Characterization of human taurine transporter expressed in insect cells using a recombinant baculovirus

Expression, purification and functional characterization of human equilibrative nucleoside transporter subtype-1 (hENT1) protein from Sf9 insect cells

Human equilibrative nucleoside transporter-1 (hENT1) is the major plasma membrane transporter involved in transportation of natural nucleosides as well as nucleoside analog drugs, used in anti-cancer and anti-viral therapies. Despite extensive biochemical and pharmacological studies, little is known about the structure-function relationship of this protein. The major obstacles to purification include a low endogenous expression level, the lack of an efficient expression and purification protocol, and the hydrophobic nature of the protein. Here, we report protein expression, purification and functional characterization of hENT1 from Sf9 insect cells. hENT1 expressed by Sf9 cells is functionally active as demonstrated by saturation binding with a Kd of 1.2±0.2nM and Bmax of 110±5pmol/mg for [(3)H]nitrobenzylmercaptopurine ribonucleoside ([(3)H]NBMPR). We also demonstrate purification of hENT1 using FLAG antibody affinity resin in lauryl maltose neopentyl glycol detergent with a Kd of 4.3±0.7nM. The yield of hENT1 from Sf9 cells was ∼0.5mg active transporter per liter of culture. The purified protein is functionally active, stable, homogenous and appropriate for further biophysical and structural studies.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

A haemagglutinin from the medicinal fungus Cordyceps militaris

There are only a few reports on agglutinins from ascomycete and medicinal fungi. An HA (haemagglutinin), with an N-terminal amino acid sequence different from those of known lectins, was isolated in the present study from dried fruiting bodies of the medicinal ascomycete fungus Cordyceps militaris. The purification protocol consisted of affinity chromatography, ion-exchange chromatography and gel filtration. The haemagglutinating activity of the HA could not be inhibited by simple sugars or heparin, and was stable over the pH range 2–13 and up to 60°C. Chemical modification of tryptophan and tyrosine residues had no effect. The HA exhibited some antiproliferative activity towards hepatoma (HepG2) cells and inhibited HIV-1 reverse transcriptase (IC50=10 μM). However, it did not exhibit antifungal activity, mitogenic activity towards splenocytes, nitric oxide-inducing activity towards macrophages or RNase activity. The results of the present study add to the meagre information pertaining to agglutinins from ascomycete and medicinal mushrooms. It is revealed in this study that C. militaris HA differs from other ascomycete mushroom HAs in a variety of biochemical characteristics.

Expression, purification and characterization of the human membrane transporter protein OATP2B1 from Sf9 insect cells

OATP2B1 is an important member of the organic anion transporting polypeptides (OATP) family and is implicated in the intestinal and hepatic disposition of endo- and xenobiotics. The purpose of this work was to produce a highly purified protein for use as a reference standard for quantification of OATP2B1 in human tissue and in vitro assay systems. Here, we report the successful expression, purification and characterization of OATP2B1 in a heterologous expression system. Protein expressed by the Sf9-baculovirus expression system is functionally active as demonstrated by saturable uptake kinetics with a K(m) of 5.9+/-0.76 microM for estrone-3-sulfate. OATP2B1 was extracted from Sf9-membranes with ABS-14-4 detergent and purified using a one-step FLAG-tag purification method. Yield of OATP2B1 from Sf9 cells was 1.1mg per liter of culture, for a final recovery of 1.8%. SDS-PAGE resolution and Western blot of purified protein displayed multiple banding of OATP2B1-specific protein, which was thoroughly investigated to confirm homogeneity of the sample. C-terminal FLAG-tag purification and immunoblot detection, together with N-terminal sequencing, confirmed the presence of only full-length protein. Treatment with endoglycosidases had little effect on the migration pattern in SDS-PAGE, suggesting that multiple banding was not due to different glycosylation states of the protein. Amino acid analysis further confirmed the homogeneity of the protein with a calculated extinction coefficient of 80,387 cm(-1) M(-1). Physical, biochemical and functional characterization show that purified human OATP2B1 is pure, homogeneous and appropriate for use as a standard to quantitate expression of OATP2B1 in in vitro systems and tissue samples.

Taurine transporter in fetal T lymphocytes and platelets: differential expression and functional activity

Transplacental transfer of taurine, a β-amino acid essential for fetal and neonatal development, constitutes the primary source of taurine for the fetus. Placental transport of taurine is compromised in pregnancies complicated by intrauterine growth restriction, resulting in a reduced concentration of taurine in cord plasma. This could impact on fetal cellular metabolism as taurine represents the most abundant intracellular amino acid in many fetal cell types. In the present study, we have used pure isolates of fetal platelets and T lymphocytes from cord blood of placentas, from normal, term pregnancies, as fetal cell types to examine the cellular uptake mechanisms for taurine by the system β transporter and have compared gene and protein expression for the taurine transporter protein (TAUT) in these two cell types. System β activity in fetal platelets was 15-fold higher compared with fetal T lymphocytes ( P < 0.005), mirroring greater TAUT mRNA expression in platelets than T lymphocytes ( P < 0.005). Cell-specific differences in TAUT protein moieties were detected with a doublet of 75 and 80 kDa in fetal platelets compared with 114 and 120 kDa in fetal T lymphocytes, with relatively higher expression in platelets. We conclude that greater system β activity in fetal platelets compared with T lymphocytes is the result of relatively greater TAUT mRNA and protein expression. This study represents the first characterization of amino acid transporters in fetal T lymphocytes.

Recovery of functional peptide transporter PepT1 in budded baculovirus fraction

Transporters play a critical role in many physiological and pathological states and expression of the functional transporter protein is essential in exploring its kinetics and developing effective drugs. We describe here the recovery of functional transporter protein in the baculovirus fraction. We introduced a gene encoding human peptide transporter PepT1, important for the absorption of protein hydrolytic products or peptide-mimetic drugs, into a baculovirus vector. After infection, a large amount of PepT1 appeared in the budded virus fraction compared with Sf9 cells. Uptake of [14C]glycylsarcosine was markedly increased in an acidic condition and showed a clear overshoot in PepT1-expressing virus fraction. The apparent Michaelis constant for [14C]glycylsarcosine was 0.55 +/- 0.06 mM. [14C]Glycylsarcosine uptake was inhibited by di- and tripeptides and orally active beta-lactam antibiotics. These results suggest that functional PepT1 recovers efficiently in a budded virus fraction, and, thus, this expression system will be a useful tool for characterization and screening of peptide-mimetic drugs in drug discovery.

The Nramp orthologue of Cryptococcus neoformans is a pH-dependent transporter of manganese, iron, cobalt and nickel

Cryptococcus neoformans is an important human opportunistic pathogen and a facultative intracellular parasite, particularly in HIV-infected individuals. Little is known about metal ion transport in this organism. C. neoformans encodes a single member of the Nramp (natural resistance-associated macrophage protein) family of bivalent cation transporters, known as Cramp, which we have cloned and expressed in Xenopus laevis oocytes and Spodoptera frugiperda Sf 21 insect cells. Cramp induces saturable transport of a broad range of bivalent transition series cations, including Mn2+, Fe2+, Co2+ and Ni2+. Maximal cation transport occurs at pH 5.5-6.0, consistent with the proton gradient-based energetics of other Nramp orthologues. Mn2+ transport is diminished in the presence of 140 mM Na+, compatible with a Na+ slippage mechanism proposed for the Saccharomyces cerevisiae Nramp orthologue Smf1p. Cramp resembles Smf1p with respect to predicted membrane topology, substrate specificity and pH dependence, but differs in terms of its apparent affinity for Mn2+ and negligible inhibition by Zn2+. Cramp is the first Nramp orthologue from a fungal pathogen to be functionally characterized. Insights afforded by these findings will allow the formulation of new hypotheses regarding the role of metal ions in the pathophysiology of cryptococcosis.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Regulation of taurine transport at the blood-brain barrier by tumor necrosis factor-alpha, taurine and hypertonicity

Taurine is the abundant sulfur-containing beta-amino acid in brain where it exerts a neuroprotective effect. Although it is known that the blood-brain barrier (BBB) mediates taurine transport, the regulation of taurine transport have not been clarified yet. A conditionally immortalized rat brain capillary endothelial cells (TR-BBB13), an in vitro model of the BBB, exhibited [3H]taurine uptake, which was dependent on both Na+ and Cl-, and inhibited by beta-alanine. Taurine transporter (TAUT) mRNA was detected in TR-BBB13 cells, and TAUT protein was also expressed at 70 kDa. TR-BBB13 cells exposed to 20 ng/mL TNF-alpha and under hypertonic conditions showed a 1.7-fold and 3.2-fold increase in [3H]taurine uptake, respectively. In contrast, lipopolysaccharide and diethyl maleate did not significantly affect taurine uptake. The taurine uptake was reduced by pre-treatment with excess taurine (50 mm). The mRNA level of the TAUT in TNF-alpha and following hypertonic treatment was greater than that in control cells, whereas that under excess taurine conditions was lower than in controls. Therefore, taurine transport activity at the BBB appears to be regulated at the transcriptional level by cell damage, osmolality and taurine in the brain.