An improved rhodopsin/EGFP fusion protein for use in the generation of transgenic Xenopus laevis

AbstractPrevious studies by Papermaster and coworkers introduced the use of rhodopsin–green fluorescent protein (rho–GFP) fusion proteins in the construction of transgenic Xenopus laevis with retinal rod photoreceptor cell-specific transgene expression [Moritz et al., J. Biol. Chem. 276 (2001) 28242–28251]. These pioneering studies have helped to develop the Xenopus system not only for use in the investigation of rhodopsin biosynthesis and targeting, but for studies of the phototransduction cascade as well. However, the rho–GFP fusion protein used in the earlier work had only 50% of the specific activity of wild-type rhodopsin for activation of transducin and only 10% of the activity of wild-type in rhodopsin kinase assays. While not a problem for the biosynthesis studies, this does present a problem for investigation of the phototransduction cascade. We report here an improved rhodopsin/EGFP fusion protein in which placement of the EGFP domain at the C-terminus of rhodopsin results in wild-type activity for activation of transducin, wild-type ability to serve as a substrate for rhodopsin kinase, and wild-type localization of the protein to the rod photoreceptor cell outer segment in transgenic X. laevis

Role of isosafrole as complexing agent and inducer of P-450LM4 in rabbit liver microsomes

Isosafrole serves as an excellent inducing agent in rabbits for P-450LM4, the same isozyme as that which is induced by 3-methylcholanthrene and 5,6-benzoflavone. Since the isosafrole adduct formed with isozyme 4 is unstable, the uncomplexed cytochrome may be purified in very good yield (over 30%, based on total P-450) from liver microsomes of animals given this agent. Isosafrole forms adducts with both isozyme 2 (the phenobarbital-inducible form of P-450) and isozyme 4 in the reconstituted system containing NADPH-cytochrome P-450 reductase, phosphatidylcholine, and NADPH under aerobic conditions, but with the latter cytochrome the reaction has a lower Km and maximal rate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23864/1/0000103.pd

A Marine Snail Neurotoxin Shares with Scorpion Toxins a Convergent Mechanism of Blockade on the Pore of Voltage-Gated K Channels

κ-Conotoxin-PVIIA (κ-PVIIA) belongs to a family of peptides derived from a hunting marine snail that targets to a wide variety of ion channels and receptors. κ-PVIIA is a small, structurally constrained, 27-residue peptide that inhibits voltage-gated K channels. Three disulfide bonds shape a characteristic four-loop folding. The spatial localization of positively charged residues in κ-PVIIA exhibits strong structural mimicry to that of charybdotoxin, a scorpion toxin that occludes the pore of K channels. We studied the mechanism by which this peptide inhibits Shaker K channels expressed in Xenopus oocytes with the N-type inactivation removed. Chronically applied to whole oocytes or outside-out patches, κ-PVIIA inhibition appears as a voltage-dependent relaxation in response to the depolarizing pulse used to activate the channels. At any applied voltage, the relaxation rate depended linearly on the toxin concentration, indicating a bimolecular stoichiometry. Time constants and voltage dependence of the current relaxation produced by chronic applications agreed with that of rapid applications to open channels. Effective valence of the voltage dependence, zδ, is ∼0.55 and resides primarily in the rate of dissociation from the channel, while the association rate is voltage independent with a magnitude of 107–108 M−1 s−1, consistent with diffusion-limited binding. Compatible with a purely competitive interaction for a site in the external vestibule, tetraethylammonium, a well-known K-pore blocker, reduced κ-PVIIA's association rate only. Removal of internal K+ reduced, but did not eliminate, the effective valence of the toxin dissociation rate to a value <0.3. This trans-pore effect suggests that: (a) as in the α-KTx, a positively charged side chain, possibly a Lys, interacts electrostatically with ions residing inside the Shaker pore, and (b) a part of the toxin occupies an externally accessible K+ binding site, decreasing the degree of pore occupancy by permeant ions. We conclude that, although evolutionarily distant to scorpion toxins, κ-PVIIA shares with them a remarkably similar mechanism of inhibition of K channels

Stability of the Neurotensin Receptor NTS1 Free in Detergent Solution and Immobilized to Affinity Resin

Purification of recombinant membrane receptors is commonly achieved by use of an affinity tag followed by an additional chromatography step if required. This second step may exploit specific receptor properties such as ligand binding. However, the effects of multiple purification steps on protein yield and integrity are often poorly documented. We have previously reported a robust two-step purification procedure for the recombinant rat neurotensin receptor NTS1 to give milligram quantities of functional receptor protein. First, histidine-tagged receptors are enriched by immobilized metal affinity chromatography using Ni-NTA resin. Second, remaining contaminants in the Ni-NTA column eluate are removed by use of a subsequent neurotensin column yielding pure NTS1. Whilst the neurotensin column eluate contained functional receptor protein, we observed in the neurotensin column flow-through misfolded NTS1.To investigate the origin of the misfolded receptors, we estimated the amount of functional and misfolded NTS1 at each purification step by radio-ligand binding, densitometry of Coomassie stained SDS-gels, and protein content determination. First, we observed that correctly folded NTS1 suffers damage by exposure to detergent and various buffer compositions as seen by the loss of [(3)H]neurotensin binding over time. Second, exposure to the neurotensin affinity resin generated additional misfolded receptor protein.Our data point towards two ways by which misfolded NTS1 may be generated: Damage by exposure to buffer components and by close contact of the receptor to the neurotensin affinity resin. Because NTS1 in detergent solution is stabilized by neurotensin, we speculate that the occurrence of aggregated receptor after contact with the neurotensin resin is the consequence of perturbations in the detergent belt surrounding the NTS1 transmembrane core. Both effects reduce the yield of functional receptor protein

The Roles of Transmembrane Domain Helix-III during Rhodopsin Photoactivation

Background: Rhodopsin, the prototypic member of G protein-coupled receptors (GPCRs), undergoes isomerization of 11- cis-retinal to all-trans-retinal upon photoactivation. Although the basic mechanism by which rhodopsin is activated is well understood, the roles of whole transmembrane (TM) helix-III during rhodopsin photoactivation in detail are not completely clear. Principal Findings: We herein use single-cysteine mutagenesis technique to investigate conformational changes in TM helices of rhodopsin upon photoactivation. Specifically, we study changes in accessibility and reactivity of cysteine residues introduced into the TM helix-III of rhodopsin. Twenty-eight single-cysteine mutants of rhodopsin (P107C-R135C) were prepared after substitution of all natural cysteine residues (C140/C167/C185/C222/C264/C316) by alanine. The cysteine mutants were expressed in COS-1 cells and rhodopsin was purified after regeneration with 11-cis-retinal. Cysteine accessibility in these mutants was monitored by reaction with 4, 49-dithiodipyridine (4-PDS) in the dark and after illumination. Most of the mutants except for T108C, G109C, E113C, I133C, and R135C showed no reaction in the dark. Wide variation in reactivity was observed among cysteines at different positions in the sequence 108–135 after photoactivation. In particular, cysteines at position 115, 119, 121, 129, 131, 132, and 135, facing 11-cis-retinal, reacted with 4-PDS faster than neighboring amino acids. The different reaction rates of mutants with 4-PDS after photoactivation suggest that the amino acids in different positions in helix-III are exposed to aqueous environment to varying degrees. Significance: Accessibility data indicate that an aqueous/hydrophobic boundary in helix-III is near G109 and I133. The lack of reactivity in the dark and the accessibility of cysteine after photoactivation indicate an increase of water/4-PDS accessibility for certain cysteine-mutants at Helix-III during formation of Meta II. We conclude that photoactivation resulted in water-accessible at the chromophore-facing residues of Helix-III.National Institutes of Health (U.S.) (grant GM28289)National Eye Institute (Grant Grant EY11716)National Science Foundation (U.S.) (grant EIA-0225609

Thermal Stability of the Human Immunodeficiency Virus Type 1 (HIV-1) Receptors, CD4 and CXCR4, Reconstituted in Proteoliposomes

BACKGROUND: The entry of human immunodeficiency virus (HIV-1) into host cells involves the interaction of the viral exterior envelope glycoprotein, gp120, and receptors on the target cell. The HIV-1 receptors are CD4 and one of two chemokine receptors, CCR5 or CXCR4. METHODOLOGY/PRINCIPAL FINDINGS: We created proteoliposomes that contain CD4, the primary HIV-1 receptor, and one of the coreceptors, CXCR4. Antibodies against CD4 and CXCR4 specifically bound the proteoliposomes. CXCL12, the natural ligand for CXCR4, and the small-molecule CXCR4 antagonist, AMD3100, bound the proteoliposomes with affinities close to those associated with the binding of these molecules to cells expressing CXCR4 and CD4. The HIV-1 gp120 exterior envelope glycoprotein bound tightly to proteoliposomes expressing only CD4 and, in the presence of soluble CD4, bound weakly to proteoliposomes expressing only CXCR4. The thermal stability of CD4 and CXCR4 inserted into liposomes was examined. Thermal denaturation of CXCR4 followed second-order kinetics, with an activation energy (E(a)) of 269 kJ/mol (64.3 kcal/mol) and an inactivation temperature (T(i)) of 56°C. Thermal inactivation of CD4 exhibited a reaction order of 1.3, an E(a) of 278 kJ/mol (66.5 kcal/mol), and a T(i) of 52.2°C. The second-order denaturation kinetics of CXCR4 is unusual among G protein-coupled receptors, and may result from dimeric interactions between CXCR4 molecules. CONCLUSIONS/SIGNIFICANCE: Our studies with proteoliposomes containing the native HIV-1 receptors allowed an examination of the binding of biologically important ligands and revealed the higher-order denaturation kinetics of these receptors. CD4/CXCR4-proteoliposomes may be useful for the study of virus-target cell interactions and for the identification of inhibitors

Kinetics of Reduction of Cytochrome P-450Lm4 and Nadph-Cytochrome P-450 Reductase.

The reduction of cytochrome P-450 in liver microsomal suspensions by NADPH is known from the work of others to exhibit biphasic kinetics, but the interpretation of this finding is complicated by the occurrence of multiple forms of the cytochrome. In the present study, the kinetics of reduction of highly purified P-450LM4 (5,6-benzoflavone-inducible form of the cytochrome, isolated in the high spin state from rabbit liver microsomes) was examined in a reconstituted system containing highly purified NADPH-cytochrome P-450 reductase from the same source, phosphatidycholine, NADPH, and carbon monoxide under anaerobic conditions. When a mixture of the two enzymes was rapidly mixed with NADPH in a stopped flow spectrophotometer, the reduction reaction, monitored by the appearance of the ferrous carbonyl complex of P-450LM4 at 448 nm, exhibited biphasic kinetics irrespective of the NADPH concentration or the molar ratio of reductase to cytochrome. The first order rate constants were about 1.0 sec('-1) and 0.2 sec('-1). The presence of a typical substrate, benzphetamine, had no effect on the results, and the omission of the phospholipid, although causing no change in the kinetics, resulted in a decrease in the extent of the fast phase. The relative magnitude of the two phases displayed complex behavior with varying NADPH concentrations. Substitution of the airstable semiquinone form of the reductase (known from earlier studies in this laboratory to contain FMN in the semiquinone state and FAD in the oxidized state) for the fully oxidized form in the reconstituted system caused no change in the kinetics of P-450LM4 reduction. When the reduction reaction was initiated by the rapid mixing of P-450LM4 and reductase, evidence was obtained that association of the two enzymes from their separate sources in bulk solution is a relatively slow process, whereas once the two enzymes have associated into large aggregates the formation of a catalytically functional complex between the two proteins proceeds very rapidly. Our results indicate that the occurrence of biphasic kinetics in the 1-electron transfer to P-450LM4 in the reconstituted system is apparently due not to the cytochrome, but to some property of the reductase. The reduction of the reductase by NADPH was found to exhibit triphasic first order kinetics. The first order rate constants for the three phases were 70 sec('-1), 7 sec('-1), and 0.05 sec('-1). A complete spectral analysis (400 to 640 nm) of the reaction mixture was carried out and the spectrum for each phase of the triphasic reaction was reconstructed from component spectra summed in proportions governed by their relative reduction potentials. For example, the first phase of the reaction with a 10-fold excess of NADPH could be spectrally described by a mixture of 31.5% disemiquinone and 68.5% (FMNH(,2),FAD). The second phase was well represented by an equimolar mixture of 2- and 4- electron-reduced species and , finally, thermodynamic relaxation in the third phase resulted in an equilibrium mixture of all possible states of the reductase. These results suggest that interflavin electron transfer, within a single reductase molecule, is much faster than the transfer of reducing equivalents from pyridine nucleotide to flavin and that the distribution of electrons between the two flavin moieties is governed by the reduction potentials for the pertinent half-cells of each flavin.Ph.D.BiochemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/157801/1/8017332.pd