Tumor protein D54 binds intracellular nanovesicles via an amphipathic lipid packing sensor (ALPS) motif

Tumor Protein D54 (TPD54) is an abundant cytosolic protein that belongs to the TPD52 family, a family of four proteins (TPD52, 53, 54 and 55) that are overexpressed in several cancer cells. Even though the functions of these proteins remain elusive, recent investigations indicate that TPD54 binds to very small cytosolic vesicles with a diameter of ca. 30 nm, half the size of classical transport vesicles (e.g. COPI and COPII). Here, we investigated the mechanism of intracellular nanovesicle capture by TPD54. Bioinformatical analysis suggests that TPD54 contains a small coiled-coil followed by several amphipathic helices, which could fold upon binding to lipid membranes. One of these helices has the physicochemical features of an Amphipathic Lipid Packing Sensor (ALPS) motif, which, in other proteins, enables membrane binding in a curvature-dependent manner. Limited proteolysis, CD spectroscopy, tryptophan fluorescence and cysteine mutagenesis coupled to covalent binding of a membrane sensitive probe show that binding of TPD54 to small liposomes is accompanied by large structural changes in the amphipathic helix region. TPD54 binding to artificial liposomes is very sensitive to liposome size and to lipid unsaturation but is poorly dependent on lipid charge. Cellular investigations confirmed the key role of the ALPS motif in vesicle targeting. Surprisingly, the vesicles selected by TPD54 poorly overlap with those captured by the golgin GMAP-210, a long vesicle tether at the Golgi apparatus, which displays a dimeric coiled-coil architecture and an N-terminal ALPS motif. We propose that TPD54 recognizes nanovesicles through a combination of ALPS-dependent and -independent mechanisms.

Discovery of Aminoglycosides as First in Class, Nanomolar Inhibitors of Heptosyltransferase I

A clinically relevant inhibitor for Heptosyltransferase I (HepI) has been sought after for many years and while many have designed novel small-molecule inhibitors, these compounds lack the bioavailability and potency necessary for therapeutic use. Extensive characterization of the HepI protein has provided valuable insight into the dynamic motions necessary for catalysis that could be targeted for inhibition. With the help of molecular dynamic simulations, aminoglycoside antibiotics were shown to be putative inhibitors for HepI and in this study, they were experimentally determined to be the first in-class nanomolar inhibitors of HepI with the best inhibitor demonstrating a Ki of 600 +/- 90 nM. Detailed kinetic analyses were performed to determine the mechanism of inhibition while circular dichroism spectroscopy, intrinsic tryptophan fluorescence, docking, and MD simulations were used to corroborate kinetic experimental findings. Kinetic analysis methods include Lineweaver-Burk, Dixon, Cornish-Bowden and Mixed-Model of Inhibition which allowed for unambiguous assignment of inhibition mechanism for each inhibitor. In this study, we show that neomycin and kanamycin b are competitive inhibitors against the sugar acceptor substrate while tobramycin exhibits a mixed inhibitory effect and streptomycin is non-competitive. MD simulations also allowed us to suggest that the inhibitors bind tightly and inhibit catalytic dynamics due to a major desolvation penalty of the enzyme active site. While aminoglycosides have long been known as a class of potent antibiotics, they also have been scientifically shown to impact cell membrane stability, and we propose that inhibition of HepI contributes to this effect by disrupting lipopolysaccharide biosynthesis.

The human TRPA1 intrinsic cold and heat sensitivity involves separate channel structures beyond the N-ARD domain

The human TRPA1 (hTRPA1) is an intrinsic thermosensitive ion channel responding to both cold and heat, depending on the redox environment. Here, we have studied purified hTRPA1 truncated proteins to gain further insight into the temperature gating of hTRPA1. We found in patch-clamp bilayer recordings that Δ1-688 hTRPA1, without the N-terminal ankyrin repeat domain (N-ARD), was more sensitive to cold and heat, whereas Δ1-854 hTRPA1 that is also lacking the S1-S4 voltage sensing-like domain (VSLD) gained sensitivity to cold but lost its heat sensitivity. The thiol reducing agent TCEP abolished the temperature sensitivity of both Δ1-688 hTRPA1 and Δ1-854 hTRPA1. Cold and heat activity of Δ1-688 hTRPA1 and Δ1-854 hTRPA1 were associated with different structural conformational changes as revealed by intrinsic tryptophan fluorescence measurements. Heat evoked major structural rearrangement of the VSLD as well as the C-terminus domain distal to the transmembrane pore domain S5-S6 (CTD), whereas cold only caused minor conformational changes. As shown for Δ1-854 hTRPA1, a sudden drop in tryptophan fluorescence occurred within 25-20°C indicating a transition between heat and cold conformations of the CTD, and thus it is proposed that the CTD contains a bidirectional temperature switch priming hTRPA1 for either cold or heat. In whole-cell patch clamp electrophysiology experiments, replacement of the cysteines 865, 1021 and 1025 with alanine modulated the cold sensitivity of hTRPA1 when heterologously expressed in HEK293T cells. It is proposed that the hTRPA1 CTD harbors cold and heat sensitive domains allosterically coupled to the S5-S6 pore region and the VSLD, respectively.

Pyridoxal 5'-phosphate supplementation modulates the heterologous expression and activity of a PLP dependent model protein in E.coli

PLP is a biologically active form of Vitamin B6 and is required for carbohydrates, amino acids and fatty acid metabolism. Many of the PLP dependent proteins are important drug targets and effector molecules, and thus, their heterologous overexpression is of industrial importance and has commercial value. We have predicted the docking site of PLP on O-acetyl serine sulfhydrylase protein (OASS) of H.contortus and determined that lysine-47 is very important for the binding of PLP in the enzyme pocket. We have used this protein as a model protein for testing the effect of PLP on the expression of PLP dependent proteins by E.coli. We have tested the effect of supplementation of PLP in the media on the expression of PLP dependent model protein in E.coli. Soluble recombinant protein could be purified from each of the culture vials grown with variable amount of PLP [0 mM (Group I), 0.01mM (Group II), 0.025mM (Group III), 0.05mM (Group IV) and 0.1mM (Group V)]. There was approximately 4.2%, 7.2%, 10.5% and 18% increase in purified protein yield in Group II, III, IV and V, respectively, in comparison to group I. We studied the relative incorporation of PLP into the purified protein by scanning the changes in internal fluorescence of purified proteins. There was a significant quenching of tryptophan fluorescence emission in groups II, III, IV and V compared to group I (Purified protein without PLP addition). There was a linear increase in the activity of protein purified from cultures of group I to group V. This was due to greater availability of PLP, thus, allowing higher incorporation of the cofactor in the apoenzyme to form holoenzyme complexes. PLP is not known to be directly imported into E.coli. We could find a PLP concentration-dependent increase in expression and catalytic activity of the enzyme signifying the probable transport of PLP across the membrane. The mechanism of transport of PLP in the light of the current experiment is still unknown and should be a subject of future studies.

Identification and functional analysis of cadmium-binding protein in the visceral mass of Crassostrea gigas

The Pacific oyster, Crassostrea gigas, is a traditional food worldwide. The soft body of the oyster can easily accumulate heavy metals such as cadmium (Cd). To clarify the molecular mechanism of Cd accumulation in the viscera of C. gigas, we identified Cd-binding proteins. 5,10,15,20-Tetraphenyl-21H,23H-porphinetetrasulfonic acid, disulfuric acid, tetrahydrate, and Cd-binding competition experiments using immobilized metal ion affinity chromatography revealed the binding of water-soluble high molecular weight proteins to Cd, including C. gigas protein disulfide isomerase (cgPDI). Liquid chromatography–tandem mass spectrometry (LC–MS/MS) analyses revealed two CGHC motifs in cgPDI. The binding between Cd and rcgPDI was confirmed through a Cd-binding experiment using the TPPS method. Isothermal titration calorimetry (ITC) revealed the binding of two Cd ions to one molecule of rcgPDI. Circular dichroism (CD) spectrum and tryptophan fluorescence analyses demonstrated that the rcgPDI bound to Cd. The binding markedly changed the two-dimensional or three-dimensional structures. The activity of rcgPDI measured by a PDI Activity Assay Kit was more affected by the addition of Cd than by human PDI. Immunological analyses indicated that C. gigas contained cgPDI at a concentration of 1.0 nmol/g (viscera wet weight). The combination of ITC and quantification results revealed that Cd-binding to cgPDI accounted for 20% of the total bound Cd in the visceral mass. The findings provide new insights into the defense mechanisms of invertebrates against Cd.

Discovery of Kasugamycin as a Potent Inhibitor of Glycoside Hydrolase Family 18 Chitinases

Kasugamycin, a well-known aminoglycoside antibiotic, has been used widely in agriculture and medicine to combat microbial pathogens by binding the ribosome to inhibit translation. Here, kasugamycin was discovered to be a competitive inhibitor of glycoside hydrolase family 18 (GH18) chitinases from three different organisms (bacterium, insect and human). Results from tryptophan fluorescence spectroscopy and molecular docking revealed that kasugamycin binds to the substrate-binding clefts in a similar mode as the substrate. An electrostatic interaction between the amino group of kasugamycin and the carboxyl group of a conserved aspartate in GH18 chitinase (one of the catalytic triad residues) was found to be vital for the inhibitory activity. This paper not only reports new molecular targets of kasugamycin, but also expands our thinking about GH inhibitor design by using a scaffold unrelated to the substrate.