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

Isolation of a second yeast Saccharomyces cerevisiae gene (GPA2) coding for guanine nucleotide-binding regulatory protein: studies on its structure and possible functions

In a previous paper, we demonstrated that a gene coding for a protein homologous to the alpha subunit of mammalian guanine nucleotide-binding regulatory (G) proteins occurs in Saccharomyces cerevisiae. The gene, designated GPA1, encodes a protein (GP1 alpha) of 472 amino acids with a calculated Mr of 54,075. Here we report the isolation of another G-protein-homologous gene, GPA2, which encodes an amino acid sequence of 449 amino acid residues with a Mr of 50,516. The predicted primary structure of the GPA2-encoded protein (GP2 alpha) is homologous to mammalian G proteins [inhibitory and stimulatory G proteins (Gi and Gs, respectively), a G protein of unknown function (Go), and transducins (Gt)] as well as yeast GP1 alpha. When aligned with the alpha subunit of Gi (Gi alpha) to obtain maximal homology, GP2 alpha was found to contain a stretch of 83 additional amino acid residues near the NH2 terminus. The gene was mapped in chromosome V, close to the centromere. Haploid cells carrying a disrupted GPA2 gene are viable. Cells carrying a high copy number of plasmid GPA2 (YEpGPA2) had markedly elevated levels of cAMP and could suppress a temperature-sensitive mutation of RAS2. These results suggest that GPA2 may be involved in the regulation of cAMP levels in S. cerevisiae.

SUF12 suppressor protein of yeast. A fusion protein related to the EF-1 family of elongation factors

Mutations at the suf12 locus were isolated in Saccharomyces cerevisiae as extragenic suppressors of +1 frameshift mutations in glycine (GGX) and proline (CCX) codons, as well as UGA and UAG nonsense mutations. To identify the SUF12 function in translation and to understand the relationship between suf12-mediated misreading and translational frameshifting, we have isolated an SUF12+ clone from a centromeric plasmid library by complementation. SUF12+ is an essential, single-copy gene that is identical with the omnipotent suppressor gene SUP35+. The 2.3 x 10(3) base SUF12+ transcript contains an open reading frame sufficient to encode a 88 x 10(3) Mr protein. The pattern of codon usage and transcript abundance suggests that SUF12+ is not a highly expressed gene. The linear SUF12 amino acid sequence suggests that SUF12 has evolved as a fusion protein of unique N-terminal domains fused to domains that exhibit essentially co-linear homology to the EF-1 family of elongation factors. Beginning internally at amino acid 254, homology is more extensive between the SUF12 protein and EF-1 alpha of yeast (36% identity; 65% with conservative substitutions) than between EF-1 alpha of yeast and EF-Tu of Escherichia coli. The most extensive regions of SUF12/EF-1 alpha homology are those regions that have been conserved in the EF-1 family, including domains involved in GTP and tRNA binding. It is clear that SUF12 and EF-1 alpha are not functionally equivalent, since both are essential in vivo. The N-terminal domains of SUF12 are unique and may reflect, in part, the functional distinction between these proteins. These domains exhibit unusual amino acid composition and extensive repeated structure. The behavior of suf12-null/SUF12+ heterozygotes indicates that suf12 is co-dominantly expressed and suggests that suf12 allele-specific suppression may result from functionally distinct mutant proteins rather than variation in residual wild-type SUF12+ activity. We propose a model of suf12-mediated frameshift and nonsense suppression that is based on a primary defect in the normal process of codon recognition.

Three-dimensional structure of an oncogene protein: catalytic domain of human c-H-ras p21

The crystal structure at 2.7 A resolution of the normal human c-H-ras oncogene protein lacking a flexible carboxyl-terminal 18 residue reveals that the protein consists of a six-stranded beta sheet, four alpha helices, and nine connecting loops. Four loops are involved in interactions with bound guanosine diphosphate: one with the phosphates, another with the ribose, and two with the guanine base. Most of the transforming proteins (in vivo and in vitro) have single amino acid substitutions at one of a few key positions in three of these four loops plus one additional loop. The biological functions of the remaining five loops and other exposed regions are at present unknown. However, one loop corresponds to the binding site for a neutralizing monoclonal antibody and another to a putative "effector region"; mutations in the latter region do not alter guanine nucleotide binding or guanosine triphosphatase activity but they do reduce the transforming activity of activated proteins. The data provide a structural basis for understanding the known biochemical properties of normal as well as activated ras oncogene proteins and indicate additional regions in the molecule that may possibly participate in other cellular functions.

Screening of cDNA libraries with oligonucleotides as applied to signal transducing G proteins, receptors and effectors

Screening of cDNA libraries constructed in phage or plasmids with oligonucleotide probes has become one of the preferred cloning techniques with the least number of false positive failures. In this article we present our current protocols for designing the procedure to detect cDNA inserts and isolate them. We illustrate with primary screens for G protein subunits and membrane receptors.

Structural and functional relationships of guanosine triphosphate binding proteins

Information available at present documents the existence of three well-defined classes of guanine nucleotide binding proteins functioning as signal transducers: Gs and Gi which stimulate and inhibit adenylate cyclase, respectively, and transducin which transmits and amplifies the signal from light-activated rhodopsin to cGMP-dependent phosphodiesterase in ROS membranes. Go is a fourth member of this family. Its function is the least known among GTP binding signal transducing proteins. The family of G proteins has a number of properties in common. All are heterotrimers consisting of three subunits, alpha, beta, and gamma. Each of the subunits may be heterogeneous depending on species and tissue of origin and may be posttranslationally modified covalently. The alpha subunits vary in size from 39 to 52 kDa. The sequences for Gs alpha and transducin alpha have 42% overall homology and those of Gi alpha and Gs alpha 43%, whereas those of Gi alpha and transducin alpha have a higher degree (68%) of homology. All alpha subunits bind guanine nucleotides and are ADP-ribosylated by either pertussis toxin (Gi, transducin, Go) or cholera toxin (Gs, Gi, transducin). Thus, transducin and Gi, which have the highest degree of sequence homology, are also ADP-ribosylated by both toxins. The beta subunits have molecular weights of 36 and 35 kDa, respectively. While Gs, Gi, and Go contain a mixture of both, transducin contains only the larger (36-kDa) beta-polypeptide. The relationship of the 36- and the 35-kDa beta subunits is not defined. Although the complete sequence of the 36-kDa beta subunit of transducin has been deduced from the cDNA sequence, complete sequences of other beta subunits are not yet available so that detailed comparisons cannot be made at present. However, the proteolytic profiles of each class of the beta subunits of different G proteins are indistinguishable. The gamma subunit of bovine transducin has been completely sequenced. It has a Mr of 8400. Again complete sequences of other gamma subunits are not yet available. While the gamma subunits of Gs, Gi, and Go have identical electrophoretic mobility in SDS gels, they differ significantly in this respect from the gamma subunit of transducin. Moreover, crossover experiments point to functional differences between gamma subunits from G protein and transducin complexes. In addition, a role for beta, gamma in anchoring guanine nucleotide binding proteins to membranes has been postulated.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterologous expression and characterization of the human R-ras gene product

We directly expressed human R-ras 23,000-dalton protein (p23) cDNA in Escherichia coli under the control of the trp promoter. GTP-dependent phosphorylation of a p23 threonine 85 substitution mutant was observed. This result is in direct analogy to the autokinase activity of H-ras and K-ras threonine 59 substitution mutants. Normal p23 protein was detected in the human fibrosarcoma cell line HT1080 by immunoprecipitation with rabbit antibodies raised against an E. coli-expressed R-ras fusion protein. The R-ras p23 protein was found to be 3H labeled in the presence of [9,10(n)-3H]palmitic acid and is associated with the P100 membrane fraction of HT1080 cells. These data suggest that human R-ras p23 has biochemical properties very similar to those of the p21 products of the H-, K-, and N-ras proto-oncogenes. We constructed an R-ras minigene and engineered the expression of normal and mutant alleles from the simian virus 40 early region promoter. Normal and mutant R-ras gene products were authenticated by transient expression in COS-7 cells and immunoprecipitation. The valine 38-substituted R-ras p23 displayed reduced electrophoretic mobility. R-ras p21-like proteins, made by eliminating the first 26 R-ras codons, displayed evident mobility differences between the pro form and mature form, along with a valine 12 substitution-dependent change in electrophoretic mobility. Rat-1 fibroblasts were transfected with normal and mutant R-ras alleles and normal and activated H-ras alleles. Unlike the human T24 bladder oncogene-encoded p21, mutant R-ras alleles do not cause monolayer focus formation or growth in soft agar of rat fibroblasts.

A small multigene family encodes Gi signal-transduction proteins

The guanine nucleotide-binding regulatory proteins known as G proteins are receptor-associated signal-transduction molecules that are implicated in the control of a variety of metabolic processes. Recent evidence suggests that G proteins may mediate B-lymphocyte responses to bacterial lipopolysaccharide and may also transduce signals from the T-cell antigen receptor. Since these receptors are uniquely expressed on lymphoid cells, we used molecular cloning strategies to ask whether lymphocytes contain specialized G-protein alpha subunits to assist in signal transduction. Comparison of our two deduced human alpha i amino acid sequences with those previously determined for bovine and rodent G proteins permits the identification of three closely related but distinct types of alpha i molecules that comprise a small multigene family. Using gene-specific probes, we found that both of our alpha i genes are expressed in most cell types but in differing ratios. Our data support the view that a modest repertoire of extremely closely related G proteins mediates the transduction of signals derived from multiple different receptor molecules.

Dynamic fatty acylation of p21N-ras

To study the acylation of p21N-ras with palmitic acid we have used cells which express the human N-ras gene to high levels under control of the steroid-inducible MMTV--LTR promoter. Addition of [3H]palmitate to these cells resulted in detectable incorporation of label into p21N-ras within 5 min, which continued linearly for 30-60 min. Inhibition of protein synthesis for up to 24 h before addition of [3H]palmitate had no effect on acylation of p21N-ras, suggesting that this can occur as a late post-translational event. Acylated p21N-ras with a high SDS--PAGE mobility is found only in the membrane fraction, whereas approximately 50% of the [35S]methionine-labelled p21N-ras is cytoplasmic and has a lower mobility. Conversion of the acylated high mobility form to a deacylated form of slightly lower mobility can be achieved with neutral hydroxylamine, which is known to cleave thioesters. This treatment also results in partial removal of p21N-ras from the membranes. A remarkably high rate of turnover of the palmitate moiety can be demonstrated by pulse--chase studies (t1/2 approximately 20 min in serum-containing medium) which cannot be attributed to protein degradation. The data suggest an active acylation--deacylation cycle for p21N-ras, which may be involved in its proposed function as a signal transducing protein.

Resolution of transducin subunits by chromatography on blue sepharose

The retinal guanine nucleotide-binding protein, transducin (TD), was subjected to chromatography on Blue Sepharose (BLS). A simple two-step protocol was developed, allowing the resolution of the alpha-subunit and the beta gamma-complex of the protein extracted from bovine retina by the use of a poorly hydrolysable GTP analogue. If TD was applied to BLS in a divalent cation-containing buffer, the beta gamma-complex did not bind to the resin, whereas the alpha-subunit was retained; elution of the latter was achieved by removing the divalent cation from the buffer. Binding of the alpha-subunit to BLS was not affected by nucleotides or by ADP ribosylation catalysed by bacterial toxins. However, adsorption of the alpha-subunit by BLS or by a strong cation exchanger (Mono S) depended strictly on divalent cations. In contrast to previous reports, the data suggest the formation of a complex between a sulphonyl residue of Cibacron Blue, a divalent metal ion, and the alpha-subunit as the relevant binding mechanism causing adsorption of the alpha-subunit to BLS.

Differential distribution of signal-transducing G-proteins in retina

We used specific antibodies in immunoblot studies of membrane fractions derived from bovine retina, and in immunohistochemical studies of sections of rat retina to determine the distribution of two guanine nucleotide binding proteins Go (a G-protein of unknown function discovered in the brain) and transducin, in retina. Both Go and transducin were readily detected in membranes derived from whole retina, and in crude rod outer segment membranes. Purification of rod outer segment membranes by sucrose density gradient centrifugation resulted in enrichment in transducin and depletion of Go immunoreactivity. Transducin-alpha immunoreactivity was localized to photoreceptor inner and outer segments and the outer nuclear layer. In contrast, Go-alpha immunoreactivity was localized in the inner and outer plexiform layers and ganglion cell layer. The results indicate that Go unlike transducin, is not associated with rod outer segment membranes and is therefore unlikely to function in phototransduction. Go is, however, relatively abundant in neural layers of retina where it may be involved in signal transduction.

mRNAs coding for proteins of the cGMP cascade in the degenerative retina of the rd mouse

A lesion in cGMP metabolism has been hypothesized to cause retinal degeneration in rd mice. Available cloned cDNAs coding for proteins involved in the cGMP cascade have been used to compare the corresponding retinal RNAs in the degenerative (rd/rd) mouse at 8-11 days with those in the 8-11-day-old morphologically normal (rd/+) and adult normal (+/+) mice. Northern analysis of these RNAs hybridized to the specific 32P-labeled cDNA probes for G-protein, 48,000 MW protein and opsin, indicates in each case, that the corresponding transcripts are made in the rd/rd mouse retina and that there are no overt differences in their size compared to the transcripts hybridized in the rd/+ or +/+ mouse retinas. Although a defect in the phosphorylation of opsin has been described in rd mice, no difference was found in the transcripts hybridized by a cDNA probe corresponding to the region of the opsin molecule on which phosphorylation occurs. We do find, however, that labeled bovine-derived opsin cDNA recognizes five different RNA size classes in mouse and bovine retinas. Control experiments were performed to confirm that the RNA hybridized by opsin cDNA was not due to non-specific hybridization to unrelated RNAs.

Localization of mRNAs encoding the alpha-subunits of signal-transducing G-proteins within rat brain and among peripheral tissues

The sequence of the mRNAs which encode the alpha-subunits of the signal-transducing G-proteins Gs, Go and two forms of Gi (termed Gi1 and Gi2) have recently been reported. Based on rat sequences we prepared oligodeoxynucleotide probes for measurement of these mRNAs in rat brain and peripheral tissues. The relative abundance of these mRNA species in brain was Gs greater than Go approximately Gi2 greater than Gi1. The Gs and Gi2 mRNAs had somewhat lower levels in heart, kidney and liver than in brain, and Go and Gi1 mRNAs were not detected in the peripheral tissues. Using in situ hybridization we localized each of these mRNAs within slices of the rat brain. The patterns of distribution of Gs and Gi2 mRNA were very similar, but very different from that of Go and Gi1 mRNA. These data illustrate that receptor-effector coupling G-proteins are regionally specialized in their expression. This regional specialization may reflect a selective coupling of individual G-proteins with the various neurotransmitter receptors and effector pathways.

GPA1, a haploid-specific essential gene, encodes a yeast homolog of mammalian G protein which may be involved in mating factor signal transduction

GPA1 protein of Saccharomyces cerevisiae is homologous to the alpha subunit of mammalian G protein. GPA1 transcript was found in haploid cells but was not detected in diploid cells. Disruption of GPA1 resulted in a haploid-specific lethal phenotype, indicating that GPA1 is a haploid-specific essential gene for cell growth. Upon regulation of expression of GPA1 by the galactose-inducible GAL1 promoter, the loss of GPA1 function was found to lead to cell-cycle arrest at the late G1 phase. Mutants that suppress the lethality of the gpa1::HIS3 mutation showed a sterile phenotype that was not cell-type-specific. These results suggest that GPA1 protein may control the signal for mating-factor-mediated cell-cycle arrest.

The yeast SCG1 gene: a G alpha-like protein implicated in the a- and alpha-factor response pathway

We have identified the SCG1 gene by its ability to suppress the pheromone-supersensitive sst2-1 mutation. The nucleotide sequence of SCG1 suggests that it encodes a 54 kd protein homologous to the alpha subunit of the vertebrate G proteins transducin, Gs, Gi, and Go. SCG1 expression and function are haploid-specific; haploid scg1 cells grow into very small colonies consisting of large, abnormally shaped cells, whereas a/alpha scg1/scg1 diploids show wild-type morphology, growth, and sporulation. We postulate that the SCG1 product is involved in the pheromone response pathway, and propose two models for the function of the SCG1 product. Expression of the rat alpha s gene in yeast partially complements both the sst2 and scg1 defects, indicating a high level of conservation of sequence and function between SCG1 and mammalian G alpha subunits.

Fatty acylation is important but not essential for Saccharomyces cerevisiae RAS function

Two proteins in the yeast Saccharomyces cerevisiae that are encoded by the genes RAS1 and RAS2 are structurally and functionally homologous to proteins of the mammalian ras oncogene family. We examined the role of fatty acylation in the maturation of yeast RAS2 protein by creating mutants in the putative palmitate addition site located at the carboxyl terminus of the protein. Two mutations, Cys-318 to an opal termination codon and Cys-319 to Ser-319, were created in vitro and substituted in the chromosome in place of the normal RAS2 allele. These changes resulted in a failure of RAS2 protein to be acylated with palmitate and a failure of RAS2 protein to be localized to a membrane fraction. The mutations yielded a Ras2- phenotype with respect to the ability of the resultant mutants to grow on nonfermentable carbon sources and to complement ras1- mutants. However, overexpression of the ras2Ser-319 product yielded a Ras+ phenotype without a corresponding association of the mutant protein with the membrane fraction. We conclude that the presence of a fatty acyl moiety is important for localizing RAS2 protein to the membrane where it is active but that the fatty acyl group is not an absolute requirement of RAS2 protein function.

S-antigen: characterization of a pathogenic epitope which mediates experimental autoimmune uveitis and pinealitis in Lewis rats

S-antigen (48K protein) is a photoreceptor cell protein highly pathogenic for the induction of experimental autoimmune uveitis (EAU) and intimately involved in the visual process. EAU is characterized, in part, as a T-cell mediated autoimmune disease which results in a severe inflammation of the uveal tract, and pineal gland. In order to determine specific sites in S-antigen responsible for its pathogenicity we synthesized twenty-three peptides, corresponding to the entire 404 amino acid sequence, and tested each peptide for its ability to induce EAU in Lewis rats. One peptide, peptide M (18 amino acids in length), was found to be highly pathogenic and consistently induced an EAU that was identical to the disease caused by native S-antigen. Clinically, the disease that developed in the eye was characterized by iris and pericorneal hyperemia, followed by inflammatory exudates in the anterior and vitreous chambers. Histopathologically a severe inflammatory response was observed which resulted in the complete destruction of the photoreceptor cell layer of the retina. In order to more fully characterize this pathogenic site, 14 additional smaller peptides (eight to eighteen amino acids in length) corresponding to the left and right portions of peptide M were synthesized. Of these peptides, peptide M16L, M15L, and M12L induced EAU, further localizing this pathogenic site to a small well-characterized region of S-antigen consisting of twelve amino acids. In addition, animals with ocular inflammatory disease had an associated pinealitis characterized by a lymphocytic infiltration of the subcapsular and central area of the pineal gland. The significance of these findings and the relationship of S-antigen in the pathogenesis of EAU and other autoimmune diseases is discussed.

Hydroxylamine-stable covalent linkage of myristic acid in G0 alpha, a guanine nucleotide-binding protein of bovine brain

G0 alpha, a guanine nucleotide-binding protein with a strong homology to the G1 alpha and Gs alpha regulatory proteins of adenylate cyclase, is shown to contain myristic acid. The attachment of myristate to the protein is stable to hydroxylamine treatment, and since the amino-terminal sequence of G0 alpha is typical of proteins with amino-terminal myristate, the inference is strong that G0 alpha is also myristylated at its amino-terminal glycine.

A structural model for the alpha-subunit of transducin. Implications of its role as a molecular switch in the visual signal transduction mechanism

Transducin is a GTP-binding protein which mediates the light activation signal from photolyzed rhodopsin to cGMP phosphodiesterase and is pivotal in the visual excitation process. Biochemical studies suggest that the T alpha subunit of transducin is composed of three functional domains, one for rhodopsin/T beta gamma interaction, another for guanine nucleotide binding, and a third for the activation of phosphodiesterase. The integration of the primary sequence of T alpha along with secondary structure, hydropathy and folding topology predictions, and a comparison with homologous proteins have led to the construction of a three-dimensional model of the T alpha subunit. A molecular mechanism which underlies the coupling action of T alpha is suggested on the basis of this model.

Heterologous expression and characterization of the human R-ras gene product

We directly expressed human R-ras 23,000-dalton protein (p23) cDNA in Escherichia coli under the control of the trp promoter. GTP-dependent phosphorylation of a p23 threonine 85 substitution mutant was observed. This result is in direct analogy to the autokinase activity of H-ras and K-ras threonine 59 substitution mutants. Normal p23 protein was detected in the human fibrosarcoma cell line HT1080 by immunoprecipitation with rabbit antibodies raised against an E. coli-expressed R-ras fusion protein. The R-ras p23 protein was found to be 3H labeled in the presence of [9,10(n)-3H]palmitic acid and is associated with the P100 membrane fraction of HT1080 cells. These data suggest that human R-ras p23 has biochemical properties very similar to those of the p21 products of the H-, K-, and N-ras proto-oncogenes. We constructed an R-ras minigene and engineered the expression of normal and mutant alleles from the simian virus 40 early region promoter. Normal and mutant R-ras gene products were authenticated by transient expression in COS-7 cells and immunoprecipitation. The valine 38-substituted R-ras p23 displayed reduced electrophoretic mobility. R-ras p21-like proteins, made by eliminating the first 26 R-ras codons, displayed evident mobility differences between the pro form and mature form, along with a valine 12 substitution-dependent change in electrophoretic mobility. Rat-1 fibroblasts were transfected with normal and mutant R-ras alleles and normal and activated H-ras alleles. Unlike the human T24 bladder oncogene-encoded p21, mutant R-ras alleles do not cause monolayer focus formation or growth in soft agar of rat fibroblasts.

Human cDNA clones for an alpha subunit of Gi signal-transduction protein

Two cDNA clones were obtained from a lambda gt11 cDNA human brain library that correspond to alpha i subunits of G signal-transduction proteins (where alpha i subunits refer to the alpha subunits of G proteins that inhibit adenylate cyclase). The nucleotide sequence of human brain alpha i is highly homologous to that of bovine brain alpha i [Nukada, T., Tanabe, T., Takahashi, H., Noda, M., Haga, K., Haga, T., Ichiyama, A., Kangawa, K., Hiranaga, M., Matsuo, H. & Numa, S. (1986) FEBS Lett. 197, 305-310] and the predicted amino acid sequences are identical. However, human and bovine brain alpha i cDNAs differ significantly from alpha i cDNAs from human monocytes, rat glioma, and mouse macrophages in amino acid (88% homology) and nucleotide (71-75% homology) sequences. In addition, the nucleotide sequences of the 3' untranslated regions of human and bovine brain alpha i cDNAs differ markedly from the sequences of human monocyte, rat glioma, and mouse macrophage alpha i cDNAs. These results suggest there are at least two classes of alpha i mRNA.

Regulatory GTP-binding proteins: emerging concepts on their role in cell function

The last few years have evidenced a tremendous expansion in our appreciation of the role of regulatory GTP-binding proteins in cellular activation. The availability of cholera and pertussis toxins to detect G proteins as well as methodological advances in the study of cellular function has afforded the opportunity to examine G protein participation in many cellular events. Regulation of adenylyl cyclase and cyclic GMP phosphodiesterase by G proteins has been demonstrated. Phosphatidylinositol-4,5-biphosphate specific phospholipase C activity appears to be subject to G protein control. G proteins regulate inward K+ and Ca2+ channels through a mechanism which may be independent of effects on the above mentioned enzymes. Certainly, the number of G proteins which have been identified from sequencing of complementary DNA affords the potential for G protein involvement in many cellular events. Only three G proteins have however been isolated and functionally characterized, Gs, Gi and transducin. Whether all the functions of these proteins have been identified remains to be seen.

Molecular cloning of a new human G protein. Evidence for two Gi alpha-like protein families

The amino acid sequence of a novel G protein alpha subunit (Gx alpha) has been deduced from the nucleotide sequence of a human cDNA clone isolated from a differentiated HL-60 cDNA library. The cDNA encodes a polypeptide of 354 amino acids (Mr 40,519) which is closely related to Gi alpha proteins. The amino acid sequence homology between Gx alpha and human myeloid Gi alpha is 86% with 15 nonconservative substitutions. Gx alpha also shares 86% homology with both rat brain and mouse macrophage Gi alpha but is more homologous (94%) to bovine brain Gi alpha with only 5 nonconservative amino acid differences. G proteins previously termed Gi alpha may fall into at least two distinct groups, with one including human myeloid Gi alpha, rat brain Gi alpha and mouse macrophage Gi alpha; and other Gx alpha and bovine brain Gi alpha. One group probably contains true Gi and the other a new class of G protein whose function remains to be determined.

Fatty acylation is important but not essential for Saccharomyces cerevisiae RAS function

Two proteins in the yeast Saccharomyces cerevisiae that are encoded by the genes RAS1 and RAS2 are structurally and functionally homologous to proteins of the mammalian ras oncogene family. We examined the role of fatty acylation in the maturation of yeast RAS2 protein by creating mutants in the putative palmitate addition site located at the carboxyl terminus of the protein. Two mutations, Cys-318 to an opal termination codon and Cys-319 to Ser-319, were created in vitro and substituted in the chromosome in place of the normal RAS2 allele. These changes resulted in a failure of RAS2 protein to be acylated with palmitate and a failure of RAS2 protein to be localized to a membrane fraction. The mutations yielded a Ras2- phenotype with respect to the ability of the resultant mutants to grow on nonfermentable carbon sources and to complement ras1- mutants. However, overexpression of the ras2Ser-319 product yielded a Ras+ phenotype without a corresponding association of the mutant protein with the membrane fraction. We conclude that the presence of a fatty acyl moiety is important for localizing RAS2 protein to the membrane where it is active but that the fatty acyl group is not an absolute requirement of RAS2 protein function.

Cell type-specific expressions of c-ras gene products in the normal rat

Expression of proteins encoded by the ras proto-oncogenes was examined immunohistochemically in formalin-fixed, paraffin-embedded tissues of the normal rat using anti-ras p21 antibodies generated against synthetic peptides. Cell type specific expressions of ras gene products were detected in distal tubules of kidney, megakaryocytes in spleen, neural cells in cerebrum, Purkinje cells in cerebellum, cells lining the pulmonary alveoli and cells in the epithelium of intestinal villi. Region specific expressions of the ras proteins were observed in spleen and thymus, where the ras proteins were detected in splenic nodules including germinal centers and thymic medulla, respectively. These findings suggest that the c-ras gene products in normal rat organs are expressed in specific cell-types within a tissue and may be associated with degree of cellular differentiation.

Deduced amino acid sequence of bovine retinal Go alpha: similarities to other guanine nucleotide-binding proteins

A bovine retinal cDNA clone encoding the complete sequence (354 amino acids) of Go alpha, a guanine nucleotide-binding protein (G protein), was isolated by using oligonucleotide probes complementary to published sequences in two putative clones for the alpha subunit of bovine transducin (T alpha). The deduced amino acid sequence contained sequences identical to those in seven tryptic peptides (total 63 amino acids) from bovine brain Go alpha. The cDNA for bovine retinal Go alpha exhibits greater than 90% identity in both coding and 3' untranslated regions with a recently described partial cDNA clone for Go alpha from rat brain [Itoh, H., Kozasa, T., Nagata, S., Nakamura, S., Katada, T., Ui, M., Iwai, S., Ohtsuka, E., Kawasaki, H., Suzuki, K. & Kaziro, Y. (1986) Proc. Natl. Acad. Sci. USA 83, 3776-3780]. Comparison of the nucleotide and deduced amino acid sequences of the bovine Go alpha clone with those previously reported for other G proteins of bovine origin (Gs alpha, Gi alpha, and T alpha) reveals extensive regions identical to those surrounding the amino acids modified by cholera toxin and pertussis toxin. There are also marked similarities of sequence in regions of the G proteins, elongation factors, and the ras p21 gene products that are believed to be involved in guanine nucleotide binding and GTP hydrolysis.

GTP binding proteins: a key role in cellular communication

One of the major steps in the understanding of the hormonal and sensory transduction mechanisms in eukaryotic cells has been the discovery of a family of GTP binding proteins which couple receptors to specific cellular effectors. The absolute requirement of GTP for hormonal stimulation of adenylate cyclase was the initial observation which led to the purification of the protein involved: Gs. Gs couples stimulatory receptors to adenylate cyclase. It is a heterotrimer composed of an alpha chain (45 or 52 kDa), a beta chain (35-36 kDa) and a gamma chain (8 kDa). Several other G proteins of known functions have been purified: Gi, which couples inhibitory receptors to adenylate cyclase, and transducin which couples photoexcited rhodopsin to cyclic GMP phosphodiesterase. Some G proteins of uncertain function have also been purified: Go, a G protein mainly localized in nervous tissues and Gp, a G protein isolated from placenta and platelets. All these G proteins have a common design. Like Gs they all consist of 3 chains: alpha, beta and gamma. The beta chains are nearly identical, whereas the gamma chains are more variable. The alpha chains are different, but share common domains (especially at the level of the GTP binding site). These domains of homologies are also similar to those of other GTP binding proteins, such as the product of the ras gene (p21) and the initiation or elongation factors. alpha Chains are also ADP ribosylated by bacterial toxins. Gs and transducin are targets for cholera toxin, whereas Gi, Go and transducin are targets for pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Occurrence in Saccharomyces cerevisiae of a gene homologous to the cDNA coding for the alpha subunit of mammalian G proteins

From cross-hybridization studies with cDNAs that code for the alpha subunits of rat brain guanine nucleotide-binding regulatory (G) proteins, we have isolated a gene from yeast Saccharomyces cerevisiae encoding an amino acid sequence that is highly homologous to the alpha subunit of the G protein that mediates inhibition of adenylate cyclase (Gi alpha) from rat brain. The gene, tentatively designated as GPA1, contains a contiguous, single open reading frame of 1416 nucleotides that codes for a protein of 472 amino acids with a calculated Mr of 54,075. The predicted amino acid sequence of the protein encoded by the GPA1 gene (tentatively designated as G protein 1 alpha or GP1 alpha) is remarkably homologous to the amino acid sequence of rat brain Gi alpha and the alpha subunit of the G protein of unknown function (Go alpha); the primary structure of the sites for GTP hydrolysis as well as GTP interaction are nearly identical. The main difference in the molecular sizes of yeast GP1 alpha (472 amino acids) and rat brain Gi alpha (355 amino acids) is due to the presence of a stretch of 110 extra amino acid residues in yeast GP1 alpha, which are inserted near the NH2-terminal one-third of mammalian Gi alpha. From blot-hybridization analysis, the size of the GP1 alpha mRNA was estimated as 1.7 kilobases.

Pancreatic neoplasia induced by ras expression in acinar cells of transgenic mice

Expression of an activated human c-H-ras oncogene under control of rat elastase I regulating elements leads to neoplasia of the fetal exocrine pancreas. In most transgenic mice bearing this gene construct, massive tumors involving all the pancreatic acinar cells develop within a few days of pancreatic differentiation. Expression of the normal c-H-ras proto-oncogene in acinar cells leads to more subtle anomalies, but no tumors develop. Thus modest amounts of the mutant ras proteins are sufficient, in an otherwise normal genetic background, to lead to neoplastic transformation of differentiating pancreatic acinar cells. In contrast, a comparable elastase-myc construct produces no pancreatic tumors in transgenic mice.

Immunoelectron microscopic localization of photoreceptor-specific markers in the monkey retina

Antibodies for several molecules that function in the visual process were used to localize these molecules in primate rod and cone cells. These antibodies (monoclonal or polyclonal) were prepared against Interphotoreceptor Retinoid-binding Protein (IRBP), S-antigen (S-Ag), opsin, alpha-transducin and also against cyclic GMP (cGMP). Lowicryl-embedded tissues were labeled with secondary antibodies linked to colloidal gold. Although IRBP is predominantly an extracellular protein, the relatively small amount found intracellularly was localized mainly in rods, with little in cones. Opsin, S-Ag and cGMP were found mainly in rod cell outer segments. A polyclonal antiserum raised against transducin-alpha purified from rod outer segments predominantly labeled rod cells, but an antiserum against the carboxyterminal decapeptide of transducin-alpha labeled both rod and cone cells. Thus, most of these specialized molecules are present predominantly in rod cells, confirming major differences in components of the visual cycle in rods and cones.

Mutational analysis of a ras catalytic domain

We used linker insertion-deletion mutagenesis to study the catalytic domain of the Harvey murine sarcoma virus v-rasH transforming protein, which is closely related to the cellular rasH protein. The mutants displayed a wide range of in vitro biological activity, from those that induced focal transformation of NIH 3T3 cells with approximately the same efficiency as the wild-type v-rasH gene to those that failed to induce any detectable morphologic changes. Correlation of transforming activity with the location of the mutations enabled us to identify three nonoverlapping segments within the catalytic domain that were dispensable for transformation and six other segments that were required for transformation. Segments that were necessary for guanosine nucleotide (GDP) binding corresponded to three of the segments that were essential for transformation; two of the three segments share strong sequence homology with other purine nucleotide-binding proteins. Loss of GDP binding was associated with apparent instability of the protein. Lesions in two of the three other required regions significantly reduced GDP binding, while small lesions in the last required region did not impair GDP binding or membrane localization. We speculate that this latter region interacts with the putative cellular target of ras. The results suggest that transforming ras proteins require membrane localization, guanosine nucleotide binding, and an additional undefined function that may represent interaction with their target.

Structure of the human and murine R-ras genes, novel genes closely related to ras proto-oncogenes

The human R-ras gene was isolated by low-stringency hybridization with a v-H-ras probe. The predicted 218 amino acid R-ras protein has an amino-terminal extension of 26 residues compared with H-ras p21, and shows 55% amino acid identity; conserved domains include the p21 GTP-binding site and the carboxy-terminal membrane localization sequence. R-ras has at least six exons, with the position of the first intron conserved relative to the Drosophila ras64B and Dictyostelium ras genes; there is no similarity in the exon-intron structure of the R-ras gene and of the mammalian H-, K-, and N-ras proto-oncogenes. Cloned mouse R-ras cDNAs exhibit 88% nucleotide and 94.5% predicted amino acid identity to human R-ras. Human R-ras was localized to chromosome 19, a site different from ras p21 genes. Mouse R-ras is syntenic with c-H-ras on chromosome 7.

Direct identification of palmitic acid as the lipid attached to p21ras

p21v-H-ras, the transforming protein of Harvey murine sarcoma virus, contains a covalently attached lipid. Using thin-layer chromatography, we identified the acyl group as the 16-carbon saturated fatty acid palmitic acid. No myristic acid was detected in fatty acids released from in vivo-labeled p21v-H-ras. The p21v-K-ras protein encoded by Kirsten sarcoma virus was also palmitylated. The processing and acylation of p21v-K-ras however differed from that of p21v-H-ras. Three forms of [3H]palmitic acid-labeled p21ras proteins were detected in Kirsten sarcoma virus-transformed cells. This contrasted with Harvey sarcoma virus, in which two forms of p21v-H-ras contained palmitic acid. Analysis by partial proteolysis of p21v-H-ras labeled with [3H]palmitic acid suggested that all of the lipid found in intact p21v-H-ras was located in the C-terminal region. On sodium dodecyl sulfate-polyacrylamide gels, p21v-H-ras labeled with [3H]palmitic acid migrated slightly ahead of the majority of p21v-H-ras. Of the mature forms of p21v-H-ras, apparently only a subpopulation contains palmitic acid.

Analysis of the essential and excision repair functions of the RAD3 gene of Saccharomyces cerevisiae by mutagenesis

The RAD3 gene of Saccharomyces cerevisiae, which is involved in excision repair of DNA and is essential for cell viability, was mutagenized by site-specific and random mutagenesis. Site-specific mutagenesis was targeted to two regions near the 5' and 3' ends of the coding region, selected on the basis of amino acid sequence homology with known nucleotide binding and with known specific DNA-binding proteins, respectively. Two mutations in the putative nucleotide-binding region and one in the putative DNA-binding region inactivate the excision repair function of the gene, but not the essential function. A gene encoding two tandem mutations in the putative DNA-binding region is defective in both excision repair and essential functions of RAD3. Seven plasmids were isolated following random mutagenesis with hydroxylamine. Mutations in six of these plasmids were identified by gap repair of mutant plasmids from the chromosome of strains with previously mapped rad3 mutations, followed by DNA sequencing. Three of these contain missense mutations which inactivate only the excision repair function. The other three carry nonsense mutations which inactivate both the excision repair and essential functions. Collectively our results indicate that the RAD3 excision repair function is more sensitive to inactivation than is the essential function. Overexpression of wild-type Rad3 protein and a number of rad3 mutant proteins did not affect the UV resistance of wild-type yeast cells. However, overexpression of Rad3-2 protein rendered wild-type cells partially UV sensitive, indicating that excess Rad3-2 protein is dominant to the wild-type form. These and other results suggest that Rad3-2 protein retains its affinity for damaged DNA or other substrates, but is not catalytically active in excision repair.

Genes for two homologous G-protein alpha subunits map to different human chromosomes

Signal transduction across biological membranes is modulated by a family of related GTP-binding proteins termed G proteins. These G proteins have a heterotrimeric structure composed of alpha, beta, and gamma subunits. The alpha subunits of the G proteins bind GTP and appear to determine the biochemical specificity of the protein. We have recently cloned and characterized cDNA encoding two G-protein alpha subunits, alpha i and alpha h. The former is a substrate for ADP-ribosylation by pertussis toxin. The protein corresponding to alpha h has not yet been identified. These cDNAs encode proteins, which demonstrate 90% sequence identity to one another and also show marked similarity to other G proteins. The present studies were designed to determine whether the genes for these related proteins are clustered on a single human chromosome. Genomic DNA isolated from a panel of mouse-human hybrid cell lines was analyzed by hybridization to cDNAs for alpha i and alpha h. Based on the distribution patterns of alpha i and alpha h in cell hybrids, the gene for alpha i was assigned to human chromosome 7, and the gene for alpha h assigned to chromosome 12. These data suggest that the G-protein gene family may be distributed over at least two human chromosomes.