase, etc.) and DNA-binding proteins (Marschner, 1995;Guerinot and Eide, 1999). Zn 21 deficiency leads to a reduction of internodal growth with a consequent rosette-like development and also produces an impaired response to oxidative stress, likely due to a reduction in superoxide dismutase levels (Hacisalihoglu et al., 2003). Thus, Zn 21 deficiency is a significant agricultural problem, particularly in cereals, limiting crop production and quality (Guerinot and Eide, 1999;Hacisalihoglu et al., 2003 Silva and Williams, 2001;Hall, 2002). Consequently, plants and other organisms have developed molecular chaperones, chelators, and specific transmembrane transporters to (1) absorb and distribute metal micronutrients throughout the entire organism and (2) prevent high cytoplasmic concentrations of free heavy metals ions (Fox and Guerinot, 1998;Rauser, 1999;Guerinot, 2000;Williams et al., 2000;Clemens, 2001;Cobbett and Goldsbrough, 2002;Hall, 2002). These processes require the metal to be transported through permeability barriers and compartments delimited by lipid membranes. Several types of heavy metal transmembrane transporters have been identified in plants (Rea, 1999;Guerinot, 2000;Maser et al., 2001;Baxter et al., 2003). Since metal ions must be transported against electrochemical gradients at some point during plant distribution, metal pumps involved in contragradient transport should play key roles in metal homeostasis. The presence of plant genes encoding proteins that specifically perform this function (mainly P IB -ATPases) is known and their potential importance has been repeatedly noted (Williams et al., 2000;Clemens, 2001;Hall, 2002 (Lutsenko and Kaplan, 1995;Axelsen and Palmgren, 1998;Argü ello, 2003). Initial reports named these proteins CPxATPases (Solioz and Vulpe, 1996). They confer metal tolerance to microorganisms (Solioz and Vulpe, 1996;Rensing et al., 1999) and are essential for the absorption, distribution, and bioaccumulation of metal micronutrients by higher organisms (Bull and Cox, 1994; 1 This work was supported by the U.S. Department of Agriculture (grant no. 2001-35106-10736) and by the National Science Foundation (grant no. MCM-0235165).* Corresponding author; e-mail arguello@wpi.edu; fax 508-831-5933.Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

Cu؉ -ATPases drive metal efflux from the cell cytoplasm. Paramount to this function is the binding of Cu ؉ within the transmembrane region and its coupled translocation across the permeability barrier. Copper is an essential micronutrient (1, 2). It has critical catalytic and electron transfer roles in a number of key proteins (tyrosinase, lysyl oxidase, ferroxidase ceruloplasmin, plastocyanin, etc.). However, when free, copper participates in the production of reactive oxygen species leading to cellular damage. Toward sustaining intracellular copper balance, transmembrane transport systems maintain the copper cell quota, Cu ϩ chaperone proteins traffic the bound metal to specific cellular targets, and metal-sensing transcription factors control copper dependent protein expression (3-5). The metal coordination geometry in these proteins is central to the efficiency of the Cu ϩ mobilization processes. In this direction, the coordination should ensure the specificity and prevent the release of free Cu ϩ to the cytoplasm. Canonical copper metalloproteins have long been characterized and classified based on spectroscopic and magnetic properties (Types I, II, and III) (6 -8). Their study has provided great detail on copper coordination in "permanent" sites where copper is bound during the functional life of the proteins. Cu ϩ linear coordination by invariant Cys residues of chaperone proteins has been described, providing insight into the mechanism of copper trafficking and exchange among similar domains (9, 10). More recently, trigonal coordination by Cys 2 -His sites has been observed, for instance, in Mycobacterium tuberculosis transcription factor CsoR (11). Alternatively, Met n His was found in several Cu ϩ -trafficking proteins located in the oxidizing periplasm of prokaryotes (12)(13)(14). Despite this progress, Cu ϩ distribution and balance cannot be understood without describing the selective coordination during compartmental transmembrane transport.In eukaryotic cells, members of the Ctr family of proteins transport Cu ϩ inside the cell (15). Ctr1 organizes as homotrimers forming transmembrane pores that facilitate Cu ϩ transmembrane translocation by an apparently energy-independent undefined mechanism (16, 17). Although relevant Cu ϩ -binding Met have been observed in the extracellular loops of Ctr1 (15); none of the invariant transmembrane residues appear to be required for transport, and no direct coordination is evident (17).As a counterpart to influx systems, Cu ϩ -ATPases are responsible for cytoplasmic Cu ϩ efflux. Mutations of the human Cu ϩ -ATPase genes, ATP7A and ATP7B, lead to Menkes syndrome and Wilson disease, respectively (18,19). Cu ϩ -ATPases are members of the superfamily of P-type ATPases (20, 21). These couple Cu ϩ transport to the hydrolysis of ATP, following a classical Post catalytic/transport cycle (19,21). In this mechanism, transmembrane metal-binding sites (TMMBSs) 4 are responsible for handling the ion during transmembrane translocation (22). These transmembrane sites are expos...

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