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Wildlife conservation efforts have traditionally prioritized protection of species in the wild over protection in zoos and other captive states. This emphasis mirrors a long-held and more general Western view of the wild and wilderness as antidote to the ills of civilization. However, recent philosophical treatments have posited that with the rise of human dominance in the world we have reached the end of nature as distinct from human culture, true wilderness no longer exists, and nature and wilderness are merely social constructs. With the putative disappearance of wild nature, its status as an organizing principle in conservation is called into question. While debate over the objective existence of wild nature continues, this commentary argues that regardless of one’s opinion on the philosophical issue, there are important practical reasons to continue to prioritize conservation of species in the wild. These are: the reality that this is the only practical hope for the vast majority of endangered species, the value of relatively wild habitats in accommodating species’ evolved requirements, the value to both ecosystems and humans of species remaining functional components of ecosystems, and the role of the wild in inspiring conservation action by humans. Ironically, as the world becomes ever less wild, conservation of species in the wild becomes more important.

Auxins are hormones that have central roles and control nearly all aspects of growth and development in plants 1 – 3 . The proteins in the PIN-FORMED (PIN) family (also known as the auxin efflux carrier family) are key participants in this process and control auxin export from the cytosol to the extracellular space 4 – 9 . Owing to a lack of structural and biochemical data, the molecular mechanism of PIN-mediated auxin transport is not understood. Here we present biophysical analysis together with three structures of Arabidopsis thaliana PIN8: two outward-facing conformations with and without auxin, and one inward-facing conformation bound to the herbicide naphthylphthalamic acid. The structure forms a homodimer, with each monomer divided into a transport and scaffold domain with a clearly defined auxin binding site. Next to the binding site, a proline–proline crossover is a pivot point for structural changes associated with transport, which we show to be independent of proton and ion gradients and probably driven by the negative charge of the auxin. The structures and biochemical data reveal an elevator-type transport mechanism reminiscent of bile acid/sodium symporters, bicarbonate/sodium symporters and sodium/proton antiporters. Our results provide a comprehensive molecular model for auxin recognition and transport by PINs, link and expand on a well-known conceptual framework for transport, and explain a central mechanism of polar auxin transport, a core feature of plant physiology, growth and development. Structural and biophysical analysis of the Arabidopsis thaliana auxin transporter PIN8 reveal that PIN transporters export auxin using an elevator mechanism.

The crystal structure of the bacterial potassium import complex KdpFABC shows how ATP hydrolysis is coupled to potassium transport to maintain cellular homeostasis under low potassium conditions. Potassium pump Potassium homeostasis is fundamental for regulating osmotic pressure, pH and membrane potential. K + import into the cells of many bacteria occurs through several mechanisms, though at low concentrations they employ KdpFABC, a transport protein complex that couples the functionality of a pump-like subunit with a channel-like subunit. Here David Stokes and colleagues report the crystal structure of KdpFABC and show how this complex acts as a potassium pump, coupling ATP hydrolysis to the transport of K + into the cytosol. A tunnel connects the KdpA and KdpB subunits, coupling the ion-binding site to the hydrolysis of ATP. This work offers new insights into the regulation of transporter and channel gating. Cellular potassium import systems play a fundamental role in osmoregulation, pH homeostasis and membrane potential in all domains of life. In bacteria, the kdp operon encodes a four-subunit potassium pump that maintains intracellular homeostasis, cell shape and turgor under conditions in which potassium is limiting 1 . This membrane complex, called KdpFABC, has one channel-like subunit (KdpA) belonging to the superfamily of potassium transporters and another pump-like subunit (KdpB) belonging to the superfamily of P-type ATPases. Although there is considerable structural and functional information about members of both superfamilies, the mechanism by which uphill potassium transport through KdpA is coupled with ATP hydrolysis by KdpB remains poorly understood. Here we report the 2.9 Å X-ray structure of the complete Escherichia coli KdpFABC complex with a potassium ion within the selectivity filter of KdpA and a water molecule at a canonical cation site in the transmembrane domain of KdpB. The structure also reveals two structural elements that appear to mediate the coupling between these two subunits. Specifically, a protein-embedded tunnel runs between these potassium and water sites and a helix controlling the cytoplasmic gate of KdpA is linked to the phosphorylation domain of KdpB. On the basis of these observations, we propose a mechanism that repurposes protein channel architecture for active transport across biomembranes.

YiiP is a dimeric antiporter from the cation diffusion facilitator family that uses the proton motive force to transport Zn2+ across bacterial membranes. Previous work defined the atomic structure of an outward-facing conformation, the location of several Zn2+ binding sites, and hydrophobic residues that appear to control access to the transport sites from the cytoplasm. A low-resolution cryo-EM structure revealed changes within the membrane domain that were associated with the alternating access mechanism for transport. In the current work, the resolution of this cryo-EM structure has been extended to 4.1 Å. Comparison with the X-ray structure defines the differences between inward-facing and outward-facing conformations at an atomic level. These differences include rocking and twisting of a four-helix bundle that harbors the Zn2+ transport site and controls its accessibility within each monomer. As previously noted, membrane domains are closely associated in the dimeric structure from cryo-EM but dramatically splayed apart in the X-ray structure. Cysteine crosslinking was used to constrain these membrane domains and to show that this large-scale splaying was not necessary for transport activity. Furthermore, dimer stability was not compromised by mutagenesis of elements in the cytoplasmic domain, suggesting that the extensive interface between membrane domains is a strong determinant of dimerization. As with other secondary transporters, this interface could provide a stable scaffold for movements of the four-helix bundle that confers alternating access of these ions to opposite sides of the membrane.

Under osmotic stress, bacteria express a heterotetrameric protein complex, KdpFABC, which functions as an ATP-dependent K + pump to maintain intracellular potassium levels. The subunit KdpA belongs to the superfamily of K + transporters and adopts pseudo fourfold symmetry with a membrane-embedded selectivity filter as seen in K + channels. KdpB belongs to the superfamily of P-type ATPases with a conserved binding site for ions within the membrane domain and three cytoplasmic domains that orchestrate ATP hydrolysis via an aspartyl phosphate intermediate. Previous work hypothesized that K + moves parallel to the membrane plane through a 40 Å long tunnel that connects the selectivity filter of KdpA with a canonical binding site in KdpB. In the current work, we have reconstituted KdpFABC into lipid nanodiscs and used cryo-EM to image the wild-type pump under turnover conditions. We present a 2.1 Å structure of the E1~P·ADP conformation, which reveals new features of the conduction pathway. This map shows strong densities within the selectivity filter and at the canonical binding site, consistent with K + bound at each of these sites in this conformation. Many water molecules occupy a vestibule and the proximal end of the tunnel, which becomes markedly hydrophobic and dewetted at the subunit interface. We go on to use ATPase and ion transport assays to assess effects of numerous mutations along this proposed conduction pathway. The results confirm that K + ions pass through the tunnel and support the existence of a low-affinity site in KdpB for releasing these ions to the cytoplasm. Taken together, these data shed new light on the unique partnership between a transmembrane channel and an ATP-driven pump in maintaining the large electrochemical K + gradient essential for bacterial survival.

Tafazzin, the mitochondrial transacylase that is deficient in Barth syndrome, selects lipid substrates in the inverted hexagonal phase but does not react with bilayer lipids. Cardiolipin is a mitochondrial phospholipid with a characteristic acyl chain composition that depends on the function of tafazzin, a phospholipid-lysophospholipid transacylase, although the enzyme itself lacks acyl specificity. We incubated isolated tafazzin with various mixtures of phospholipids and lysophospholipids, characterized the lipid phase by 31 P-NMR and measured newly formed molecular species by MS. Substantial transacylation was observed only in nonbilayer lipid aggregates, and the substrate specificity was highly sensitive to the lipid phase. In particular, tetralinoleoyl-cardiolipin, a prototype molecular species, formed only under conditions that favor the inverted hexagonal phase. In isolated mitochondria, <1% of lipids participated in transacylations, suggesting that the action of tafazzin was limited to privileged lipid domains. We propose that tafazzin reacts with non–bilayer-type lipid domains that occur in curved or hemifused membrane zones and that acyl specificity is driven by the packing properties of these domains.

Human preferences will increasingly determine many species' prospects for survival. However, aside from a small number of survey-based studies of preference among disparate taxa, human species preferences have received little attention. I determined human aesthetic preferences among a relatively homogenous group, the penguins, from representation in all recently published, comprehensive, popular books of photographs of penguins (n=4 books; 304 photographs). Representation of visually distinguishable types of penguins, measured by total photograph area, was highly skewed and rankings were highly concordant across books, suggesting large and commonly held differences in aesthetic appeal. Multiple regression analysis indicated that amount of warm color was the only significant determinant of representation, and warm color was highly correlated (r2=0.96) with mean representation of the penguin types. Body size and neotenic form, traits found to influence human preferences among other animals, were not significant, suggesting that the bases of human species preferences differ by species type. The results of this study indicate that human aesthetic preferences discriminate finely among species and may be based on minor features. Conservationists must be vigilant to the potential for aesthetic responses to influence conservation efforts.

YiiP from Shewanella oneidensis is a prokaryotic Zn 2+ /H + antiporter that serves as a model for the Cation Diffusion Facilitator (CDF) superfamily, members of which are generally responsible for homeostasis of transition metal ions. Previous studies of YiiP as well as related CDF transporters have established a homodimeric architecture and the presence of three distinct Zn 2+ binding sites named A, B, and C. In this study, we use cryo-EM, microscale thermophoresis and molecular dynamics simulations to address the structural and functional roles of individual sites as well as the interplay between Zn 2+ binding and protonation. Structural studies indicate that site C in the cytoplasmic domain is primarily responsible for stabilizing the dimer and that site B at the cytoplasmic membrane surface controls the structural transition from an inward facing conformation to an occluded conformation. Binding data show that intramembrane site A, which is directly responsible for transport, has a dramatic pH dependence consistent with coupling to the proton motive force. A comprehensive thermodynamic model encompassing Zn 2+ binding and protonation states of individual residues indicates a transport stoichiometry of 1 Zn 2+ to 2–3 H + depending on the external pH. This stoichiometry would be favorable in a physiological context, allowing the cell to use the proton gradient as well as the membrane potential to drive the export of Zn 2+ .

We simulate deformable red blood cells in the microcirculation using the immersed boundary method with a cytoskeletal model that incorporates structural details revealed by tomographic images. The elasticity of red blood cells is known to be supplied by both their lipid bilayer membranes, which resist bending and local changes in area, and their cytoskeletons, which resist in-plane shear. The cytoskeleton consists of spectrin tetramers that are tethered to the lipid bilayer by ankyrin and by actin-based junctional complexes. We model the cytoskeleton as a random geometric graph, with nodes corresponding to junctional complexes and with edges corresponding to spectrin tetramers such that the edge lengths are given by the end-to-end distances between nodes. The statistical properties of this graph are based on distributions gathered from three-dimensional tomographic images of the cytoskeleton by a segmentation algorithm. We show that the elastic response of our model cytoskeleton, in which the spectrin polymers are treated as entropic springs, is in good agreement with the experimentally measured shear modulus. By simulating red blood cells in flow with the immersed boundary method, we compare this discrete cytoskeletal model to an existing continuum model and predict the extent to which dynamic spectrin network connectivity can protect against failure in the case of a red cell subjected to an applied strain. The methods presented here could form the basis of disease- and patient-specific computational studies of hereditary diseases affecting the red cell cytoskeleton.

KdpFABC is an ATP-dependent K + pump that ensures bacterial survival in K + -deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down-regulation when K + levels are restored has not been described. Here, we show that KdpFABC is inhibited when cells return to a K + -rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation abolishes the K + -dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.