Sequence of the alpha subunit of photoreceptor G protein: homologies between transducin, ras, and elongation factors

Molecular evolutionary analysis of vertebrate transducins: a role for amino acid variation in photoreceptor deactivation

Transducin is a heterotrimeric G protein that plays a critical role in phototransduction in the rod and cone photoreceptor cells of the vertebrate retina. Rods, highly sensitive cells that recover from photoactivation slowly, underlie dim-light vision, whereas cones are less sensitive, recover more quickly, and underlie bright-light vision. Transducin deactivation is a critical step in photoreceptor recovery and may underlie the functional distinction between rods and cones. Rods and cones possess distinct transducin α subunits, yet they share a common deactivation mechanism, the GTPase activating protein (GAP) complex. Here, we used codon models to examine patterns of sequence evolution in rod (GNAT1) and cone (GNAT2) α subunits. Our results indicate that purifying selection is the dominant force shaping GNAT1 and GNAT2 evolution, but that GNAT2 has additionally been subject to positive selection operating at multiple phylogenetic scales; phylogeny-wide analysis identified several sites in the GNAT2 helical domain as having substantially elevated dN/dS estimates, and branch-site analysis identified several nearby sites as targets of strong positive selection during early vertebrate history. Examination of aligned GNAT and GAP complex crystal structures revealed steric clashes between several positively selected sites and the deactivating GAP complex. This suggests that GNAT2 sequence variation could play an important role in adaptive evolution of the vertebrate visual system via effects on photoreceptor deactivation kinetics and provides an alternative perspective to previous work that focused instead on the effect of GAP complex concentration. Our findings thus further the understanding of the molecular biology, physiology, and evolution of vertebrate visual systems.

Organ-specific and light-induced expression of plant genes

Light plays a pivotal role in the development of plants. The photoregulation of plant genes involves recognition of light quality and quantity by phytochrome and other light receptors. Two gene families, rbcS and Cab, which code for abundant proteins active in photosynthesis, the small subunit of ribulose bisphosphate carboxylase and the chlorophyll a/b binding protein, show a 20-to 50-fold increase in transcript abundance in the light. Analyses in calli and transgenic plants of deletions of the rbcS gene and of chimeric constructions has allowed localization of two regions involved in light-induced transcription. One element is confined to a 33-base pair region surrounding the TATA box. In addition, an enhancer-like element contained within a 240-base pair fragment can confer phytochrome-induced transcription and organ specificity on nonregulated promoters.

Gustducin and transducin: a tale of two G proteins

In the vertebrate taste cell, heterotrimeric guanine nucleotide-binding proteins (G proteins) are involved in the transduction of both bitter and sweet taste stimulants. The bitter compound denatonium raises the intracellular Ca2+ concentration in rat taste cells, apparently via G protein-mediated increases in inositol trisphosphate. Sucrose causes a G protein-dependent generation of cAMP in rat taste bud membranes; elevation of cAMP levels leads to taste cell depolarization. To identify and characterize those proteins involved in the taste transduction process, we have cloned G protein alpha subunit (G alpha) cDNAs from rat taste cells. Using degenerate primers corresponding to conserved regions of G proteins, we used the polymerase chain reaction to amplify and clone taste cell G alpha cDNAs. Eight distinct G alpha cDNAs were isolated, cloned and sequenced from a taste cell library. Among these clones was alpha gustducin, a novel taste G alpha closely related to the transducins. In addition to alpha gustducin, we cloned rod and cone transducins from taste cells. This is the first identification of transducin expression outside photoreceptor cells. The primary sequence of alpha gustducin shows similarities to the transducins in the receptor interaction domain and the phosphodiesterase activation site. These sequence similarities suggest that gustducin and transducin regulate taste cell phosphodiesterase, probably in bitter taste transduction.

G-Proteins and GPCRs: From the Beginning

From the point of view of a participant observer, I tell the discovery stories of trimeric G-proteins and GPCRs, beginning in the 1970s. As in most such stories, formidable obstacles, confusion, and mistakes make eventual triumphs even more exciting. Because these pivotally important signaling molecules were discovered before the recombinant DNA revolution, today's well-trained molecular biologist may find it amazing that we learned anything at all.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

Why photoreceptors die (and why they don't)

Light can kill the photoreceptors of the eye, not only very bright direct sunlight, but more moderate illumination if the light is present continuously. Recent experiments show that rod apoptosis can be triggered by strong and constant activation of transduction, and that death can be prevented if transduction is inhibited even though the eye is illuminated. Vitamin A deficiency and genetically inherited diseases, such as some forms of retinitis pigmentosa and Leber congenital amaurosis, appear to kill like this: transduction is activated at a high rate and continuously, and this causes the rods to die. Why does transduction kill? Our best guess is that continuous activation produces a prolonged lowering of the Ca(2+) concentration, which is also thought to kill neurons in tissue culture and during the development of the nervous system. To prevent death in constant light, rods have evolved protective mechanisms including modulation of channels and ion transport to keep the Ca(2+) from going too low. Prolonged light exposure also causes migration of transduction proteins from one part of the cell to another and a reversible shortening of the rod outer segments, the part of the cell that contains the pigment rhodopsin. All of these mechanisms are at work in the normal eye to reduce transduction and prevent the Ca(2+) concentration from dropping too low for too long a time. That most of us retain our vision our entire lives is a testament to their effectiveness.

Molecular evolution of proteins involved in vertebrate phototransduction

Vision is one of the most important senses for vertebrates. As a result, vertebrates have evolved a highly organized system of retinal photoreceptors. Light triggers an enzymatic cascade, called the phototransduction cascade, that leads to the hyperpolarization of photoreceptors. It is expected that a systematic comparison of phototransduction cascades of various vertebrates can provide insights into the diversity of vertebrate photoreceptors and into the evolution of vertebrate vision. However, only a few attempts have been made to compare each phototransduction protein participating in this cascade. Here, we determine phylogenetic trees of the vertebrate phototransduction proteins and compare them. It is demonstrated that vertebrate opsin sequences fall into five fundamental subfamilies. It is speculated that this is crucial for the diversity of the spectral sensitivity observed in vertebrate photoreceptors and provides the vertebrates with the molecular tools to discriminate the color of incident light. Other phototransduction proteins can be classified into only a few subfamilies. Cones generally share isoforms of phototransduction proteins that are different from those found in rods. The difference in sensitivity to light between rods and cones is likely due to the difference in the molecular properties of these isoforms. The phototransduction proteins seem to have co-evolved as a system. Switching the expression of these isoforms may characterize individual vertebrate photoreceptors.

Molecular cloning of a novel membrane glycoprotein, pal, specifically expressed in photoreceptor cells of the retina and containing leucine-rich repeat

We have isolated a novel retina-specific gene in a screen for genes of which expression is not apparent neonatally in rat retina but is abundant postnatally on day 14 (P14). This gene, named Pal, encodes a putative type I transmembrane protein containing five leucine-rich repeats (LRRs), a single C2-type Ig-like domain, and a single fibronectin type III domain and is considered to be a new member of the LRR and Ig superfamily. No expression of Pal was found in rat retina at P1, but it was detected at P7 and markedly increased with subsequent development. These expression patterns of Pal appeared to be correlated with the development of the photoreceptor outer segments, because in the adult rat retina it was specifically localized in these segments. Ultrastructually, Pal immunoreactivity was distributed diffusely on the disk membrane in the lamellar regions. On the basis of its structural features and localization pattern, Pal may act as a receptor for a certain trophic factor or for an adhesion molecule participating in morphogenesis. The human homolog of Pal was mapped to chromosome 10q23.2-23.3 using fluorescence in situ hybridization.

Piscine (Sparus aurata) alpha subunit of the G-protein transducin is homologous to mammalian cone and rod transducin

A novel cDNA encoding alpha subunit of the GTP-binding protein, transducin, has been cloned from a marine fish, Sparus aurata. The cDNA contains an open reading frame of 1050 nt (encoding 350 amino acid residues). A high degree of identity was found with known mammalian transducin proteins of cones (Gt2 alpha) or rods (Gt1 alpha): human Gt2 alpha (80.2%), bovine Gt2 alpha (79.3%), mouse Tt1 alpha (78.2%), mouse Gt2 alpha (78%) and bovine Gt1 alpha (77.9%). Northern blot analysis of different tissues revealed a transcript of about 2.5 kb, which is expressed only in the fish eye and not in other tissues from adult fish, supporting its identification as transducin. Ontogeny of transducin mRNA expression during early development of Sparus aurata, determined by Northern blot analysis, showed very low levels in larvae 3 days after hatching but not earlier. Levels increased 3- and 6-fold on days 4 and 6 (respectively) compared with those on day 3 and remained essentially unchanged thereafter, until day 21 after hatching (the last day studied). Our results suggest that in fish only one alpha subunit of transducin is found, which shows similar identity with cone and rod alpha subunits of mammals.

Pancreatic beta-cell receptors and G proteins coupled to adenylyl cyclase

Glucagon and tGLP-1 receptors can be either coexpressed or selectively expressed in beta-cell models. Our results indicate that both these peptides can regulate insulin secretion from beta-cells through their own specific receptors. The finding of a selective expression of G proteins in insulin and glucagon cells indicates a clear difference in their transduction pathways. A key role of the G alpha s family in beta-cell function is further supported by its conserved cell distribution between different species. In conclusion, one could postulate that in the human beta-cells, tGLP-1 and glucagon receptors could mediate their action through different G protein alpha-subunits of the G alpha s family.

Sheep linkage mapping: RFLP markers for comparative mapping studies

Restriction fragment length polymorphisms (RFLPs) detected using cDNA probes for conserved genes provide an important set of markers that anchor or link syntenic groups in a range of divergent mammalian species. DNA probes from sheep, cattle, pig, human and mouse were screened against sheep DNA samples and 24 new RFLP markers for sheep were identified. Among the loci tested, 22 had a homologue that has been mapped in humans. An RFLP for fibronectin (FN1) was linked to alpha-inhibin (INHA) at a distance of 5cM. The FN1 locus has been assigned to sheep chromosome 2q41-q44 and linkage between FN1 and INHA assigns INHA to the same chromosome in sheep. In addition to the new loci reported here, 28 RFLPs have been published previously by this group and these are collated together with RFLPs published from other laboratories. RFLPs have been reported for 86 loci in sheep. Fifty-four loci have been mapped to 16 different chromosomes.

Coupling of bitter receptor to phosphodiesterase through transducin in taste receptor cells

The rod and cone transducins are specific G proteins originally thought to be present only in photoreceptor cells of the vertebrate retina. Transducins convert light stimulation of photoreceptor opsins into activation of cyclic GMP phosphodiesterase (reviewed in refs. 5-7). A transducin-like G protein, gustducin, has been identified and cloned from rat taste cells. We report here that rod transducin is also present in vertebrate taste cells, where it specifically activates a phosphodiesterase isolated from taste tissue. Furthermore, the bitter compound denatonium in the presence of taste-cell membranes activates transducin but not Gi. A peptide that competitively inhibits rhodopsin activation of transducin also blocks taste-cell membrane activation of transducin, arguing for the involvement of a seven-transmembrane-helix G-protein-coupled receptor. These results suggest that rod transducin transduces bitter taste by coupling taste receptor(s) to taste-cell phosphodiesterase. Phosphodieterase-mediated degradation of cyclic nucleotides may lead to taste-cell depolarization through the recently identified cyclic-nucleotide-suppressible conductance.

Nobel Lecture. G proteins and regulation of adenylyl cyclase

The function and structures of G proteins and their role in the regulation of adenylyl cyclase is reviewed.

Molecular cloning of G proteins and phosphodiesterases from rat taste cells

To identify and characterize those proteins involved in taste transduction, we cloned G proteins and phosphodiesterases from rat taste tissue. Using degenerate primers corresponding to conserved regions of G protein alpha subunits, the polymerase chain reaction was used to amplify and clone eight distinct cDNAs: alpha i-2, alpha i-3, alpha 12, alpha 14, a(s), alpha t-rod, alpha t-cone and alpha gustducin. alpha i-3, alpha 14, alpha s, and alpha t-rod are more highly expressed in taste tissue than in the surrounding nonsensory tissue. alpha gustducin is only expressed in taste cells. Rod transducin had previously been found only in the rod cells of the retina, where it converts light stimulation of rhodopsin into activation of cGMP phosphodiesterase. The primary sequence of alpha gustducin shows striking similarities to rod transducin in the receptor interaction domain and the phosphodiesterase activation site. We propose that gustducin and transducin regulate phosphodiesterase activity in taste cells and that this may promote bitter transduction and inhibit sweet transduction. Consistent with this proposal, we cloned two types of cAMP PDE from taste tissue: dnc-1 and PDE-3.

SK-N-BE: a human neuroblastoma cell line containing two subtypes of delta-opioid receptors

A human neuroblastoma cell line, SK-N-BE, was shown to express a substantial amount of opioid receptors (200-300 fmol/mg of protein). A ligand binding profile of these receptors revealed that they could belong to two distinct subtypes of delta-opioid receptors. Results from sucrose-gradient sedimentation experiments were compared with similar data obtained with the mu-opioid receptor of the rabbit cerebellum and the delta-opioid receptor of the hybrid NG108-15 cell line and have shown that the opioid receptor of the SK-N-BE cell line behaved hydrodynamically as an intermediate between mu- and delta-opioid receptors. Taken together, pharmacological and hydrodynamic studies suggest that the opioid receptors present in the SK-N-BE cell membranes could belong to two delta-opioid receptor subtypes interacting allosterically. Functional experiments suggest that at least one of these subtypes of delta-opioid receptor is negatively coupled to the adenylate cyclase via a Gi protein and that the opiate receptors of the SK-N-BE neuroblastoma cell line undergo a rapid down-regulation when preincubated in the presence of the high-affinity opioid, etorphine.

The molecular biology of taste transduction

Taste cells respond to a wide variety of chemical stimuli: certain ions are perceived as salty (Na+) or sour (H+); other small molecules are perceived as sweet (sugars) and bitter (alkaloids). Taste has evolutionary value allowing animals to respond positively (to sweet carbohydrates and salty NaCl) or aversively (to bitter poisons and corrosive acids). Recently, some of the proteins involved in taste transduction have been cloned. Several different G proteins have been identified and cloned from taste tissue: gustducin is a taste cell specific G protein closely related to the transducins. Work is under way to clone additional components of the taste transduction pathways. The combination of electrophysiology, biochemistry and molecular biology is being used to characterize taste receptor cells and their sensory transduction mechanisms.

An immunohistochemical study of ras oncoprotein expression in gastric carcinoma

BACKGROUND

Previous studies have shown that ras oncoprotein is overexpressed in gastric carcinoma. It is unsettled whether there is differential expression of this oncoprotein in the two major subtypes of gastric carcinoma--intestinal type and diffuse type.

METHODS

Forty-four cases of gastric carcinomas (24 intestinal type, 18 diffuse type, and 2 mixed according to the Lauren classification) were analyzed for ras oncoprotein expression using commercially available antibodies in routinely formaldehyde-fixed, paraffin-embedded tissue sections.

RESULTS

Altogether, 13 cases stained unequivocally, and these appeared as granular cytoplasmic staining in tumor cells. Only five were intestinal-type carcinomas; the remaining eight were diffuse type and included two intramucosal carcinomas. When the comparison between the two subtypes was limited only to the poorly differentiated carcinomas, the difference was statistically significant (P < 0.05). ras Oncoprotein also was strongly expressed in normal duodenal mucosa as well as areas of gastric intestinal metaplasia.

CONCLUSIONS

These results suggest that the strong expression of ras oncoprotein in gastric intestinal metaplasia may be merely a reflection of metaplastic change to an intestinal epithelial phenotype. The differential expression of ras oncoprotein between diffuse-type and poorly differentiated intestinal-type gastric carcinoma implies that these are two distinct subtypes of gastric carcinoma.

Tryptophan207 is involved in the GTP-dependent conformational switch in the alpha subunit of the G protein transducin: chymotryptic digestion patterns of the GTP gamma S and GDP-bound forms

The limited proteolytic pattern of transducin, Gt, and its purified subunits with chymotrypsin were analyzed and the cleavage sites on the alpha t subunit were identified. The alpha t subunit in the GTP gamma S bound form was cleaved into a major 38 kD fragment, whereas alpha t-GDP was progressively digested into 38, 23, 21, and 15 kD fragments. The beta gamma t subunit was not very sensitive to proteolytic digestion with chymotrypsin. The gamma t subunit was not cleaved and only a small portion of beta t was digested into several fragments. In order to determine which proteolytic fragment of alpha t still contained the carboxyl terminal region, chymotrypsinization was carried out using Gt previously 32P-labeled at Cys347 by pertussis toxin-catalyzed ADP-ribosylation. The 32P-label was mainly associated with the alpha t subunit and a 15 kD fragment. The 23 and 21 kD fragments were not 32P-labeled. Analysis of amino terminal sequences of 38, 21, and 15 kD proteolytic bands allowed the identification of the major cleavage sites. Chymotrypsin had two cleavage sites in the amino terminal region of alpha t, at Leu15 and Leu19. Chymotrypsin removed 15-19 amino acid residues from the amino terminus of alpha t, generating two peptides (38 kD) which comigrates in gel electrophoresis. Chymotrypsin also cleaved at Trp207 in a conformation-dependent manner. Trp207 of alpha t-GTP gamma S was resistant to proteolysis but alpha t-GDP and the 38 kD fragments of alpha t-GDP produced the 23 and 21 kD fragments, respectively, and a 15 kD fragment containing the carboxyl terminus.(ABSTRACT TRUNCATED AT 250 WORDS)

Amplification and kinetics of the activation steps in phototransduction

We can summarize our investigation of amplification in the activation steps of vertebrate phototransduction as follows. (1) A theoretical analysis of the activation steps of the cGMP cascade shows that after a brief flash of phi photoisomerizations the number of activated PDE molecules should rise as a delayed ramp with slope proportional to phi, and that, as a consequence, the cGMP-activated current should decay as a delayed Gaussian function of time (Eqn. 20). (i) Early in the response to a flash, the normalized response R(t) can be approximated as rising as 1/2 phi At2 (after a short delay), where A is the amplification constant characteristic of the individual photoreceptor. (ii) The delayed ramp behavior of PDE activation and the consequent decline of current in the form of the delayed Gaussian are confirmed by experiments in a variety of photoreceptors; the analysis thus yields estimates of the amplification constant from these diverse photoreceptors. (iii) Eqn. 20 further predicts that the response-intensity relation at any fixed time should saturate exponentially, as has been found experimentally. (2) The amplification constant A can be expressed as the product of amplification factors contributed by the individual activation steps of phototransduction, i.e., A = nu RG cGP beta sub n (Eqns. 9 and 21), where (i) nu RG is the rate of G* production per Rh*; (ii) cGP is the efficiency of the coupling between G* production and PDE* production; (iii) beta sub is the increment in hydrolytic rate constant produced by one PDE*, i.e., a single activated catalytic subunit of PDE; and (iv) n is the Hill coefficient of opening of the cGMP-activated channels. (3) The amplification factor beta sub includes the ratio kcat/Km, which characterizes the hydrolytic activity of the PDE in vivo where cG << Km. Two different analyses based upon photocurrents were developed which provide lower bounds for kcat/Km in vivo; these analyses establish that kcat/Km probably exceeds 10(7) M-1 s-1 (and is likely to be higher) in both amphibian and mammalian rods. Few biochemical studies (other than those using trypsin activation) have yielded such high values. A likely explanation of many of the relatively low biochemical estimates of kcat/Km is that Km may have been overestimated by a factor of about 4 in preparations in which stacks of disks are left intact, due to diffusion with hydrolysis in the stacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogene-transformed granulosa cells as a model system for the study of steroidogenic processes

Highly steroidogenic granulosa cell lines were established by transfection of primary granulosa cells from preovulatory follicles with SV40 DNA and Ha-ras oncogene. Progesterone production in these cells was enhanced to levels comparable to normal steroidogenic cells, by prolonged (> 12 h) stimulation with 8-Br-cAMP, forskolin and cholera toxin, which elevate intracellular cAMP. The steroidogenic capacity of individual lines correlated with the expression of the ras oncogene product (p21) and the morphology of the cells. Formation of the steroid hormones was associated with de novo synthesis of the mitochondrial cytochrome P450scc system proteins. Since cholesterol import into mitochondria is essential for steroidogenesis, the expression of the peripheral benzodiazepine receptor (PBR) and the sterol carrier protein 2 was characterized in these cells. The induction of the expression of the genes coding for both proteins appeared to be mediated, at least in part, by cAMP. Stimulation of the PBR by specific agonists enhanced progesterone production in these cells. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) dramatically suppressed the cAMP-induced steroidogenesis, in spite of enhanced intracellular cAMP levels, suggesting that TPA can modify the effects of cAMP. cAMP stimulation suppressed growth of transformed cells concomitantly with induction of steroidogenesis. The transformed cells lacked receptors for the native stimulants, the gonadotropic hormones. After transfection of the cells with a lutropin (LH) receptor expression plasmid, the LH and hCG response was reconstituted. In these newly established cell lines gonadotropins were able to stimulate the formation of cAMP and progesterone in a dose-dependent manner with an ED₅₀ characteristic of the native receptor. High doses caused desensitization to gonadotropins as observed in normal cells. These newly established oncogene-transformed granulosa cell lines can serve as a useful model to study inducible steroidogenesis and the effect of oncogene expression on this process.

Lipid modification at the N terminus of photoreceptor G-protein alpha-subunit

Myristate is a fatty acid (fourteen-carbon chain with no double bonds, C14:0) linked to the amino-terminal glycine of several proteins, including alpha-subunits of heterotrimeric (alpha/beta gamma) G proteins. We report here a novel modification at the N terminus of the alpha-subunit of the photoreceptor G protein transducin, T alpha, with heterogeneous fatty acids composed of laurate (C12:0), unsaturated C14:2 and C14:1 fatty acids, and a small amount (approximately 5%) of myristate. Both the GTPase activity of T alpha/T beta gamma and the T beta gamma-dependent ADP-ribosylation of T alpha catalysed by pertussis toxin were inhibited by the lauroylated and myristoylated N-terminal peptide of T alpha. The myristoylated peptide gave 50% inhibition at a 3.5 to approximately 4.5-fold lower concentration than the lauroylated peptide in each assay, indicating that the strength of the interaction between T alpha and T beta gamma is altered by heterogeneous fatty acids linked to T alpha. This suggests that a looser subunit interaction in transducin which is due to an abundance of N-linked fatty acids other than myristate would favour the rapid turnover and catalysis essential for the visual excitation in photoreceptor cells.

Selective amplification and partial sequencing of cDNAs encoding G protein alpha subunits from cochlear tissues

An approach utilizing the polymerase chain reaction (PCR) was devised to clone members of a family of cDNAs encoding the alpha subunit of G proteins in the cochlea. RNA was extracted from the whole cochlea of the mouse and from the organ of Corti or the lateral wall of the cochlea microdissected from the guinea pig cochlea. The RNA was reverse-transcribed to cDNA which was selectively amplified by PCR using degenerate primers corresponding to two conserved regions of the G protein coding sequence. PCR products were cloned into a plasmid for sequencing. The following seven cDNA clones of particular interest were obtained: three clones putatively coding for part of the alpha-subunit of a stimulatory G protein (Gs), one clone putatively coding for part of the alpha-subunit of an inhibitory G protein (Gi) and three clones putatively coding for part of the alpha-subunit of a transducin (Gi)-like protein. Possible functions in the cochlea of putative G proteins with alpha-subunits partly encoded by these cDNA clones are briefly discussed and future studies are suggested.

Phylogeny and evolutionary rates of G protein alpha subunit genes

Rooted phylogenetic trees for a total of 34 genes encoding the stimulatory (s alpha), inhibitory (i alpha), transducin (t alpha), Gx (x alpha), Gz (z alpha), G11 (alpha 11), G12 (alpha 12), G13 (alpha 13), G16 (alpha 16), Gq (q alpha), and other (o alpha) G protein alpha subunits have been constructed. The analysis shows that the G12 (alpha 12 and alpha 13), Gq (alpha 11, alpha 16, and q alpha), and Gs (s alpha genes) groups form one cluster, and the Gx (x alpha and z alpha genes), G(i) (i alpha genes), Gt (t alpha 1 and t alpha 2), and G(o) (o alpha genes) groups form another cluster. During mammalian evolution, the rates of synonymous substitutions for these genes were estimated to be between 1.77 x 10(-9)/site/year and 5.63 x 10(-9)/site/year, whereas those of non-synonymous substitutions were between 0.008 x 10(-9)/site/year and 0.067 x 10(-9)/site/year. These evolutionary rates are similar to those for histone genes, suggesting equally important biological functions of the G protein alpha subunits.

The beta subunit of cyclic GMP phosphodiesterase mRNA is deficient in canine rod-cone dysplasia 1

Irish setter dogs affected with rod-cone dysplasia 1 have elevated levels of retinal cGMP resulting from deficient rod-specific cGMP phosphodiesterase (cGMP PDE) activity. We investigated the mRNAs coding for the three subunits of cGMP PDE and for the proteins involved in the activation/deactivation of this enzyme in the retinas of developing affected and control dogs. While the photoreceptor cells are viable in the diseased retinas, opsin, transducin alpha 1 and beta 1, 48 and 33 kd proteins, and cGMP PDE alpha and gamma mRNAs have normal transcript sizes and levels. In contrast, a different pattern of cGMP PDE beta mRNAs with lower than normal concentrations is present in the developing affected retinas prior to degeneration. Our observations suggest that an abnormality involving cGMP PDE beta expression is implicated in rod-cone dysplasia 1.

Gustducin is a taste-cell-specific G protein closely related to the transducins

A novel G protein alpha-subunit (alpha-gustducin) has been identified and cloned from taste tissue. alpha-Gustducin messenger RNA is expressed in taste buds of all taste papillae (circumvallate, foliate and fungiform); it is not expressed in non-sensory portions of the tongue, nor is it expressed in the other tissues examined. alpha-Gustducin most closely resembles the transducins (the rod and cone photoreceptor G proteins), suggesting that gustducin's role in taste transduction is analogous to that of transducin in light transduction.

Identification of effector-activating residues of Gs alpha

The heterotrimeric G proteins transduce extracellular signals by interacting with specific intracellular effectors. We have used a scanning mutagenesis approach to identify amino acids of alpha S, the alpha subunit of Gs, that determine the specificity of its interaction with its effector, adenylyl cyclase. In alpha subunit chimeras, residues 236-356 of alpha S comprise the shortest linear stretch that is required for activation of adenylyl cyclase. Within these 121 residues, we identified four clusters of residues in which substitutions prevented effector activation. Mutations in three of these regions did not affect alpha subunit expression or the GTP-induced conformational change. The identified alpha S residues in the NH2-terminal half of the 121-residue region endowed the cognate alpha i2 segment with the ability to activate effector, while those in the COOH-terminal half did not. In a three-dimensional G alpha model, based on the structure of p21ras, the effector-activating residues of alpha S form a surface on the membrane-facing side of the molecule; this surface includes a region that changes conformation upon binding GTP.

Characterization of Gs alpha mRNA transcripts in primary cultures of rat brain astrocytes

A cDNA clone encoding a stimulatory G-protein alpha subunit (Gs alpha) was isolated from a cDNA library derived from cultured rat astrocytes. The nucleotide sequence of the cDNA indicated that it corresponds to the Gs alpha-2 form of Gs alpha mRNA, one of four Gs alpha mRNAs known to be derived by alternative splicing from the human Gs alpha gene. A ribonuclease protection assay using cRNA from this clone allowed distinction between the Gs alpha-1 and Gs alpha-2 mRNAs, which encode the 52-kDa (Gs-L) forms of Gs alpha. Astrocytes express relatively high amounts of Gs alpha-1 mRNA, much lower amounts of the Gs alpha-2 mRNA, and no detectable amounts of the mRNAs (Gs alpha-3 and Gs alpha-4) encoding the two 45-kDa forms of Gs alpha (Gs alpha-S). Similar results were obtained with RNA samples isolated from whole brain. The 45-kDa form of Gs alpha protein was not detectable by immunoblot analysis of a membrane preparation from rat cerebral cortex (the source of the astrocyte cultures). These results indicate that the expression of Gs alpha forms in astrocytes is similar to that found in whole brain.

Molecular cloning and sequence analysis of cDNA and genomic DNA for the human cone transducin alpha subunit

A novel GTP binding protein (G protein) alpha subunit cDNA was isolated from a T cell leukemia cell line, Jurkat, utilizing polymerase chain reaction (PCR). The predicted amino acid sequence of this G protein alpha subunit showed the highest identity (96.6%) to bovine cone cell-specific transducin (Tc alpha). The organization of the coding region of this G protein alpha subunit gene was composed of 8 exons and 7 introns. Northern hybridization revealed the presence of this G protein message in a retinoblastoma cell line, Y79. In Jurkat, however, the message was detectable only by reverse transcription/PCR. Taken together, this novel G protein alpha subunit must be human Tc.

Structural heterogeneity of membrane receptors and GTP-binding proteins and its functional consequences for signal transduction

Recent information obtained, mainly by recombinant cDNA technology, on structural heterogeneity of hormone and transmitter receptors, of GTP-binding proteins (G-proteins) and, especially, of G-protein-linked receptors is reviewed and the implications of structural heterogeneity for diversity of hormone and transmitter actions is discussed. For the future, three-dimensional structural analysis of membrane proteins participating in signal transmission and transduction pathways is needed in order to understand the molecular basis of allosteric regulatory mechanisms governing the interactions between these proteins including hysteretic properties and cell-cybernetic aspects.

G-protein beta gamma forms: identity of beta and diversity of gamma subunits

Signal-transducing G-proteins are heterotrimers composed of GTP-binding alpha subunits in association with a tightly bound complex of beta and gamma subunits. While the alpha subunits are recognized as a family of diverse structures, beta and gamma subunits have also been found as heterogeneous isoforms. To investigate the diversity and tissue specificity of the beta gamma complexes, we have examined homogeneous oligomeric G-proteins from a variety of sources. The beta and gamma subunits isolated from the major-abundance G-proteins from bovine brain, bovine retina, rabbit liver, human placenta, and human platelets were purified and subjected to biochemical and immunological analysis. Protease mapping and immune recognition revealed an identical profile for each of the two distinctly migrating beta isoforms (beta 36 and beta 35) regardless of tissue or G-protein origin. Digestion with V8 protease revealed four distinct, clearly resolved terminal fragments for beta 36 and two for beta 35. Trypsin and chymotrypsin digestion yielded numerous bands, but again each form had a unique profile with no tissue specificity. Tryptic digestion was found to be conformationally specific with the most resistant structure being the native beta gamma complex. With increasing trypsin, the complex was digested but in a pattern distinct from that for denatured beta. In contrast to the two highly homologous beta structures, examination of this set of proteins revealed at least six distinct gamma peptides. Two unique gamma peptides were found in bovine retinal Gt and three gamma peptides in samples of bovine brain derived Go/Gi. Human placental and platelet Gi samples each contained a unique gamma.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-specific and developmental regulation of rod opsin chimeric genes in transgenic mice

Chimeric gene fusions between 4.4 kb of rod opsin 5' flanking sequence fused to a diphtheria toxin gene and 4.4 kb or 500 bp of rod opsin 5' flanking sequence fused to the E. coli IacZ gene were used to generate transgenic mice for analysis of cell type-specific expression and temporal and spatial distribution of reporter gene product during retinal development. Opsin-diphtheria toxin transgene expression evoked photoreceptor-specific cell death. The 4.4 kb opsin-IacZ transgene followed temporal and spatial gradients of expression that approximate opsin expression. The 500 bp opsin fragment targeted expression to photoreceptors, but expression was weaker and nonuniform, suggesting that elements located upstream may be required for enhanced and uniform spatial expression.

Elevation of glucose transporter, c-myc, and transin RNA levels by Ha-rasT24 is independent of its effect on the cell cycle

Elevation of the steady-state mRNA levels of glucose transporter and c-myc are among the earliest changes in gene expression observed after Ha-rasT24 stimulation of Rat-1 fibroblasts to enter the cell cycle. Since the expression of these genes may be the result of either increased cell proliferation or a specific response to rasT24, we evaluated the expression of glucose transporter and c-myc and their induction during the cell cycle in both parental Rat-1 cells and cell lines bearing a metallothionein rasT24 fusion gene (MTrasT24). We showed that, although levels of glucose transporter and c-myc mRNAs in Rat-1 cells underwent a transient increase within hours of the addition of serum, epidermal growth factor, or 12-O-tetradecanoylphorbol-13-acetate to quiescent (G0) cells, the levels of glucose transporter and c-myc mRNA otherwise remained constant throughout the normal cell cycle. In cells carrying MTrasT24 (MR5 cells), induction of rasT24 expression by ZnSO4 led to a rapid induction of glucose transporter and c-myc mRNA expression in both quiescent (density-arrested) and G1/S-synchronized (aphidicolin-blocked) cells. These increases exceeded the constitutive levels expressed in rapidly proliferating Rat-1 cells, indicating that the ras oncogene has an effect on these genes that is independent of growth status. In addition, the transin gene, which is not expressed in proliferating Rat-1 cells in the continuous presence of serum growth factors, was also induced after increased expression of the mutant ras gene. These results suggest that the induction of glucose transporter, c-myc, and transin is the direct result of rasT24-mediated alterations in cellular gene expression and is distinct from normal cell-cycle events.

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.

On the origins and present state of the art of G protein research

G proteins are central to the transduction of many receptor signals. I review the "firsts" that have led to our current knowledge and pose some of the questions presently investigated in several laboratories around the world.

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.

The p38 and p34 polypeptides of growth cone particle membranes are the alpha- and beta-subunits of G proteins

Growth cone particle (GCP) membranes prepared from fetal day 17 rat brain are comprised of 5 major polypeptides as analyzed by SDS-PAGE: tubulin (p52), actin (p42), pp46/GAP-43 and two unidentified species, p38 and p34. Antibodies specific for the alpha- and beta-subunits of G proteins recognize p38 and p34, respectively, on immunoblots following one- and two-dimensional electrophoretic separation. That G protein subunits comprise major species of GCP membrane-associated polypeptides suggests a role for G proteins in transmembrane signaling in nerve growth cones.

Alternative splicing produces transcripts encoding two forms of the alpha subunit of GTP-binding protein Go

The alpha subunit of the guanine nucleotide-binding protein Go ("o" for other) is believed to mediate signal transduction between a variety of receptors and effectors. cDNA clones encoding two forms of Go alpha subunit were isolated from a mouse brain library. These two forms, which we call GoA alpha and GoB alpha, appear to be the products of alternative splicing. GoA alpha differs from GoB alpha over the C-terminal third of the deduced protein sequence. Both forms are predicted to be substrates for ADP-ribosylation by pertussis toxin. GoA alpha transcripts are present in a variety of tissues but are most abundant in brain. The GoB alpha transcript is expressed at highest levels in brain and testis. It is possible that GoA alpha and GoB alpha have different functions.

Multiple forms of Go alpha mRNA: analysis of the 3'-untranslated regions

Go, a guanine nucleotide binding protein found predominantly in neural tissues, interacts in vitro with rhodopsin, muscarinic, and other receptors and has been implicated in the regulation of ion channels. Despite the virtual identity of reported cDNA sequences for the alpha subunit of Go (Go alpha), multiple molecular weight forms of mRNA have been identified in tissues from all species examined. To investigate the molecular basis for the size heterogeneity of Go alpha mRNAs, four cDNA clones were isolated from the same retinal lambda gt10 cDNA library that was used earlier to isolate lambda GO9, a clone encompassing the complete coding region of Go alpha. These clones were identified as Go alpha clones based on nucleotide sequence identity with lambda GO9 in the coding region; they diverge, however, from lambda GO9 in the 3'-untranslated region 28 nucleotides past the stop codon. An oligonucleotide probe complementary to a portion of the 3'-untranslated region of lambda GO9 that differs from the newly isolated clones hybridized with 3.0- and 4.0-kb mRNAs present in bovine brain and retina whereas a similar probe for the unique region of the new clones hybridized with a 4.0-kb mRNA in both tissues and with a 2.0-kb mRNA found predominantly in retina. A similar hybridization pattern was observed when brain poly(A+) RNA from other species was hybridized with the different 3'-untranslated region probes. It appears that differences in the 3'-untranslated regions could, in part, be the basis for the observed heterogeneity in Go alpha mRNAs.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.