Molecular characterization of Drosophila Gene Encoding G0 α Subunit Homolog

Cloning and expression analysis of a G-protein α subunit--Gαo in the rice water weevil Lissorhoptrus oryzophilus Kuschel

The open reading frame (ORF) encoding a novel G protein α subunit, Lo Gα(o), was cloned from the parthenogenetic rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae). The Lo Gα(o) ORF encodes a protein of 354 amino acid residues. The deduced protein sequence shares high homology with Gα(o) from other species. The expression patterns of Lo Gα(o) in various adult tissues were indicated by real-time quantitative PCR and Western blot. The results showed that Lo Gα(o) mRNA was expressed at similar levels in tissues except relative high levels in the antennae of adult, and Lo Gα(o) protein of an apparent molecular mass of about 40 kDa was expressed in various tissues of the adult. Immunocytochemical localization showed that Lo Gα(o) was mainly expressed in the dendrites of the trichoid sensilla in the antenna of the weevil. The tissue and cellular localization of Lo Gα(o) suggests that Lo Gα(o) may take a part in signal transduction of olfactory/gustatory.

Go α is involved in sugar perception in Drosophila

Detection of chemical compounds in food sources is based on the activation of 7 transmembrane gustatory receptors (GRs) in mammals and in insects such as Drosophila, although the receptors are not conserved between the classes. Different combinations of Drosophila GRs are involved in the detection of sugars, but the activated signaling cascades are largely unknown. Because 7 transmembrane receptors usually couple to G-proteins, we tried to unravel the intracellular signaling cascade in taste neurons by screening heterotrimeric G-protein mutant flies for gustatory deficits. We found the subunit Goα to be involved in feeding behavior and cell excitability by different transgenic and pharmacological approaches. Goα is involved in the detection of sucrose, glucose, and fructose, but not with trehalose and maltose. Our studies reveal that Goα plays an important role in the perception of some sweet tastants. Because the perception of other sweet stimuli was not affected by mutations in Goα, we also found strong indication for the existence of multiple signaling pathways in the insect gustatory system.

Galpha encoding gene family of the malaria vector mosquito Anopheles gambiae: expression analysis and immunolocalization of AGalphaq and AGalphao in female antennae

To initiate a comprehensive investigation of chemosensory signal transduction downstream of odorant receptors, we identified and characterized the complete set of genes that encode G-protein alpha subunits in the genome of the malaria vector mosquito An. gambiae. Data are provided on the tissue-specific expression patterns of 10 corresponding aga-transcripts in adult mosquitoes and pre-imago developmental stages. Specific immunoreactivity in chemosensory hairs of female antennae provides evidence in support of the participation of a subset of AGalphaq isoforms in olfactory signal transduction in this mosquito. In contrast, AGalphao is localized along the flagellar axon bundle but is absent from chemosensory sensilla, which suggests that this G-protein alpha subunit does not participate in olfactory signal transduction.

Expression and localization of three G protein alpha subunits, Go, Gq, and Gs, in adult antennae of the silkmoth (Bombyx mori)

In insect olfactory receptor neurons, rapid and transient increases in inositol triphosphate (IP3) and Ca2+ are detected upon stimulation with pheromone or nonpheromonal odorants. This suggests that heterotrimeric guanine nucleotide binding proteins (G proteins) may transduce some odorant responses in insects. We obtained cDNA clones encoding three classes of G protein alpha subunits, Bm Go, Bm Gq, and Bm Gs, from the antennae of the adult male silkmoth (Bombyx mori). RT-PCR experiments showed that the mRNA of these G protein alpha subunits was also present in the various tissues of adult and larval insects. We used immunocytochemistry to localize these G protein alpha subunits in adult male and female antennae. In the adult male antennae, anti-Go antiserum stained the nerve bundles. In contrast, anti-Gq and anti-Gs antisera stained the inner and outer dendritic segments of the putative olfactory receptor neuron. The localizations of Bm Go, Bm Gq, and Bm Gs in the female antennae were the same as in the male antennae. The localizations of Bm Gq and Bm Gs suggest that each subunit mediates a subset of the odorant response.

Nerfin-1 is required for early axon guidance decisions in the developing Drosophila CNS

Many studies have focused on the mechanisms of axon guidance; however, little is known about the transcriptional control of the navigational components that carryout these decisions. This report describes the functional analysis of Nerfin-1, a nuclear regulator of axon guidance required for a subset of early pathfinding events in the developing Drosophila CNS. Nerfin-1 belongs to a highly conserved subfamily of Zn-finger proteins with cognates identified in nematodes and man. We show that the neural precursor gene prospero is essential for nerfin-1 expression. Unlike nerfin-1 mRNA, which is expressed in many neural precursor cells, the encoded Nerfin-1 protein is only detected in the nuclei of neuronal precursors that will divide just once and then transiently in their nascent neurons. Although nerfin-1 null embryos have no discernible alterations in neural lineage development nor in neuronal or glial identities, CNS pioneering neurons require nerfin-1 function for early axon guidance decisions. Furthermore, nerfin-1 is required for the proper development of commissural and connective axon fascicles. Our studies also show that Nerfin-1 is essential for the proper expression of robo2, wnt5, derailed, G-oalpha47A, Lar, and futsch, genes whose encoded proteins participate in these early navigational events.

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

The aim of this work was to sample the diversity of G protein alpha subunits in lepidopteran insect cell lines. Here we report the amplification by degenerate PCR of partial sequences representing six G protein alpha subunits from three different lepidopteran insect cell lines. Sequence comparisons with known G protein alpha subunits indicate that the Sf9, Ld and High Five cell lines each contain (at least) one Galpha(q)-like and one Galpha(i)-like Galpha subunit. All six PCR products are unique at the nucleotide level, but the translation products of the three Galpha q-like partial clones (Sf9-Galpha 1, Ld-Galpha 1, and Hi5-Galpha 1) are identical, as are the translation products of the three Galpha i-like partial clones (Sf9-Galpha 2, Ld-Galpha 2, and Hi5-Galpha 2). Both the Galpha(q)-like and Galpha(i)-like translation products are identical to known Galpha subunits from other Lepidoptera, are highly similar (88-98%) to Galpha subunits from other invertebrates including mosquitoes, fruit flies, lobsters, crabs, and snails, and are also highly similar (88-90%) to known mammalian Galpha subunits. Identification of G protein alpha subunits in lepidopteran cell lines will assist in host cell line selection when insect cell lines are used for the pharmacological analysis of human GPCRs.

Structure, chromosomal localization, and expression of the Drosophila melanogaster gene encoding sepiapterin reductase

We have isolated and characterized a Drosophila melanogaster gene encoding the sepiapterin reductase (SR). The gene does not have introns. The 5'- and 3'-RACE analysis, which determined the transcription start point (tsp) and polyadenylation site, respectively, showed that the gene produces single mRNA species. The potential promoter region lacks distinct TATAAA or CCAAT box consensus sequences. RNA blot analysis revealed that the gene encodes a 1.4kb transcript that could be detected throughout development and in both heads and bodies of adults. The Drosophila SR gene maps to 15A on the X chromosome.

The heterotrimeric protein Go is required for the formation of heart epithelium in Drosophila

The gene encoding the alpha subunit of the Drosophila Go protein is expressed early in embryogenesis in the precursor cells of the heart tube, of the visceral muscles, and of the nervous system. This early expression coincides with the onset of the mesenchymal-epithelial transition to which are subjected the cardial cells and the precursor cells of the visceral musculature. This gene constitutes an appropriate marker to follow this transition. In addition, a detailed analysis of its expression suggests that the cardioblasts originate from two subpopulations of cells in each parasegment of the dorsal mesoderm that might depend on the wingless and hedgehog signaling pathways for both their determination and specification. In the nervous system, the expression of Goalpha shortly precedes the beginning of axonogenesis. Mutants produced in the Goalpha gene harbor abnormalities in the three tissues in which the gene is expressed. In particular, the heart does not form properly and interruptions in the heart epithelium are repeatedly observed, henceforth the brokenheart (bkh) name. Furthermore, in the bkh mutant embryos, the epithelial polarity of cardial cells was not acquired (or maintained) in various places of the cardiac tube. We predict that bkh might be involved in vesicular traffic of membrane proteins that is responsible for the acquisition of polarity.

Isolation and characterization of the Drosophila melanogaster cDNA encoding the sepiapterin reductase

We have isolated and characterized the cDNA encoding Drosophila melanogaster sepiapterin reductase (SR). The amino acid sequence deduced from the cDNA sequence was 29% identical to those of mammalian SRs. The active site residues proposed from the three-dimensional structure of mouse SR are well conserved in Drosophila SR. The protein-coding region of the cDNA was expressed in Escherichia coli as a histidine fusion protein, and the resulting recombinant protein proved to have SR activity. The SR activity of the recombinant protein was inhibited by two indoleamines, N-acetyl serotonin and melatonin. Southern analysis suggests that the Drosophila SR gene is encoded by a single copy gene. RNA blot analysis revealed that the gene expresses 1.5 kb mRNA in both adult heads and bodies.

Molecular characterization of the Drosophila melanogaster gene encoding the pterin 4alpha-carbinolamine dehydratase

We have isolated and characterized the cDNA and the genomic DNA encoding Drosophila melanogaster pterin 4alpha-carbinolamine dehydratase (PCD). The amino acid sequence deduced from the cDNA sequence was very similar to those of PCDs previously reported in other species (19-57% identity). The protein coding region of the cDNA was expressed in E. coli as a histidine fusion protein, and the expressed protein proved to have PCD activity. The characterization of the Drosophila genomic clone revealed that the Drosophila PCD gene is interrupted by two introns. The potential promoter region, deduced from the determination of the transcription start point (tsp), lacks the distinct TATAAA box consensus sequence.

G alpha(o) is necessary for muscarinic regulation of Ca2+ channels in mouse heart

Heterotrimeric G proteins, composed of G alpha and G betagamma subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The G alpha(o) protein is the most abundant G alpha subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca2+ channels in GH3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of G alpha(o), we have generated mice lacking both isoforms of G alpha(o) by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that G alpha(o) makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane. The G alpha(o)-/- mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, G alpha(o)-/- mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other G alpha subunits (G alpha(s), G alpha(q), and G alpha(i)) are unchanged in the hearts of G alpha(o)-/- mice, but the amount of G betagamma is decreased. Whichever subunit, G alpha(o) or G betagamma, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca2+ channels requires coupling of the muscarinic receptor to G alpha(o). Other cardiac G alpha subunits cannot substitute.

Molecular characterization of two Drosophila guanylate cyclases expressed in the nervous system

We have isolated, by interspecies hybridization, two classes of Drosophila cDNA each encoding a different guanylate cyclase (GC). One of them encodes an alpha subunit homolog of soluble GC, designated DGC alpha 1, and the other encodes a receptor-type GC, designated DrGC. The dgc alpha 1 cDNA encodes a protein of 676 amino acids and maps to 99B. In situ hybridization to adult tissue sections showed that dgc alpha 1 mRNA is found mainly in the cell bodies of the optic lobe, central brain, and thoracic ganglia. The DGC alpha 1 protein was also localized primarily to the nervous system by immunocytochemical staining, consistent with results of in situ hybridization. However, no detectable expression of this protein was found in the retina. The other class of cDNA, drgc, maps to 76C and encodes a 1525-amino acid protein displaying structural features similar to other known receptor-type guanylate cyclases. However, it has a C-terminal 430 amino acid region that has no homology to any known proteins. drgc RNA is expressed at low levels throughout development and in adult heads and bodies. In situ hybridizations to adult tissue sections showed that drgc mRNA is expressed in a wide range of tissues, including the optic lobe, central brain, thoracic ganglia, digestive tract, and the oocyte.

Isolation and characterization of the gene encoding the Drosophila melanogaster transcriptional elongation factor, TFIIS

We have characterized a genomic clone encoding the Drosophila melanogaster transcriptional elongation factor, TFIIS. The coding region of the TFIIS gene is interrupted by a short intron. The potential promoter region, deduced from the determination of the transcription start point (tsp), lacks distinct TATAAA or CCAAT box consensus sequences. Southern analysis and the in situ hybridization to chromosomes suggests that it is single-copy gene which is localized to the 35B region on the left arm of the second chromosome.

A Drosophila gene that encodes a member of the protein disulfide isomerase/phospholipase C-alpha family

Screening of a Drosophila genomic DNA library at reduced stringency hybridization conditions using a rat PLC alpha cDNA probe yielded a gene which encodes a member of the protein disulfide isomerase/PLC alpha family. The gene has been localized to band 74C on the left arm of the third chromosome and has been designated dpdi. Northern analysis shows that the dpdi gene encodes a transcript that is 2.3 kb in length and is present throughout development as well as in both heads and bodies of adults. The deduced dpdi protein is 496 amino acids in length and contains two domains exhibiting high similarity to thioredoxin, two regions that are similar to the hormone binding domain of human estrogen receptor, and a sequence of four amino acids (KDEL) at the C-terminus which has been described by others as being responsible for retention of proteins in the endoplasmic reticulum. Overall, dpdi contains a higher similarity to rat protein disulfide isomerase (53% identical) than to rat PLC alpha (30% identical). However, it is unclear whether dpdi functions in vivo as a PDI or as a PLC, or both. Drosophila, with its well characterized genetics and the ability to generate mutants in a gene that has been cloned, provides an excellent system in which to resolve this issue.

Developmental expression of heterotrimeric G proteins in the nervous system of Manduca sexta

The heterotrimeric G proteins are a conserved family of guanyl nucleotide-binding proteins that appear in all eukaryotic cells but whose developmental functions are largely unknown. We have examined the developmental expression of representative G proteins in the developing nervous system of the moth Manduca sexta. Using affinity-purified antisera against different G alpha subunits, we found that each of the G proteins exhibited distinctive patterns of expression within the developing central nervous system (CNS), and that these patterns underwent progressive phases of spatial and temporal regulation that corresponded to specific aspects of neuronal differentiation. Several of the G proteins examined (including Gs alpha and G(o) alpha) were expressed in an apparently ubiquitous manner in all neurons, but other proteins (including Gi alpha) were ultimately confined to a more restricted subset of cells in the mature CNS. Although most of the G proteins examined could be detected within the central ganglia, only G(o) alpha-related proteins were seen in the developing peripheral nerves; manipulations of G protein activity in cultured embryos suggested that this class of G protein may contribute to the regulation of neuronal motility during axonal outgrowth. G(o) alpha-related proteins were also localized to the developing axons and terminals of the developing adult limb during metamorphosis. These intracellular signaling molecules may, therefore, play similar developmental roles in both the embryonic and postembryonic nervous system.

G(O), a guanine nucleotide binding protein, is expressed during neurite extension in the embryonic mouse

The developmental pattern of expression of the G protein alpha o subunit and GAP43 were compared by immunohistochemical staining of mouse embryos. Staining for alpha o and GAP43 was identical and detected throughout the developing nervous system, and the antigens first appeared in neurons at the beginning of neuronal differentiation. GAP43 and alpha o were not detected in regions containing only neuroblasts. These observations suggest that alpha o and GAP43 may not be required for the decision to pass from neuroblast to differentiated neuron, but may play a role in signal transduction during early neuronal development.

A G-protein beta-subunit is essential for Dictyostelium development

Recent studies have demonstrated that G-protein-linked signal transduction pathways play a significant role in the developmental program of the simple eukaryotic organism Dictyostelium. We have reported previously the isolation of a G-protein beta-subunit and present here a more complete analysis of this gene. Low-stringency Southern blots and RFLP mapping studies suggest that the beta-subunit is a unique gene found on linkage group II. Its deduced amino acid sequence of 347 residues is approximately 60% identical to those of the human, Drosophila, and Caenorhabditis elegans beta-subunits. The carboxy-terminal 300 residues are about 70% identical; the amino-terminal 50 residues are quite divergent, containing only 10 identities. At all stages of growth and development, a single 1.9-kb beta-subunit mRNA is present at a high level, and a specific antibody detects a single 37-kD protein. We propose that G-protein heterotrimers are formed when this beta-subunit couples with each of the eight distinct G-protein alpha-subunits that are transiently expressed during development. Targeted disruption of the beta-subunit gene had no effect on the viability of haploid cells, but resulted in the inability of cells to aggregate.

Pertussis toxin expression in Drosophila alters the visual response and blocks eating behaviour

Pertussis toxin inactivates certain G-proteins by introducing an ADP-ribose group near the carboxyl-terminus of the alpha-subunit. The major pertussis toxin substrate in Drosophila tissues is Go alpha. We introduced a pertussis toxin gene under control of the hsp70 heat-shock promoter into the Drosophila genome. When heat-shocked, transformed flies produce active pertussis toxin which ADP-ribosylates endogenous Go alpha. Pertussis toxin is expressed in photoreceptors, in the lamina of the eye and in epithelial cells lining the gut. As expected from the absence of Go alpha in photoreceptors, pertussis toxin does not affect the photoreceptor component of the Drosophila visual response. However, it abolishes light on- and off-transients in the electroretinogram. These transients normally arise from the lamina, a tissue where Go alpha transcripts have been detected. Pertussis toxin expression also blocks embryonic development and shortens the lifetime of adult Drosophila. Following heat-shock, transformed adults are active, but they fail to take up nutrients because they stop eating. High energy metabolites are significantly depleted shortly after pertussis toxin expression is induced and the flies die within 48 h.

Molecular cloning of G protein alpha subunits from the central nervous system of the mollusc Lymnaea stagnalis

The central nervous system of the pond snail, Lymnaea stagnalis, contains many large, identified neurons which can be easily manipulated making it an advantageous model system to elucidate in vivo the architecture of neuronal signal transduction pathways. We have isolated three cDNA clones encoding G protein alpha subunits that are expressed in the Lymnaea CNS, i.e. G alpha o, G alpha s and G alpha i. The deduced proteins exhibit a very high degree of sequence identity to their vertebrate and invertebrate counterparts. The strong conservation of G protein alpha subunits suggests that functional insights into G protein-mediated signalling routes obtained through the experimental amenability of the Lymnaea CNS will have relevance for similar pathways in the mammalian brain.

Evolution of the mammalian G protein alpha subunit multigene family

Heterotrimeric guanine nucleotide binding proteins (G proteins) transduce extracellular signals received by transmembrane receptors to effector proteins. The multigene family of G protein alpha subunits, which interact with receptors and effectors, exhibit a high level of sequence diversity. In mammals, 15 G alpha subunit genes can be grouped by sequence and functional similarities into four classes. We have determined the murine chromosomal locations of all 15 G alpha subunit genes using an interspecific backcross derived from crosses of C57BL/6J and Mus spretus mice. These data, in combination with mapping studies in humans, have provided insight into the events responsible for generating the genetic diversity found in the mammalian alpha subunit genes and a framework for elucidating the role of the G alpha subunits in disease.

Probable occurrence of toxin-susceptible G proteins in the nematode Caenorhabditis elegans

Pertussis toxin, islet-activating protein (IAP), and cholera toxin ADP-ribosylated 40 kDa and 45 kDa proteins in membrane preparations from Caenorhabditis elegans. Proteins with the same molecular weights were recognized in the same membranes by an antibody that had been raised against a peptide common to alpha-subunits of mammalian alpha beta gamma-heterotrimeric G proteins. The antibody produced immunoprecipitation with the 40 kDa protein 32P-labeled by IAP. A 35 kDa protein immunochemically indistinguishable from the beta-component of mammalian G proteins was also found in C. elegans membranes. The membranes displayed adenylate cyclase activity which was highly sensitive to forskolin and GTP analogues, whose action was antagonized by GDP beta S. Receptor-coupled regulation of adenylate cyclase thus appears to be mediated by mammalian-type G proteins in C. elegans as well.

Evidence for a novel splice variant of the alpha subunit of Go in rat male haploid germ cells

Northern analysis shows that Go alpha mRNA is highly expressed in immature germ cells from rat. Whereas alpha o2 mRNA is the major form in pachytene spermatocytes, a message of shorter chain length is present in large amount in haploid germ cells. This mRNA was detected with an oligonucleotide specific for the 3'-coding region of alpha o2, but did not hybridize to oligonucleotides specific for the 5'-untranslated and 5'-coding regions. The results indicate the presence of a novel splice variant of alpha o mRNA, which may code for a Go alpha protein important for germ cell development.

Identification of a retinal protein in Drosophila with antibody to the alpha subunit of bovine brain G(o) protein

An antibody directed against the alpha(o) subunit of bovine brain G(o) (R4) was used to identify a Drosophila retinal protein which may be the analogue of vertebrate transducin. The immunoreactivity appears predominantly in the retinal and occellar rhabdomeres. On a Western blot, the antibody recognizes a 41 kDa protein that is present in the heads of yellow white flies, but not in the heads of eyeless mutant flies, eyes absent. This protein is not recognized by an antibody raised against Drosophila alpha(o). Antibody R4 intensely stains rhabdomeres and, to a lesser extent, the neuropil of the central nervous system in tissue sections of adult flies. Antibody to Drosophila alpha(o) stains the neuropil of the central nervous system, but does not stain rhabdomeres. In developing flies, faint immunoreactivity appears in the retinal rhabdomeres at about 70% of the time through pupal development and increases to its apparent adult maximal level about 1 day after eclosion. Tissue sections from a phototransduction mutant, norp A, have retinal immunoreactivity at normal levels up to about 1 week after eclosion, but by 2 weeks, immunoreactivity has largely disappeared. This disappearance parallels the degeneration of the retina in norp A mutants. In Drosophila and other invertebrates, light activates a phospholipase C in the retina. The identification of a protein in Drosophila rhabdomeres with an antibody raised against a mammalian G protein alpha subunit thought to be involved in phospholipase C activation suggests that there may be common structural features between the putative Drosophila transducin and alpha(o). The identification of regions common to mammalian alpha(o) and Drosophila transducin may then provide clues to the structural requirements for PLC activation.

Diversity of G proteins in signal transduction

The heterotrimeric guanine nucleotide-binding proteins (G proteins) act as switches that regulate information processing circuits connecting cell surface receptors to a variety of effectors. The G proteins are present in all eukaryotic cells, and they control metabolic, humoral, neural, and developmental functions. More than a hundred different kinds of receptors and many different effectors have been described. The G proteins that coordinate receptor-effector activity are derived from a large gene family. At present, the family is known to contain at least sixteen different genes that encode the alpha subunit of the heterotrimer, four that encode beta subunits, and multiple genes encoding gamma subunits. Specific transient interactions between these components generate the pathways that modulate cellular responses to complex chemical signals.

The Drosophila gastrulation gene concertina encodes a G alpha-like protein

Gastrulation is a complex process requiring the coordination of cell shape changes and cell movements. In Drosophila, gastrulation begins immediately upon cellularization of the blastoderm stage embryo with the formation of the ventral furrow and posterior midgut. Cells that form both of these invaginations change their shape via apical constriction. Embryos from mothers homozygous for mutations in the concertina (cta) gene begin furrow formation by forming a zone of tightly apposed cells, constrict some cells, and then fail to constrict enough cells to form an organized groove. The cta gene has been cloned, and sequence analysis suggests that it encodes an alpha subunit of a G protein. G proteins have a role in cell-cell communication as mediators of signals between membrane-bound receptors and intracellular effectors. The phenotype of embryos from homozygous cta mothers suggests that the cta gene plays a role in a signal transduction pathway used during gastrulation.

Structure and function of G protein coupled receptors

Application of a molecular genetic techniques has allowed the isolation and identification of more than 50 members of the G protein-coupled receptor family. Their specificities range from sensory receptors such as the opsins and odorant receptors through those for the amines, peptides and other small molecules to those for glycoprotein hormones. These studies make it clear that traditional pharmacological methods, often underestimate receptor diversity. G protein-coupled receptors share a common structure consisting of 7 transmembrane alpha helical segments. Receptor structure-function relationships are discussed in the light of results obtained by site-directed mutagenesis and the construction of chimeric receptors. Studies which have allowed the identification of ligand-binding domains, and of sequences defining G protein specificity as well as those involved in receptor desensitization and downregulation are also discussed.

Structure and function of G proteins

G proteins are heterotrimeric proteins involved in the transduction of a variety of external signals in all eukaryotic organisms. This review analyzes the molecular aspects of G protein structure and function. The cloning of cDNAs coding for a great variety of G protein subunits has allowed us to deduce the primary and secondary structure of the subunits. Emphasis is given to the dissection of the molecular regions of the G alpha subunits implicated in the binding and hydrolysis of GTP and in the interaction with the receptor, with the effector and with the beta gamma dimer. The localization of these regions in a two-dimensional model of the G alpha subunit is attempted to provide a more comprehensive view of the structure and function of G proteins.

The transduction signalling protein Go during embryonic development of Drosophila melanogaster

G proteins are heterotrimeric proteins that play a key role in signalling transduction conveying signals from cell surface receptors to intracellular effector proteins. In particulate preparations from Drosophila melanogaster embryos, only one substrate of 39,000-40,000 molecular weight could be ADP-ribosylated with pertussis toxin. This substrate reacted in immunoblotting and immunoprecipitation experiments with a polyclonal antibody directed against the carboxy-terminal sequence of the alpha subunit of the mammalian Go protein. The Drosophila Go alpha protein was present at all stages of embryonic development; however, its expression markedly increased after 10 h embryogenesis, a period of time during which there is an active development of axonal tracts. Immunolocalization on whole mount embryos has indicated that this protein is principally localized in the CNS and is mainly restricted to the neuropil without any labelling of the cell bodies. In contrast, all the axon tracts of the CNS appeared to be highly labelled. The distribution of the Go alpha protein was also examined in several neurogenic mutants. The Go alpha protein expression was not altered in any of them but the pattern of labelling was disorganized as was the neuronal network. These results suggest a possible role for the Go protein during axonogenesis.

Occurrence, quaternary structure and function of G protein subunits in an insect endocrine gland

The occurrence, structure and function of the alpha and beta subunits of GTP-binding proteins (G proteins) were investigated in the Manduca sexta prothoracic gland, a tissue which possesses a hormonally regulated adenylate cyclase. Subunit-specific antibodies were utilized in immunoblotting studies of tissue from Manduca prothoracic glands, brain, eyes and antennae, and compared to the substrates present in the heads of Drosophila, as well as in a mammalian cell line. All Manduca tissues examined showed putative G beta subunits of 37 and 38 kDa, an unidentified alpha subunit of 41 kDa, in addition to an eye specific alpha subunit of 42 kDa. Manduca tissues also produced putative Gs alpha subunits of 48 and 51 kDa which were coupled to prothoracic gland adenylate cyclase as demonstrated by immunoprecipitation. Prothoracic gland G proteins have a definite and limited quaternary structure, consistent with a heterotrimeric model, as demonstrated by crosslinking of prothoracic gland membrane preparations followed by immunoblotting. These studies also yielded data on relative titers of alpha subunits, and suggest that Gs alpha is present in lower amounts than other alpha subunits. The G protein subunits studied in the prothoracic gland appear strikingly similar in molecular weight, function and structure to their mammalian counterparts.

Identification of two distinct isoforms of the guanine nucleotide binding protein G0 in neuroblastoma X glioma hybrid cells: independent regulation during cyclic AMP-induced differentiation

Three distinct antipeptide antisera generated against synthetic peptides that represent parts of the primary sequence of the alpha-subunit of the (pertussis toxin-sensitive) guanine nucleotide binding protein G0 were used in two-dimensional immunoblots of membranes of neuroblastoma X glioma (NG108-15) cells. Each antiserum identified two distinct polypeptides of some 39 kDa. These had apparent isoelectric points of 5.5 and 5.8. Differentiation of NG108-15 cells in response separately to dibutyryl cyclic AMP (cAMP), 8-bromo cAMP, forskolin, and prostaglandin E1 produced elevated levels of G0 alpha, as has previously been noted in one-dimensional immunoblots. Two-dimensional analysis demonstrated that the cAMP-induced increases in levels of G0 alpha were only of the more acidic isoform. The two isoforms were both substrates for pertussis toxin-catalysed ADP-ribosylation and did not appear to represent differentially phosphorylated forms of the same polypeptide. Separation of the two forms of G0 alpha could be achieved in one-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis when 4 M deionized urea was included in the resolving gel. The more slowly migrating band was the acidic form and corresponded exactly in mobility with the major form of G0 from both rat and mouse brain. There was no equivalent in brain of the more rapidly migrating form of G0 from the cells. In agreement with the data from two-dimensional gels, only the more slowly migrating form was expressed in considerably higher amounts following cAMP-induced differentiation of NG108-15 cells. Of these two forms of "G0," the acidic species is equivalent to G0 from brain, but the basic form is not identical with G0*, which has been purified from bovine brain.

dgq: a drosophila gene encoding a visual system-specific G alpha molecule

We describe the isolation and preliminary characterization of a new G alpha gene (dgq) in Drosophila. The dgq gene is differentially spliced, yielding two putative proteins, both of which contain guanine nucleotide binding and hydrolysis domains and share 50% identity with transducins and other G proteins. These proteins represent a new class of G alpha subunits because they lack both high amino acid identity with other G alpha proteins and the pertussis toxin ADP ribosylation site. The dgq mRNA is detected by RNA-RNA Northern hybridization in wild-type heads but not in wild-type bodies or in the mutant eyes absent heads. Tissue in situ hybridization detects dgq expression only in the retina and ocellus of the adult head, making it a prime candidate for encoding the Drosophila transducin analog, the G protein required for phototransduction.

Molecular cloning, sequencing and expression of cDNA encoding a G0-protein from insect

A locust cDNA clone encoding the complete sequence of a guanine nucleotide-binding protein was isolated and its nucleotide sequence determined. Comparing the deduced amino acid sequence with primary structures of other G-proteins revealed striking homologies with the vertebrate G0-protein. The cloned cDNA was expressed and the translation product detected by specific antibodies. Northern blot analysis revealed that the corresponding mRNA exists in two forms, preferentially expressed in the nervous tissue.