Genetic identification of residues involved in association of alpha and beta G-protein subunits

The beta subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the gamma subunit

The Kluyveromyces lactis heterotrimeric G protein is a canonical Galphabetagamma complex; however, in contrast to Saccharomyces cerevisiae, where the Ggamma subunit is essential for mating, disruption of the KlGgamma gene yielded cells with almost intact mating capacity. Expression of a nonfarnesylated Ggamma, which behaves as a dominant-negative in S. cerevisiae, did not affect mating in wild-type and DeltaGgamma cells of K. lactis. In contrast to the moderate sterility shown by the single DeltaKlGalpha, the double DeltaKlGalpha DeltaKlGgamma mutant displayed full sterility. A partial sterile phenotype of the DeltaKlGgamma mutant was obtained in conditions where the KlGbeta subunit interacted defectively with the Galpha subunit. The addition of a CCAAX motif to the C-end of KlGbeta, partially suppressed the lack of both KlGalpha and KlGgamma subunits. In cells lacking KlGgamma, the KlGbeta subunit cofractionated with KlGalpha in the plasma membrane, but in the DeltaKlGalpha DeltaKlGgamma strain was located in the cytosol. When the KlGbeta-KlGalpha interaction was affected in the DeltaKlGgamma mutant, most KlGbeta fractionated to the cytosol. In contrast to the generic model of G-protein function, the Gbeta subunit of K. lactis has the capacity to attach to the membrane and to activate mating effectors in absence of the Ggamma subunit.

Distinct roles for two Galpha-Gbeta interfaces in cell polarity control by a yeast heterotrimeric G protein

Saccharomyces cerevisiae mating pheromones trigger dissociation of a heterotrimeric G protein (Galphabetagamma) into Galpha-guanosine triphosphate (GTP) and Gbetagamma. The Gbetagamma dimer regulates both mitogen-activated protein (MAP) kinase cascade signaling and cell polarization. Here, by independently activating the MAP kinase pathway, we studied the polarity role of Gbetagamma in isolation from its signaling role. MAP kinase signaling alone could induce cell asymmetry but not directional growth. Surprisingly, active Gbetagamma, either alone or with Galpha-GTP, could not organize a persistent polarization axis. Instead, following pheromone gradients (chemotropism) or directional growth without pheromone gradients (de novo polarization) required an intact receptor-Galphabetagamma module and GTP hydrolysis by Galpha. Our results indicate that chemoattractant-induced cell polarization requires continuous receptor-Galphabetagamma communication but not modulation of MAP kinase signaling. To explore regulation of Gbetagamma by Galpha, we mutated Gbeta residues in two structurally distinct Galpha-Gbeta binding interfaces. Polarity control was disrupted only by mutations in the N-terminal interface, and not the Switch interface. Incorporation of these mutations into a Gbeta-Galpha fusion protein, which enforces subunit proximity, revealed that Switch interface dissociation regulates signaling, whereas the N-terminal interface may govern receptor-Galphabetagamma coupling. These findings raise the possibility that the Galphabetagamma heterotrimer can function in a partially dissociated state, tethered by the N-terminal interface.

Scanning mutagenesis of regions in the Galpha protein Gpa1 that are predicted to interact with yeast mating pheromone receptors

The mechanism by which receptors activate heterotrimeric G proteins was examined by scanning mutagenesis of the Saccharomyces cerevisiae pheromone-responsive Galpha protein (Gpa1). The juxtaposition of high-resolution structures for rhodopsin and its cognate G protein transducin predicted that at least six regions of Galpha are in close proximity to the receptor. Mutagenesis was targeted to residues in these domains in Gpa1, which included four loop regions (beta2-beta3, alpha2-beta4, alpha3-beta5, and alpha4-beta6) as well as the N and C termini. The mutants displayed a range of phenotypes from nonsignaling to constitutive activation of the pheromone pathway. The constitutive activity of some mutants could be explained by decreased production of Gpa1, which permits unregulated signaling by Gbetagamma. However, the constitutive activity caused by the F344C and E335C mutations in the alpha2-beta4 loop and F378C in the alpha3-beta5 loop was not due to decreased protein levels, and was apparently due to defects in sequestering Gbetagamma. The strongest loss of the function mutant, which was not detectably induced by a pheromone, was caused by a K314C substitution in the beta2-beta3 loop. Several other mutations caused weak signaling phenotypes. Altogether, these results suggest that residues in different interface regions of Galpha contribute to activation of signaling.

The Gbeta(KlSte4p) subunit of the heterotrimeric G protein has a positive and essential role in the induction of mating in the yeast Kluyveromyces lactis

In the yeast Saccharomyces cerevisiae the Gbetagamma dimer of the heterotrimeric G protein transduces a pheromone signal from serpentine receptor to a MAP kinase cascade that activates the mating response pathway. Haploid cells lacking the Gbeta subunit do not respond to sexual pheromone, leading to sterility. In this work we demonstrate that the beta-subunit of Kluyveromyces lactis, encoded by the KlSTE4 gene, is a component of the G protein, and that its disruption gives rise to sterile cells. However, unlike Ste4p in S. cerevisiae, its overexpression does not induce growth arrest or promote mating. It has been shown that in K. lactis, the Galpha subunit has a positive role in the mating process, hence the resulting double GalphaDelta GbetaDelta mutant was viable and sterile. Here we show that the overproduction of Gbeta subunit fails to rescue GalphaDelta mutant from sterility and that expression of a constitutive active allele of Galpha enhances transcription of the KlSTE4 gene. The mating pathway triggered by the Gbeta-subunit requires a functional KlSte12p transcription factor. Gbeta has a 10-fold higher association rate with the Galpha1 subunit involved in pheromone response than with Galpha2, the protein involved in cAMP regulation in K. lactis. Additionally, the Gbeta-subunit from K. lactis is able to interact with the Galpha-subunit from S. cerevisiae but fails to restore the mating deficiency of Scste4Delta mutant. The data presented indicate that the mating pathway of K. lactis is positively and cooperatively regulated by both the Galpha and the Gbeta subunits.

The G-beta subunit MGB1 is involved in regulating multiple steps of infection-related morphogenesis in Magnaporthe grisea

Trimeric G-proteins transmit extracellular signals to various downstream effectors (e.g. MAP kinases) in eukaryotes. In the rice blast fungus Magnaporthe grisea, the Pmk1 MAP kinase is essential for appressorium formation and infectious growth. The pmk1 deletion mutant fails to form appressoria but still responds to exogenous cAMP for tip deformation. Since gene disruption mutants of three Galpha subunits still form appressoria and are phenotypically different from pmk1 mutants, it is likely that the Pmk1 pathway is activated by Gbeta in M. grisea. In this study, we isolated and characterized the MGB1 gene that encodes the G subunit in M. grisea. Mutants disrupted in MGB1 were reduced in conidiation. Conidia from mgb1 mutants were defective in appressorium formation and failed to penetrate or grow invasively on rice leaves. Exogenous cAMP induced appressorium formation in mgb1 mutants, but these appressoria were abnormal in shape and could not penetrate. The intracellular cAMP level was reduced in mgb1 mutants and the defects in conidiation and hyphal growth were partially suppressed with 1 mM cAMP. Transformants expressing multiple copies of MGB1 were able to form appressoria on hydrophilic surfaces. Our results suggest that MGB1 may be involved in the cAMP signalling for regulating conidiation, surface recognition and appressorium formation. The Pmk1 pathway may be the downstream target of MGB1 for regulating penetration and infectious hyphae growth in M. grisea.

NudE-L, a novel Lis1-interacting protein, belongs to a family of vertebrate coiled-coil proteins

The LIS1-encoded protein (Lis1) plays a role in brain development because a hemizygous deletion or mutation of the human gene causes neuronal migration disorders, such as Miller-Dieker syndrome (MDS) or isolated lissencephaly sequence (ILS). Using a yeast two-hybrid screen, we have isolated a novel protein that interacts with mouse Lis1 (mLis1) which is termed mouse NudE-like protein (mNudE-L) because of its 49% amino acid conservation with NudE, a protein involved in nuclear migration in Aspergillus nidulans. GST pull-down assays and co-immunoprecipitation of fusion proteins expressed in mammalian cells confirmed the interaction of mLis1 and mNudE-L. mNudE-L gives rise to a approximately 2.3 kb mRNA and encodes an ORF corresponding to approximately 38 kDa protein. The overall amino acid sequence of mNudE-L is 49-95% identical to proteins found in a variety of organisms, thus establishing mNudE-L as a new member of a protein family. The hallmark of this family is an N-terminal region predicted to form a coiled-coil domain. We show that mNudE-L and mLis1 are coexpressed in the postnatal and adult cerebral cortices and in the Purkinje neurons of the cerebellum. In contrast to mLis1, mNudE-L transcripts are absent in the mitral cell layer of the olfactory bulb and in the inward migrating granular neurons of the developing cerebellum. Mutant mLis1 proteins modelling mutations found in human lissencephaly patients fail to interact with mNudE-L, raising the possibility that phenotypic changes result, in part, from the inability of mutant Lis1 proteins to interact with the human NudE-L polypeptide.

Map kinases in fungal pathogens

MAP kinases in eukaryotic cells are well known for transducing a variety of extracellular signals to regulate cell growth and differentiation. Recently, MAP kinases homologous to the yeast Fus3/Kss1 MAP kinases have been identified in several fungal pathogens and found to be important for appressorium formation, invasive hyphal growth, and fungal pathogenesis. This MAP kinase pathway also controls diverse growth or differentiation processes, including conidiation, conidial germination, and female fertility. MAP kinases homologous to yeast Slt2 and Hog1 have also been characterized in Candida albicans and Magnaporthe grisea. Mutants disrupted of the Slt2 homologues have weak cell walls, altered hyphal growth, and reduced virulence. The Hog1 homologues are dispensable for growth but are essential for regulating responses to hyperosmotic stress in C. albicans and M. grisea. Overall, recent studies have indicated that MAP kinase pathways may play important roles in regulating growth, differentiation, survival, and pathogenesis in fungal pathogens.

Lissencephaly associated mutations suggest a requirement for the PAFAH1B heterotrimeric complex in brain development

Human brain malformations, such as Miller-Dieker syndrome (MDS) or isolated lissencephaly sequence (ILS) may result from abnormal neuronal migration during brain development. MDS and ILS patients have a hemizygous deletion or mutation in the LIS1 gene (PAFAH1B1), therefore, the LIS1 encoded protein (Lis1) may play a role in neuronal migration. Lis1 is a subunit of a brain platelet-activating factor acetylhydrolase (PAFAH1B) where it forms a heterotrimeric complex with two hydrolase subunits, referred to as 29 kDa (PAFAH1B3) and 30 kDa (PAFAH1B2). In order to determine whether this heterotrimer is required for the developmental functions of PAFAH1B, we examined the binding properties of 29 and 30 kDa subunits to mutant Lis1 proteins. The results defined the critical regions of Lis1 for PAFAH1B complex formation and demonstrated that all human LIS1 mutations examined resulted in abolished or reduced capacity of Lis1 to interact with the 29 and 30 kDa subunits, suggesting that the PAFAH1B complex participates in the process of neuronal migration.

The Leu-132 of the Ste4(Gbeta) subunit is essential for proper coupling of the G protein with the Ste2 alpha factor receptor during the mating pheromone response in yeast

In order to identify amino acid residues of Ste4p involved in receptor recognition and/or receptor-G protein coupling, we employed random in vitro mutagenesis and a genetic screening to isolate mutant Ste4p subunits with altered pheromone response. We generated a plasmid library containing randomly mutagenized Ste4 ORFs, followed by phenotypic selection of ste4p mutants by altered alpha pheromone response in yeast cells. Subsequently, we analyzed mutant ste4-10 which has a replacement of the almost universally conserved leucine 132 by phenylalanine. This residue lies in the first blade of the beta propeller structure proposed by crystallographic analysis. By overexpression experiments we found that mutant ste4p subunit triggers the mating pathway at wild type levels in both wild type and receptorless strains. When expressed in a ste4 background, however, the mutant G protein is activated inefficiently by mating pheromone in both a and alpha cells. The mutant ste4-10p was tested in the two-hybrid system and found to be defective in its interaction with the Gpa1p, but has a normal association with the C-termini end of the Ste2p receptor. These observations strongly suggest that the Leu-132 of the Ste4p subunit is essential for efficient activation of the G protein by the pheromone-stimulated receptor and that this domain could be an important point for physical interaction between the Gbeta and the Galpha subunits.

SWiP-1: novel SOCS box containing WD-protein regulated by signalling centres and by Shh during development

We describe a novel chick WD-protein, cSWiP-1, expressed in somitic mesoderm and developing limb buds as well as in other embryonic structures where Hedgehog signalling has been shown to play a role. Using embryonic manipulations we show that in somites cSWiP-1 expression integrates two signals originating from structures adjacent to the segmental mesoderm: a positive signal from the notochord and a negative signal from intermediate and/or lateral mesoderm. In explant cultures of somitic mesoderm, Shh protein induces cSWiP-1, while a blocking antibody to Shh inhibits the induction of cSWiP-1 by the notochord. These results show that the positive signal from the notochord is mediated by Shh. We also show that in limb buds cSWiP-1 is upregulated by ectopic Shh. This occurs in about the same time period as upregulation of BMP2, placing cSWiP-1 among the earliest markers for the change of limb pattern caused by ectopic Shh. We also describe a human homologue of cSWiP-1 and a mouse gene, mSWiP-2, that is more distantly related to SWiP-1, suggesting that SWiP-1 belongs to a novel subfamily of WD-proteins.

Sequence elements that contribute to the degradation of yeast G alpha

BACKGROUND

Gpa1 is the alpha subunit of the yeast G-protein that regulates signal transduction during mating. The stability of Galpha/Gpa1 is influenced by the ubiquitin-dependent N-end rule pathway, suggesting that the regulation of G alpha levels may be important for effective mating response and recovery.

RESULTS

The G alpha sequences that confer sensitivity to degradation by the N-end rule pathway were identified. The insertion of this degradation signal (G1-Deg) into the ordinarily stable Gpa2 protein conferred proteolytic targeting. We examined G alpha degradation under different conditions and found that it was efficiently degraded in haploid and diploid cells, but was stable if it was synthesized prior to expression of the N-end rule pathway. Interestingly, a specific mutation in G alpha that is believed to promote the GTP-bound form (N388K) caused accelerated degradation.

CONCLUSION

A region encompassing a putative effector-binding domain (G1-Deg) is required for G alpha degradation via the N-end rule pathway. Our studies have shown that G alpha is susceptible to proteolysis soon after synthesis. These results are in agreement with the idea that G alpha is more unstable in the GTP-bound form, which is the predominant state of monomeric/free G alpha soon after synthesis. It is likely that the signal transduced by Gbetagamma can be regulated by adjusting the levels of G alpha through proteolysis.

Three amino acid substitutions selectively disrupt the activation but not the repression function of the glucocorticoid receptor N terminus

A 210-amino acid region, termed enh2, near the N terminus of the rat glucocorticoid receptor, is necessary for both transcriptional activation and repression. The mechanism(s) of transcriptional regulation conferred by this region, however, are poorly understood. We screened in Saccharomyces cerevisiae a library of random mutants in the enh2 region of a constitutive glucocorticoid receptor derivative and isolated a series of multiply substituted receptors that are specifically defective in transcriptional activation. Although many substitutions in this area were tolerated, three amino acid substitutions (E219K, F220L, and W234R) within a 16-amino acid region were sufficient to disrupt the enh2 transcriptional activation function both in yeast and in mammalian cells. Although this region is rich in acidic residues, the conserved tryptophan at position 234 appears to be a more critical feature for enh2 activity; hydrophobic but not charged residues were tolerated at this position. Notably, the mutants uncoupled the activation and repression functions of enh2, as the activation defective isolates remained competent for repression of AP-1 at the composite response element plfG.

Pheromone signalling and polarized morphogenesis in yeast

Yeast cells respond to mating pheromones by activating a signal transduction pathway involving a seven transmembrane receptor/G protein complex linked to a mitogen-activated protein kinase module. Regulation of the G protein signal is controlled by the receptor and Sst2p; Sst2p may function as a GTPase-activating protein for the G protein alpha subunit. The Ste20 kinase acts in the linkage between the G protein and the MAP kinase module. Experiments suggest that binding of the Rho-like GTPase Cdc42p to Ste20p is not required for the mating response, yet is needed for the pseudohyphal growth response which involves many of the same kinases.

Structural determinants for interaction with three different effectors on the G protein beta subunit

In the yeast two-hybrid system, a 100-residue fragment (beta1A) from the N terminus of the beta1 subunit interacts with domains specific to adenylyl cyclase 2 (AC2), the muscarinic atrial potassium channel (GIRK1), and phospholipase C-beta2 (PLC-beta2). Based on the crystal structure of the G protein, beta1A is composed of an N-terminal alpha helix, a loop, and five beta strands in which the C-terminal four beta strands form a beta sheet, the first of seven sheets that make up the propeller structure of the beta subunit. A mutant of beta1A (L4P, L7P, and L14P), in which the alpha helix was potentially destroyed, interacted poorly with the G protein gamma subunit but effectively with domains of AC2, GIRK1, and PLC-beta2. In contrast, another mutant of beta1A (S72A, D76A, and W82A), in which a network of hydrogen bonds was disrupted, interacted poorly with GIRK1 and PLC-beta2 domains, but effectively with the gamma subunit and the AC2 domain. These results suggest that the proper folding of the first five beta strands in the G protein beta subunit is a requirement for appropriately positioning residues that interact with GIRK1 and PLC-beta2. Furthermore, since mutations that potentially disrupted the folding of these beta strands did not affect interaction with AC2, the structural determinants on the G protein beta subunit for interaction with various effectors may be different.

The G beta gamma complex of the yeast pheromone response pathway. Subcellular fractionation and protein-protein interactions

Genetic evidence suggests that the yeast STE4 and STE18 genes encode G beta and G gamma subunits, respectively, that the G betagamma complex plays a positive role in the pheromone response pathway, and that its activity is subject to negative regulation by the G alpha subunit (product of the GPA1 gene) and to positive regulation by cell-surface pheromone receptors. However, as yet there is no direct biochemical evidence for a G betagamma protein complex associated with the plasma membrane. We found that the products of the STE4 and STE18 genes are stably associated with plasma membrane as well as with internal membranes and that 30% of the protein pool is not tightly associated with either membrane fraction. A slower-migrating, presumably phosphorylated, form of Ste4p is enriched in the non-membrane fraction. The Ste4p and Ste18p proteins that had been extracted from plasma membranes with detergent were found to co-sediment as an 8 S particle under low salt conditions and as a 6 S particle in the presence of 0.25 M NaCl; the Ste18p in these fractions was precipitated with anti-Ste4p antiserum. Under the conditions of our assay, Gpa1p was not associated with either particle. The levels of Ste4p and Ste18p accumulation in mutant cells provided additional evidence for a G betagamma complex. Ste18p failed to accumulate in ste4 mutant cells, and Ste4p showed reduced levels of accumulation and an increased rate of turnover in ste18 mutant cells. The gpa1 mutant blocked stable association of Ste4p with the plasma membrane, and the ste18 mutant blocked stable association of Ste4p with both plasma membranes and internal membranes. The membrane distribution of Ste4p was unaffected by the ste2 mutation or by down-regulation of the cell-surface receptors. These results indicate that at least 40% of Ste4p and Ste18p are part of a G betagamma complex at the plasma membrane and that stable association of this complex with the plasma membrane requires the presence of G alpha.

G protein mechanisms: insights from structural analysis

This review is concerned with the structures and mechanisms of a superfamily of regulatory GTP hydrolases (G proteins). G proteins include Ras and its close homologs, translation elongation factors, and heterotrimeric G proteins. These proteins share a common structural core, exemplified by that of p21ras (Ras), and significant sequence identity, suggesting a common evolutionary origin. Three-dimensional structures of members of the G protein superfamily are considered in light of other biochemical findings about the function of these proteins. Relationships among G protein structures are discussed, and factors contributing to their low intrinsic rate of GTP hydrolysis are considered. Comparison of GTP- and GDP-bound conformations of G proteins reveals how specific contacts between the gamma-phosphate of GTP and the switch II region stabilize potential effector-binding sites and how GTP hydrolysis results in collapse (or reordering) of these surfaces. A GTPase-activating protein probably binds to and stabilizes the conformation of its cognate G protein that recognizes the transition state for hydrolysis, and may insert a catalytic residue into the G protein active site. Inhibitors of nucleotide release, such as the beta gamma subunit of a heterotrimeric G protein, bind selectively to and stabilize the GDP-bound state. Release factors, such as the translation elongation factor, Ts, also recognize the switch regions and destabilize the Mg(2+)-binding site, thereby promoting GDP release. G protein-coupled receptors are expected to operate by a somewhat different mechanism, given that the GDP-bound form of many G protein alpha subunits does not contain bound Mg2+.

Ste50p sustains mating pheromone-induced signal transduction in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the heterotrimeric G protein transduces the mating pheromone signal from a cell-surface receptor. Free G beta gamma then activates a mitogen-activated protein (MAP) kinase cascade. STE50 has been shown to be involved in this pheromone signal-transduction pathway. In this study, we present a functional characterization of Ste50p, a protein that is required to sustain the pheromone-induced signal which leads cells to hormone-induced differentiation. Inactivation of STE50 leads to the attenuation of mating pheromone-induced signal transduction, and overexpression of STE50 intensifies the pheromone-induced signalling. By genetic analysis we have positioned the action of Ste50p downstream of the alpha-pheromone receptor (STE2), at the level of the heterotrimeric G protein, and upstream of STE5 and the kinase cascade of STE11 and STE7. In a two-hybrid assay Ste50p interacts weakly with the G protein and strongly with the MAPKKK Ste11p. The latter interaction is absent in the constitutive mutant Ste11pP279S. These data show that a new component, Ste50p, determines the extent and the duration of signal transduction by acting between the G protein and the MAP kinase complex in S. cerevisiae.

Genetic studies of the PRP17 gene of Saccharomyces cerevisiae: a domain essential for function maps to a nonconserved region of the protein

The PRP17 gene product is required for the second step of pre-mRNA splicing reactions. The C-terminal half of this protein bears four repeat units with homology to the beta transducin repeat. Missense mutations in three temperature-sensitive prp17 mutants map to a region in the N-terminal half of the protein. We have generated, in vitro, 11 missense alleles at the beta transducin repeat units and find that only one affects function in vivo. A phenotypically silent missense allele at the fourth repeat unit enhances the slow-growing phenotype conferred by an allele at the third repeat, suggesting an interaction between these domains. Although many missense mutations in highly conserved amino acids lack phenotypic effects, deletion analysis suggests an essential role for these units. Only mutations in the N-terminal nonconserved domain of PRP17 are synthetically lethal in combination with mutations in PRP16 and PRP18, two other gene products required for the second splicing reaction. A mutually allele-specific interaction between Prp17 and snr7, with mutations in U5 snRNA, was observed. We therefore suggest that the functional region of Prp17p that interacts with Prp18p, Prp16p, and U5 snRNA is the N terminal region of the protein.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

Crystal structure of a G-protein beta gamma dimer at 2.1A resolution

Many signalling cascades use seven-helical transmembrane receptors coupled to heterotrimeric G proteins (G alpha beta gamma) to convert extracellular signals into intracellular responses. Upon nucleotide exchange catalysed by activated receptors, heterotrimers dissociate into GTP-bound G alpha subunits and G beta gamma dimers, either of which can modulate many downstream effectors. Here we use multiwavelength anomalous diffraction data to solve the crystal structure of the beta gamma dimer of the G protein transducin. The beta-subunit is primarily a seven-bladed beta-propeller that is partially encircled by an extended gamma-subunit. The beta-propeller, which contains seven structurally similar WD repeats, defines the stereochemistry of the WD repeat and the probable architecture of all WD-repeat-containing domains. The structure details interactions between G protein beta- and gamma-subunits and highlights regions implicated in effector modulation for the conserved family of G protein beta gamma dimers.

The 2.0 A crystal structure of a heterotrimeric G protein

The structure of a heterotrimeric G protein reveals the mechanism of the nucleotide-dependent engagement of the alpha and beta gamma subunits that regulates their interaction with receptor and effector molecules. The interaction involves two distinct interfaces and dramatically alters the conformation of the alpha but not of the beta gamma subunits. The location of the known sites for post-translational modification and receptor coupling suggest a plausible orientation with respect to the membrane surface and an activated heptahelical receptor.

Intersubunit surfaces in G protein alpha beta gamma heterotrimers. Analysis by cross-linking and mutagenesis of beta gamma

Heterotrimeric guanine nucleotide binding proteins (G proteins) are made up of alpha, beta, and gamma subunits, the last two forming a very tight complex. Stimulation of cell surface receptors promotes dissociation of alpha from the beta gamma dimer, which, in turn, allows both components to interact with intracellular enzymes or ion channels and modulate their activity. At present, little is known about the conformation of the beta gamma dimer or about the areas of beta gamma that interact with alpha. Direct information on the orientation of protein surfaces can be obtained from the analysis of chemically cross-linked products. Previous work in this laboratory showed that 1,6-bismaleimidohexane, which reacts with cysteine residues, specifically cross-links alpha to beta and beta to gamma (Yi, F., Denker, B. M., and Neer, E. J. (1991) J. Biol. Chem. 266, 3900-3906). To identify the residues in beta and gamma involved in cross-linking to each other or to alpha, we have mutated the cysteines in beta 1, gamma 2, and gamma 3 and analyzed the mutated proteins by in vitro translation in a rabbit reticulocyte lysate. All the mutants were able to form beta gamma dimers that could interact with the alpha subunit. We found that 1,6-bismaleimidohexane can cross-link beta 1 to gamma 3 but not to gamma 2. The cross-link goes from Cys25 in beta 1 to Cys30 in gamma 3. This cysteine is absent from any of the other known gamma isoforms and therefore confers a distinctive property to gamma 3. The beta subunit in the beta 1 gamma 2 dimer can be cross-linked to an unidentified protein in the rabbit reticulocyte lysate, generating a product slightly larger than cross-linked beta 1 gamma 3. The beta subunit can also be cross-linked to alpha, giving rise to two products on SDS-polyacrylamide gel electrophoresis, both of which were previously shown to be formed by cross-linking beta to Cys215 in alpha o (Thomas, T. C., Schmidt, C. J., and Neer, E. J. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 10295-10299). Mutation of Cys204 in beta 1 abolished one of these two products, whereas mutation of Cys271 abolished the other. Because both alpha-beta cross-linked products are formed in approximately equal amounts, Cys204 and Cys271 in beta are equally accessible from Cys215 in alpha o. Our findings begin to define intersubunit surfaces, and they pose structural constraints upon any model of the beta gamma dimer.

Separate roles for N- and C-termini of the STE4 (beta) subunit of the Saccharomyces cerevisiae G protein in the mediation of the growth arrest. Lack of growth-arresting activity of mammalian beta gamma complexes

Mating pheromone signal transduction in Saccharomyces cerevisiae involves a G protein composed to Scg1p (Gpa1p), Ste4p and Ste18p subunits, homologous to the alpha, beta and gamma subunits of mammalian G protein. Growth arrest in G1 phase is activated by the Ste4p/Ste18p complex via a downstream pathway and it is negatively controlled by the Scg1p subunit. Here we explored whether mammalian beta or gamma subunits could functionally substitute for their yeast homologues. While no evidence was obtained for functional replacement of Ste4p and Ste18p, we found that overexpression of Ste18p potentiated the effect of hybrid proteins in which the N terminus of the Ste4p subunit was replaced by that of the mammalian beta. ste4 mutants having deletions in the N terminus showed a decreased activity in signalling to the downstream effector of the pheromone response. This defect was totally cured by overexpression of Ste18p, indicating that the truncated forms of Ste4p have retained their ability to form an active complex with Ste18p. Removal of six amino acids from the C terminus of Ste4p rendered a completely inactive subunit and this defect persisted in hybrids where the C terminus was placed by that of the beta subunit, indicating that the C terminus of Ste4p is essential to trigger the effector of the yeast pheromone response pathway.

The structure of the G protein heterotrimer Gi alpha 1 beta 1 gamma 2

The crystallographic structure of the G protein heterotrimer Gi alpha 1(GDP)beta 1 gamma 2 (at 2.3 A) reveals two nonoverlapping regions of contact between alpha and beta, an extended interface between beta and nearly all of gamma, and limited interaction of alpha with gamma. The major alpha/beta interface covers switch II of alpha, and GTP-induced rearrangement of switch II causes subunit dissociation during signaling. Alterations in GDP binding in the heterotrimer (compared with alpha-GDP) explain stabilization of the inactive conformation of alpha by beta gamma. Repeated WD motifs in beta form a circularized sevenfold beta propeller. The conserved cores of these motifs are a scaffold for display of their more variable linkers on the exterior face of each propeller blade.

Truncated forms of a novel yeast protein suppress the lethality of a G protein alpha subunit deficiency by interacting with the beta subunit

In Saccharomyces cerevisiae, the mating pheromone-initiated signal is transduced by a heterotrimeric G protein and normally results in transient cell cycle arrest and differentiation. A null allele of the G alpha (GPA1/SCG1) subunit results in cell death due to unchecked signaling from the G beta gamma (STE4, STE18, respectively) heterodimer. We have identified three high copy suppressors of gpa1 lethality. Two of these genes encode known transcription factors. Mat alpha 2p and Mcm1p. The third is a truncated form of a novel gene, SYG1. Overexpressed wild type SYG1 is a weak suppressor of gpa1. In contrast, the isolated mutant allele SYG1-1 is a strong suppressor that completely blocks the cell cycle arrest and differentiation phenotypes of gpa1 cells of both mating types. One deletion mutant (SYG1 delta 340) can suppress the cell cycle arrest associated with gpa1, but the cells retain a differentiated morphology. SYG1-1 can suppress the effects of overexpressed wild type G beta but is not able to suppress the lethality of an activated G beta mutant (STE4Hpl). Consistent with these genetic observations, the suppressing form of Syg1p can interact with the STE4 gene product, as determined by a two-hybrid assay. SYG1-1 is also capable of promoting pheromone recovery in wild type cells, as judged by halo assay. The sequence of SYG1 predicts eight membrane-spanning domains. Deletion mutants of SYG1 indicate that complete gpa1 suppression requires removal of all of these hydrophobic regions. Interestingly, this truncated protein localizes to the same plasma membrane-enriched subcellular fraction as does full-length Syg1p. Three hypothetical yeast proteins, identified by their similarity to the SYG1 primary sequence within the gpa1 suppression domain, also appear to have related structures. The properties of Syg1p are consistent with those of a transmembrane signaling component that can respond to, or transduce signals through, G beta or G beta gamma.

The extra sex combs product contains WD40 repeats and its time of action implies a role distinct from other Polycomb group products

The extra sex combs (esc) gene product is a transcriptional repressor of homeotic genes. Although it is classified in the Polycomb group (PcG) on the basis of phenotypic criteria, it is distinct from most other PcG repressors in its time of action during development. We describe the temporal profile of esc mRNA expression during embryogenesis and the stage-specific rescue of esc mutants with a heat shock-inducible esc cDNA transformation construct. Both experiments support the idea that esc product plays an early, transient role in repression of homeotic genes. We also present the sequence of a full-length esc cDNA. The predicted esc protein is composed primarily of multiple copies of a repeat motif, termed the WD40 repeat, which are likely used in protein-protein contact. We provide evidence that individual copies of the esc WD40 repeats are needed for function in vivo. We suggest that esc protein is an adaptor that binds to multiple protein partners and assists in the assembly or targeting of other PcG proteins.

The extra sex combs protein is highly conserved between Drosophila virilis and Drosophila melanogaster

Extra sex combs (esc) is one of the Polycomb Group genes, whose products are required for long term maintenance of the spatially restricted domains of homeotic gene expression initially established by the products of the segmentation genes. We recently showed that the esc protein contains five copies of the WD motif, which in other proteins has been directly implicated in protein-protein interactions. Mutations affecting the WD repeats of the esc protein indicate that they are essential for its function as a repressor of the homeotic genes. We proposed that they may mediate interactions between esc and other Polycomb Group proteins, recruiting them to their target genes, perhaps by additional interactions with transiently expressed repressors such as hunchback. To further investigate the functional importance of the WD motifs and identify other functionally important regions of the esc protein, we have begun to determine its evolutionary conservation by characterizing the esc gene from Drosophila virilis, a distantly related Drosophila species. We show that the esc protein is highly conserved between these species, particularly its WD motifs. Their high degree of conservation, particularly at positions which are not conserved in the WD consensus derived from alignment of all known WD motifs, suggests that each of the WD repeats in the esc protein is functionally specialized and that this specialization has been highly conserved during evolution. Its highly charged N-terminus exhibits the greatest divergence, but even these differences are conservative of its predicted physical properties. These observations suggest that the esc protein is functionally compact, nearly every residue making an important contribution to its function.

The Drosophila extra sex combs protein contains WD motifs essential for its function as a repressor of homeotic genes

Extra sex combs is a member of the Polycomb Group genes, whose products are required for stable long term transcriptional repression of the homeotic genes of the Bithorax and Antennapedia complexes. The Pc-G proteins are required to maintain the spatially restricted domains of homeotic gene expression established by the transiently expressed repressors, e.g., hunchback, but are not required for the functioning of these early repressors. This implies two distinct modes of repression and raises the question: how does the transition from early transient repression to stable Pc-G-mediated repression occur? While other Pc-G proteins are required continuously throughout development, the esc RNA is only present transiently in early embryos, suggesting that esc may play a role in mediating this transition to stable long term Pc-G-mediated repression. The predicted esc protein contains multiple copies of the WD motif, found in G-protein beta subunits as well as non-G proteins involved in diverse cellular functions, including transcriptional repression. The sequence alterations of a number of esc mutations cause amino acid substitutions within the WD repeats, identifying them as essential for the function of the esc protein as a repressor of homeotic gene expression. Other WD proteins are components of reversible macromolecular assemblies and the WD motif has recently been directly implicated in mediating interactions with other proteins in such complexes. We propose that the esc protein is similarly involved in the initial recruitment of Pc-G repressors to the homeotic genes to establish their stable long term repression.

Biochemical analysis of the transducin-phosphodiesterase interaction

In vertebrate rod cells, the activated alpha-subunit of rod transducin interacts with the gamma (regulatory) subunits of phosphodiesterase to disinhibit the catalytic subunits. A 22-amino acid long region of rod transducin involved in phosphodiesterase activation has recently been identified. We have used peptides from this region of rod transducin and from several other G protein alpha-subunits to study the nature and specificity of the G protein alpha-effector interaction. Although peptides derived from rod transducin, cone transducin and gustducin are similar, only the rod peptide is capable of activating rod phosphodiesterase. Using substituted peptides we have identified five residues on one exposed face of rod transducin as important to phosphodiesterase activation. These results disagree with previous models which propose that loop regions of rod transducin interact with phosphodiesterase gamma.