The magnesium ion, Mg 2+ , is essential for myriad biochemical processes and remains the only major biological ion whose transport mechanisms remain unknown. The CorA family of magnesium transporters is the primary Mg 2+ uptake system of most prokaryotes 1-3 and a functional homologue of the eukaryotic mitochondrial magnesium transporter 4 . Here we determine crystal structures of the full-length Thermotoga maritima CorA in an apparent closed state and its isolated cytoplasmic domain at 3.9 Å and 1.85Å resolution, respectively. The transporter is a funnel-shaped homopentamer with two transmembrane helices per monomer. The channel is formed by an inner group of five helices and putatively gated by bulky hydrophobic residues. The large cytoplasmic domain forms a funnel whose wide mouth points into the cell and whose walls are formed by five long helices that are extensions of the transmembrane helices. The cytoplasmic neck of the pore is surrounded, on the outside of the funnel, by a ring of highly conserved positively charged residues. Two negatively charged helices in the cytoplasmic domain extend back towards the membrane on the outside of the funnel and abut the ring of positive charge. An apparent Mg 2+ ion was bound between monomers at a conserved site in the cytoplasmic domain, suggesting a mechanism to link gating of the pore to the intra-cellular concentration of Mg 2+ . The CorA magnesium transporter is a homopentamer with fivefold symmetry about a central pore and can be divided into three parts (Fig. 1). A carboxy-terminal transmembrane domain comprises two transmembrane helices from each monomer (Fig. 2). The middle portion resembles a funnel, narrow at the entrance ( 5 Å) and wide at the mouth ( 20Å), that is formed largely by a long -helix extension of the inner transmembrane helix. Finally, a large cytoplasmic domain lies exterior to the funnel.The cytoplasmic domain of CorA is a seven-stranded parallel/antiparallel -sheet ( 2 1 3 7 6 5 4 ) sandwiched between two sets of -helices ( 1, 2, 3) and ( 4, 5, 6) ( Fig. 1). The domain fold is unlike all other known structures of ion channels or transporters and constitutes a new protein fold (see Supplementary Information). This domain, solved in its soluble form at 1.85 Å resolution ( Supplementary Fig. S1), is linked to the transmembrane helices by the long 7 helix (residues 251-312), termed the stalk helix. The stalk helix kinks as it enters the membrane, extends through the membrane, forms the first transmembrane helix (TM1; residues 293-312) and harbours the 'YGMNF' signature sequence of CorA (residues 311-315) 5,6 ( Fig. 2 and Supplementary Fig. S2). The five TM1 helices (residues 293-312) form the pore. After a short extracellular seven-amino-acid loop, the TM2 helix (residues 326-345) returns back to the cytoplasm and ends in a highly conserved C-terminal KKKKWL motif (Fig. 3). In the current structure, neither the extracellular loop nor the final two amino acids could be resolved.The cytoplasmic domain shows the lowest sequence conservati...

Opioid peptides are involved in various essential physiological processes, most notably nociception. Dipeptidyl peptidase III (DPP III) is one of the most important enkephalin-degrading enzymes associated with the mammalian pain modulatory system. Here we describe the X-ray structures of human DPP III and its complex with the opioid peptide tynorphin, which rationalize the enzyme's substrate specificity and reveal an exceptionally large domain motion upon ligand binding. Microcalorimetric analyses point at an entropy-dominated process, with the release of water molecules from the binding cleft ("entropy reservoir") as the major thermodynamic driving force. Our results provide the basis for the design of specific inhibitors that enable the elucidation of the exact role of DPP III and the exploration of its potential as a target of pain intervention strategies.isothermal titration calorimetry | metallopeptidase | peptide binding | X-ray crystallography T he endogenous opioid system, composed of opioid peptides and their receptors, modulates a large number of physiological processes, such as endocrine and immune function, gastrointestinal motility, respiration, reward, stress, complex social behavior (e.g., sexual activity), vulnerability to drug addiction, and most notably the procession and transmission of pain stimuli (nociception) (1, 2). Two major types of endogenous opioid peptides are those containing enkephalin sequences at the N terminus (TyrGly-Gly-Phe-Met/Leu) (3) and, more recently identified, endomorphins 1 and 2 (Tyr-Pro-Trp/Phe-Phe-NH 2 ) (4, 5). Knowledge and control over synthesis and degradation pathways of this class of molecules is prerequisite for the development of new therapies that target pertinent physiological processes.Dipeptidyl peptidase III (DPP III), also known as enkephalinase B, is an enkephalin-degrading enzyme that cleaves dipeptides sequentially from the N termini of substrates (6). All DPP IIIs described thus far contain the unique zinc-binding motif HEXXGH characteristic of metallopeptidase family M49 (7). Enzymes from several human and animal tissues, as well as from lower eukaryotes, were purified and biochemically characterized (8, 9). DPP III is largely found as a cytosolic protein, although membrane association in rat brain and Drosophila melanogaster has been described (10, 11). The 3D structure of the yeast ortholog has recently been determined, revealing a unique protein fold with two lobes forming a wide-open substrate-binding cleft (12). The lack of structural information on peptide complexes, however, left the question of substrate specificity largely unanswered.DPP III purified from monkey brain microsomes is strongly inhibited by the neuropeptide spinorphin (Leu-Val-Val-Tyr-ProTrp-Thr), an endogenous factor isolated from bovine spinal cord that also inhibits other enkephalin-degrading enzymes, such as neutral endopeptidase (NEP, neprilysin), aminopeptidase, and angiotensin-converting enzyme (13). Because of a different mode of action compared with morphine, spinorp...

PRMT3, a protein arginine methyltransferase, has been shown to influence ribosomal biosynthesis by catalyzing the dimethylation of the 40S ribosomal protein S2. Although PRMT3 has been reported to be a cytosolic protein, it has been shown to methylate histone H4 peptide (H4 1-24) in vitro. Here, we report the identification of a PRMT3 inhibitor (1-(benzo[d][1,2,3]thiadiazol-6-yl)-3-(2-cyclohexenylethyl)urea; compound 1) with IC50 value of 2.5 μM by screening a library of 16,000 compounds using H4 (1-24) peptide as a substrate. The crystal structure of PRMT3 in complex with compound 1 as well as kinetic analysis reveals an allosteric mechanism of inhibition. Mutating PRMT3 residues within the allosteric site or using compound 1 analogs that disrupt interactions with allosteric site residues both abrogated binding and inhibitory activity. These data demonstrate an allosteric mechanism for inhibition of protein arginine methyltransferases, an emerging class of therapeutic targets.

PRMT3 catalyzes the asymmetric dimethylation of arginine residues of various proteins. It is essential for maturation of ribosomes, may have a role in lipogenesis, and is implicated in several diseases. A potent, selective, and cell- active PRMT3 inhibitor would be a valuable tool for further investigating PRMT3 biology. Here we report the discovery of the first PRMT3 chemical probe, SGC707, by structure-based optimization of the allosteric PRMT3 inhibitors we reported previously, and thorough characterization of this probe in biochemical, biophysical, and cellular assays. SGC707 is a potent PRMT3 inhibitor (IC50 = 31 ± 2 nm, KD = 53 ± 2 nm) with outstanding selectivity (selective against 31 other methyltransferases and more than 250 non-epigenetic targets). The mechanism of action studies and crystal structure of the PRMT3-SGC707 complex confirm the allosteric inhibition mode. Importantly, SGC707 engages PRMT3 and potently inhibits its methyltransferase activity in cells. It is also bioavailable and suitable for animal studies. This well- characterized chemical probe is an excellent tool to further study the role of PRMT3 in health and disease.

] These authors contributed equally to this work.Supporting information for this article (including detailed synthetic procedures and compound characterization as well as methods for scaffold hopping, crystallization, structure determination, bio-chemical assays, SPR, ITC, PRMT3 InCELL Hunter assay, cellular PRMT3 assay, cell viability assay, and mouse PK studies) is available on the WWW under http://dx

Protein arginine methyltransferases (PRMTs) play an important role in diverse biological processes. Among the nine known human PRMTs, PRMT3 has been implicated in ribosomal biosynthesis via asymmetric dimethylation of the 40S ribosomal protein S2 and in cancer via interaction with the DAL-1 tumor suppressor protein. However, few selective inhibitors of PRMTs have been discovered. We recently disclosed the first selective PRMT3 inhibitor, which occupies a novel allosteric binding site and is noncompetitive with both the peptide substrate and cofactor. Here we report comprehensive structure–activity relationship studies of this series, which resulted in the discovery of multiple PRMT3 inhibitors with submicromolar potencies. An X-ray crystal structure of compound 14u in complex with PRMT3 confirmed that this inhibitor occupied the same allosteric binding site as our initial lead compound. These studies provide the first experimental evidence that potent and selective inhibitors can be created by exploiting the allosteric binding site of PRMT3.

Abbreviations: CTBP, C-terminal binding protein; ELISA, enzyme linked immunosorbant assay; HCDR3, Heavy chain complementarity determining region 3; HPA, Human Protein Atlas; scFv, single chain Fv; PrESTs, Protein epitope signature tag; rrpAbs, recombinant renewable polyclonal antibodies; SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis; TEV, tobacco etch virusOnly a small fraction of the antibodies in a traditional polyclonal antibody mixture recognize the target of interest, frequently resulting in undesirable polyreactivity. Here, we show that high-quality recombinant polyclonals, in which hundreds of different antibodies are all directed toward a target of interest, can be easily generated in vitro by combining phage and yeast display. We show that, unlike traditional polyclonals, which are limited resources, recombinant polyclonal antibodies can be amplified over one hundred million-fold without losing representation or functionality. Our protocol was tested on 9 different targets to demonstrate how the strategy allows the selective amplification of antibodies directed toward desirable target specific epitopes, such as those found in one protein but not a closely related one, and the elimination of antibodies recognizing common epitopes, without significant loss of diversity. These recombinant renewable polyclonal antibodies are usable in different assays, and can be generated in high throughput. This approach could potentially be used to develop highly specific recombinant renewable antibodies against all human gene products.

Membrane proteins constitute ~30% of prokaryotic and eukaryotic genomes but comprise a small fraction of the entries in protein structural databases. A number of features of membrane proteins render them challenging targets for the structural biologist, among which the most important is the difficulty in obtaining sufficient quantities of purified protein. We are exploring procedures to express and purify large numbers of prokaryotic membrane proteins. A set of 280 membrane proteins from Escherichia coli and Thermotoga maritima, a thermophile, was cloned and tested for expression in Escherichia coli. Under a set of standard conditions, expression could be detected in the membrane fraction for approximately 30% of the cloned targets. About 22 of the highest expressing membrane proteins were purified, typically in just two chromatographic steps. There was a clear correlation between the number of predicted transmembrane domains in a given target and its propensity to express and purify. Accordingly, the vast majority of successfully expressed and purified proteins had six or fewer transmembrane domains. We did not observe any clear advantage to the use of thermophilic targets. Two of the purified membrane proteins formed crystals. By comparison with protein production efforts for soluble proteins, where approximately 70% of cloned targets express and approximately 25% can be readily purified for structural studies [Christendat et al. (2000) Nat. Struct. Biol., 7, 903], our results demonstrate that a similar approach will succeed for membrane proteins, albeit with an expected higher attrition rate.