Na+/Cl--dependent transporters terminate synaptic transmission by using electrochemical gradients to drive the uptake of neurotransmitters, including the biogenic amines, from the synapse to the cytoplasm of neurons and glia. These transporters are the targets of therapeutic and illicit compounds, and their dysfunction has been implicated in multiple diseases of the nervous system. Here we present the crystal structure of a bacterial homologue of these transporters from Aquifex aeolicus, in complex with its substrate, leucine, and two sodium ions. The protein core consists of the first ten of twelve transmembrane segments, with segments 1-5 related to 6-10 by a pseudo-two-fold axis in the membrane plane. Leucine and the sodium ions are bound within the protein core, halfway across the membrane bilayer, in an occluded site devoid of water. The leucine and ion binding sites are defined by partially unwound transmembrane helices, with main-chain atoms and helix dipoles having key roles in substrate and ion binding. The structure reveals the architecture of this important class of transporter, illuminates the determinants of substrate binding and ion selectivity, and defines the external and internal gates.

SummaryP2X receptors are cation selective ion channels gated by extracellular ATP and implicated in diverse physiological processes, from synaptic transmission to inflammation to the sensing of taste and pain. Because P2X receptors are not related to other ion channel proteins of known structure, there is presently no molecular foundation for mechanisms of ligand-gating, allosteric modulation and ion permeation. Here we present crystal structures of the zebrafish P2X4 receptor in its closed, resting state. The chalice-shaped, trimeric receptor is knit together by subunit-subunit contacts implicated in ion channel gating and receptor assembly. Extracellular domains, rich in β-strands, have large acidic patches that may attract cations, through fenestrations, to vestibules near the ion channel. Within the transmembrane pore, the ‘gate’ is defined by an ~8 Ǻ slab of protein. We define the location of three non-canonical, intersubunit ATP binding sites and suggest that ATP binding promotes subunit rearrangement and ion channel opening.

Formation of well-ordered crystals of membrane proteins is a bottleneck for structure determination by X-ray crystallography. Nevertheless, one can increase the probability of successful crystallization by precrystallization screening, a process by which one analyzes the monodispersity and stability of the protein-detergent complex. Traditionally, this has required microgram to milligram quantities of purified protein and a concomitant investment of time and resources. Here, we describe a rapid and efficient precrystallization screening strategy in which the target protein is covalently fused to green fluorescent protein (GFP) and the resulting unpurified protein is analyzed by fluorescence-detection size-exclusion chromatography (FSEC). This strategy requires only nanogram quantities of unpurified protein and allows one to evaluate localization and expression level, the degree of monodispersity, and the approximate molecular mass. We show the application of this precrystallization screening to four membrane proteins derived from prokaryotic or eukaryotic organisms.

SummaryAcid-sensing ion channels are proton-activated, sodium-selective channels composed of three subunits and members of the superfamily of epithelial sodium channels, mechanosensitive and FMRF peptide-gated ion channels. These ubiquitous eukaryotic ion channels play essential roles in biological activities as diverse as sodium homeostasis, taste, and pain. Despite their crucial roles in biology and their unusual trimeric subunit stoichiometry, there is little knowledge of the structural and chemical principles underlying their ion channel architecture and ion binding sites. Here we describe the structure of a functional acid sensing ion channel in a desensitized state at 3 Å resolution, the location and composition of the ~8 Å ‘thick’ desensitization gate, and the trigonal antiprism coordination of cesium ions bound in the extracellular vestibule. Comparison of the acid sensing ion channel structure with the ATP-gated P2×4 receptor reveals similarity in pore architecture and aqueous vestibules, suggesting unanticipated yet common structural and mechanistic principles.

We report the generation and characterization of transgenic mouse and zebrafish expressing green fluorescent protein (GFP) specifically in vascular endothelial cells in a relatively uniform fashion. These reporter lines exhibit fluorescent vessels in developing embryos and throughout adulthood, allowing visualization of the general vascular patterns with single cell resolution. Furthermore, we show the ability to purify endothelial cells from whole embryos and adult organs by a single step fluorescence activated cell sorting. We expect that these transgenic reporters will be useful tools for imaging vascular morphogenesis, global gene expression profile analysis of endothelial cells, and high throughput screening for vascular mutations.