Diversity of G proteins in Lepidopteran cell lines: partial sequences of six G protein alpha subunits

Advances in the application of fluorescence correlation spectroscopy to study detergent purified and encapsulated membrane proteins

Fluorescence correlation spectroscopy (FCS) is a quantitative spectroscopy technique which could potentially increase throughput and sensitivity of screening for ligand, substrate and inhibitor binding to membrane proteins in solution. However, the purification of membrane proteins in their active forms is complex, as the lipid bilayer provides stability and its removal often causes the protein to become conformationally unstable. This has limited the application of biophysical techniques such as FCS to study the function of membrane proteins. The recent application of native extraction techniques such as styrene maleic acid lipid particles (SMALPs) has resolved this issue and FCS has emerged as a powerful option for studying proteins extracted in this way. This review will discuss the application of FCS to study purified membrane proteins in detergent micelles, nanodiscs and SMALPs and its potential to be used routinely in membrane protein drug discovery.

GprotPRED: Annotation of Gα, Gβ and Gγ subunits of G-proteins using profile Hidden Markov Models (pHMMs) and application to proteomes

Heterotrimeric G-proteins form a major protein family, which participates in signal transduction. They are composed of three subunits, Gα, Gβ and Gγ. The Gα subunit is further divided in four distinct families Gs, Gi/o, Gq/11 and G12/13. The goal of this work was to detect and classify members of the four distinct families, plus the Gβ and the Gγ subunits of G-proteins from sequence alone. To achieve this purpose, six specific profile Hidden Markov Models (pHMMs) were built and checked for their credibility. These models were then applied to ten (10) proteomes and were able to identify all known G-protein and classify them into the distinct families. In a separate case study, the models were applied to twenty seven (27) arthropod proteomes and were able to give more credible classification in proteins with uncertain annotation and in some cases to detect novel proteins. An online tool, GprotPRED, was developed that uses these six pHMMs. The sensitivity and specificity for all pHMMs were equal to 100% with the exception of the Gβ case, where sensitivity equals to 100%, while specificity is 99.993%. In contrast to Pfam's pHMM which detects Gα subunits in general, our method not only detects Gα subunits but also classifies them into the appropriate Gα-protein family and thus could become a useful tool for the annotation of G-proteins in newly discovered proteomes. GprotPRED online tool is publicly available for non-commercial use at http://bioinformatics.biol.uoa.gr/GprotPRED and, also, a standalone version of the tool at https://github.com/vkostiou/GprotPRED.

Molecular Cloning and Characterization of G Alpha Proteins from the Western Tarnished Plant Bug, Lygus hesperus

The Gα subunits of heterotrimeric G proteins play critical roles in the activation of diverse signal transduction cascades. However, the role of these genes in chemosensation remains to be fully elucidated. To initiate a comprehensive survey of signal transduction genes, we used homology-based cloning methods and transcriptome data mining to identity Gα subunits in the western tarnished plant bug (Lygus hesperus Knight). Among the nine sequences identified were single variants of the Gαi, Gαo, Gαs, and Gα12 subfamilies and five alternative splice variants of the Gαq subfamily. Sequence alignment and phylogenetic analyses of the putative L. hesperus Gα subunits support initial classifications and are consistent with established evolutionary relationships. End-point PCR-based profiling of the transcripts indicated head specific expression for LhGαq4, and largely ubiquitous expression, albeit at varying levels, for the other LhGα transcripts. All subfamilies were amplified from L. hesperus chemosensory tissues, suggesting potential roles in olfaction and/or gustation. Immunohistochemical staining of cultured insect cells transiently expressing recombinant His-tagged LhGαi, LhGαs, and LhGαq1 revealed plasma membrane targeting, suggesting the respective sequences encode functional G protein subunits.

Identification of an in vitro interaction between an insect immune suppressor protein (CrV2) and G alpha proteins

The protein CrV2 is encoded by a polydnavirus integrated into the genome of the endoparasitoid Cotesia rubecula (Hymenoptera:Braconidae:Microgastrinae) and is expressed in host larvae with other gene products of the polydnavirus to allow successful development of the parasitoid. CrV2 expression has previously been associated with immune suppression, although the molecular basis for this was not known. Here, we have used time-resolved Förster resonance energy transfer (TR-FRET) to demonstrate high affinity binding of CrV2 to Gα subunits (but not the Gβγ dimer) of heterotrimeric G-proteins. Signals up to 5-fold above background were generated, and an apparent dissociation constant of 6.2 nm was calculated. Protease treatment abolished the TR-FRET signal, and the presence of unlabeled CrV2 or Gα proteins also reduced the TR-FRET signal. The activation state of the Gα subunit was altered with aluminum fluoride, and this decreased the affinity of the interaction with CrV2. It was also demonstrated that CrV2 preferentially bound to Drosophila Gα(o) compared with rat Gα(i1). In addition, three CrV2 homologs were detected in sequences derived from polydnaviruses from Cotesia plutellae and Cotesia congregata (including the immune-related early expressed transcript, EP2). These data suggest a potential mode-of-action of immune suppressors not previously reported, which in addition to furthering our understanding of insect immunity may have practical benefits such as facilitating development of novel controls for pest insect species.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

High constitutive activity and a G-protein-independent high-affinity state of the human histamine H(4)-receptor

The human histamine H(4)-receptor (hH(4)R) is expressed in mast cells and eosinophils and mediates histamine (HA)-induced chemotaxis via G(i)-proteins. For a detailed investigation of hH(4)R/G(i)-protein interaction, we coexpressed the hH(4)R with Galpha(i2) and Gbeta(1)gamma(2) as well as an hH(4)R-Galpha(i2) fusion protein with Gbeta(1)gamma(2) in Sf9 insect cells. The agonist radioligand [(3)H]HA showed a K(D) value of approximately 10 nM at hH(4)R and hH(4)R-Galpha(i2). The high-affinity states of hH(4)R and hH(4)R-Galpha(i2) were insensitive to guanosine 5'-[gamma-thio]triphosphate (GTPgammaS). The affinity of [(3)H]HA for hH(4)R was retained in the absence of mammalian G(i)-proteins. In steady-state GTPase- and [(35)S]GTPgammaS-binding assays, hH(4)R exhibited high constitutive activity and uncommon insensitivity to Na(+). Thioperamide (THIO) was only a partial inverse agonist. Addition of HA or THIO to baculovirus-infected (hH(4)R + Galpha(i2) + Gbeta(1)gamma(2)) Sf9 cells increased the B(max) in [(3)H]HA binding, but not in immunoblots, suggesting conformational instability and ligand-induced stabilization of membrane-integrated hH(4)R. No effect was observed on hH(4)R-Galpha(i2) expression, neither in [(3)H]HA binding nor in immunoblot. However, the expression level of hH(4)R-Galpha(i2) was consistently higher compared to hH(4)R, suggesting chaperone-like or stabilizing effects of Galpha(i2) on hH(4)R. In 37 degrees C stability assays, HA stabilized hH(4)R, and THIO even restored misfolded [(3)H]HA binding sites. Inhibition of hH(4)R glycosylation by tunicamycin reduced the [(3)H]HA binding B(max) value. In conclusion, (i) hH(4)R shows high constitutive activity and structural instability; (ii) hH(4)R shows a G-protein-independent high-affinity state; (iii) hH(4)R conformation is stabilized by agonists, inverse agonists and G-proteins; (iv) hH(4)R glycosylation is essential for cell-surface expression of intact hH(4)R.

GPCR-induced dissociation of G-protein subunits in early stage signal transduction

G-protein coupled receptors (GPCRs) form a ternary complex of agonist, receptor and G-proteins during primary signal transduction at the cell membrane. Downstream signalling is thought to be preceded by the process of dissociation of Galpha and Gbetagamma subunits, thus exposing new surfaces to interact with downstream effectors. We demonstrate here for the first time, the dissociation of heterotrimeric G-protein subunits (i.e., Galpha and Gbetagamma) following agonist-induced GPCR (alpha(2A)-adrenergic receptor; alpha(2A)-AR) activation in a cell-free assay system. alpha(2A)-AR membranes were reconstituted with the G-proteins (+/-hexahistidine-tagged) Galpha(i1) and Gbeta1gamma2 and functional signalling was determined following activation of the reconstituted receptor:G-protein complex with the potent agonist UK-14304, and [35S]GTPgammaS. In the presence of Ni(2+)-coated agarose beads, the activated his-tagged Galpha(i1)his-[35S]GTPgammaS complex was captured on the Ni(2+)-presenting surface. When his-tagged Gbeta1gamma2 (Gbeta1gamma2his) was used with Galpha(i1), the [35S]GTPgammaS-bound Galpha(i1) was not present on the Ni(2+)-coated beads, but rather, it was separated from the beta1gamma2(his)-beads, demonstrating receptor-induced dissociation of Galpha and Gbetagamma subunits. Treatment of the reconstituted alpha(2A)-AR membranes containing Gbeta1gamma2his:Galpha(i1) with imidazole confirmed the specificity for the Ni2+:G-protein surface dissociation of Galpha(i1) from Gbeta1gamma2his. These data demonstrate for the first time, the complete dissociation of the G-protein subunits and extend observations on the role of G-proteins in the assembly and disassembly of the ternary complex in the primary events of GPCR signalling.

Histamine H(4) receptor-RGS fusion proteins expressed in Sf9 insect cells: a sensitive and reliable approach for the functional characterization of histamine H(4) receptor ligands

The human histamine H(4) receptor (hH(4)R), co-expressed with Galpha(i2) and Gbeta(1)gamma(2) in Sf9 cells, is highly constitutively active. In the steady-state GTPase assay, the full agonist histamine (HA) induces only a relatively small signal (approximately 20-30%), resulting in a low signal-to background ratio. In order to improve this system for ligand screening purposes, the effects of the regulators of G-protein signaling (RGS) RGS4 and RGS19 (GAIP) were investigated. RGS4 and GAIP were fused to the C-terminus of hH(4)R or co-expressed with non-fused hH(4)R, always combined with Galpha(i2) and Gbeta(1)gamma(2). The non-fused RGS proteins did not significantly increase the relative effect of HA. With the hH(4)R-RGS4 fusion protein the absolute GTPase activities, but not the relative HA-induced signal were increased. Fusion of hH(4)R with GAIP caused a selective increase of the HA signal, resulting in an enhanced signal-to-noise ratio. A detailed characterization of the hH(4)R-GAIP fusion protein (co-expressed with Galpha(i2) and Gbeta(1)gamma(2)) and a comparison with the data obtained for the non-fused hH(4)R (co-expressed with Galpha(i2) and Gbeta(1)gamma(2)) led to the following results: (i) the relative agonist- and inverse agonist-induced signals at hH(4)R-GAIP are markedly increased. (ii) Compared to the wild-type hH(4)R, standard ligands show unaltered potencies and efficacies at hH(4)R-GAIP. (iii) Like hH(4)R, hH(4)R-GAIP shows high and NaCl-resistant constitutive activity. (iv) hH(4)R-GAIP shows the same G-protein selectivity profile as the non-fused hH(4)R. Collectively, hH(4)R-GAIP provides a sensitive test system for the characterization of hH(4)R ligands and can replace the non-fused hH(4)R in steady-state GTPase assays.

Functional analysis of a Drosophila melanogaster olfactory receptor expressed in Sf9 cells

Olfactory receptors (ORs) are seven transmembrane proteins that are responsible for the transduction of volatiles into neuronal signals. Their low sequence homology means that the prediction of ligands for ORs based on extrapolation from empirical data of other ORs is difficult, so an experimental approach must be used. Here, we report a functional assay for insect ORs using calcium-imaging in Sf9 cells. We find that the interaction of the odorant, ethyl butyrate, with the Drosophila melanogaster olfactory receptor Or22a is both dose-dependent and highly sensitive, with Or22a responding to ethyl butyrate with an EC(50) of (1.58+/-0.82)x10(-11)M. This degree of sensitivity does not require the addition of odorant binding proteins or downstream signal transduction elements. Furthermore, we demonstrate that Or22a expressed in Sf9 cells has a similar response profile to a range of odorants previously tested in vivo. This functional assay system will provide a useful tool for the de-orphaning of ORs from a wide range of insect species that are yet to have ligands assigned, and will help provide insight into OR specificity and mechanism of activation.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.