Do G protein subunits associate via a three-stranded coiled coil?

An epidermis-specific chitin synthase CDNA in Choristoneura fumiferana: cloning, characterization, developmental and hormonal-regulated expression

Chitin synthase catalyzes chitin synthesis in the exoskeleton, tracheal system and gut during insect development. A chitin synthase 1 (CfCHS1) cDNA was identified and cloned from the spruce budworm, Choristoneura fumiferana. The CfCHS1 cDNA is 5,300 bp in length and codes a 1,564-amino acid protein with a molecular mass of 178 kDa. The deduced protein contains 16 transmembrane helixes in its domains A and C. The single copy CfCHS1 gene expressed during each of the larval molts from the 3rd to the 6th instar. The gene expressed highly and periodically in the epidermis during each of molts, whereas no transcripts were detected in the midgut and fat body. 20-hydroxyecdysone and the ecdysone agonist RH5992 suppressed CfCHS1 expression, whereas the juvenile hormone analog methoprene induced CfCHS1 expression. These results implicate that CfCHS1 is involved in the chitin synthase and new chitin formation during molting in the insect.

Developmental expression of glutamic acid decarboxylase and of gamma-aminobutyric acid type B receptors in the ascidian Ciona intestinalis

We describe Ciona intestinalis gamma-aminobutyric acid (GABA)-ergic neurons during development, studying the expression pattern of Ci-GAD (glutamic acid decarboxylase: GABA synthesizing enzyme) by in situ hybridization. Moreover, we cloned two GABA(B) receptor subunits (Ci-GABA(B)Rs), and a phylogenetic analysis (neighbor-joining method) suggested that they clustered with their vertebrate counterparts. We compared Ci-GAD and Ci-GABA(B)Rs expression patterns in C. intestinalis embryos and larvae. At the tailbud stage, Ci-GAD expression was widely detected in central and peripheral nervous system (CNS/PNS) precursors, whereas Ci-GABA(B)Rs expression was evident at the level of the precursors of the visceral ganglion. GABA was localized by immunohistochemistry at the same developmental stage. In the larva, Ci-GAD transcripts and GABA immunofluorescence were also detected throughout the CNS and in some neurons of the PNS, whereas transcripts of both GABA(B) receptor subunits were found mainly in the CNS. The expression pattern of Ci-GABA(B)Rs appeared restricted to Ci-GAD-positive territories in the sensory vesicle, whereas, in the visceral ganglion, Ci-GABA(B)Rs transcripts were found in ventral motoneurons that did not express Ci-GAD. Insofar as GABAergic neurons are widely distributed also in the CNS and PNS of vertebrates and other invertebrate chordates, it seems likely that GABA signaling was extensively present in the protochordate nervous system. Results from this work show that GABA is the most widespread inhibitory neurotransmitter in C. intestinalis nervous system and that it can signal through GABA(B) receptors both pre- and postsynaptically to modulate different sensory inputs and subsequent swimming activity.

Genetic evidence for a microtubule-destabilizing effect of conventional kinesin and analysis of its consequences for the control of nuclear distribution in Aspergillus nidulans

Conventional kinesin is a microtubule-dependent motor protein believed to be involved in a variety of intracellular transport processes. In filamentous fungi, conventional kinesin has been implicated in different processes, such as vesicle migration, polarized growth, nuclear distribution, mitochondrial movement and vacuole formation. To gain further insights into the functions of this kinesin motor, we identified and characterized the conventional kinesin gene, kinA, of the established model organism Aspergillus nidulans. Disruption of the gene leads to a reduced growth rate and a nuclear positioning defect, resulting in nuclear cluster formation. These clusters are mobile and display a dynamic behaviour. The mutant phenotypes are pronounced at 37 degrees C, but rescued at 25 degrees C. The hyphal growth rate at 25 degrees C was even higher than that of the wild type at the same temperature. In addition, kinesin-deficient strains were less sensitive to the microtubule destabilizing drug benomyl, and disruption of conventional kinesin suppressed the cold sensitivity of an alpha-tubulin mutation (tubA4). These results suggest that conventional kinesin of A. nidulans plays a role in cytoskeletal dynamics, by destabilizing microtubules. This new role of conventional kinesin in microtubule stability could explain the various phenotypes observed in different fungi.

G protein beta2 subunit antisense oligonucleotides inhibit cell proliferation and disorganize microtubule and mitotic spindle organization

The association of G protein beta2 subunit (Gbeta2) with mitotic spindles in various mammalian cells has been demonstrated previously. Recently, we have identified the association of Gbeta2 protein with microtubules (Wu et al., [1998] J. Cell. Biochem. 70: 552-562). In the present experiment we have demonstrated the possible functional role of Gbeta2 in microtubule and mitotic spindle organization in mammalian cells. When Gbeta2 antisense phosphorothioate oligonucleotides were transfected into mammalian cells, inhibition of cell proliferation with cell death after a 4-day treatment was observed. If the transfected cells were incubated for two days and their Gbeta2 and microtubules were examined by Western blotting and immunofluorescence localization, marked reduction of the Gbeta2 protein, fragmentation and disassembly of cytoplasmic microtubules, and disorganized mitotic spindles were found. We conclude that the Gbeta2 protein is closely associated with microtubule assembly and may play a potential role in the regulation of cell proliferation and microtubule and mitotic spindle organization in mammalian cells.

Nicotiana tabacum cDNAs encoding alpha and beta subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

Heterotrimeric GTP-binding proteins (G-proteins) play important roles in signal transduction pathways in eukaryotic cells. Through differential screening of a hairy root cDNA library of tobacco (Nicotiana tabacum L.) against transcripts from non-root tissues of normal cuttings, we obtained a partial cDNA clone that showed abundant expression and high homology to the alpha subunit gene of plant G-protein. After RACE-PCR, a full-length cDNA clone was obtained, which was 1,677-bp in length and contained an open reading frame encoding a protein of 384 amino acids. A cDNA clone encoding a beta subunit of G-protein was also isolated from the same cDNA library based on PCR amplification and library screening. The clone was 1,600-bp in length and contained an open reading frame encoding 377 amino acids. The deduced amino acid sequences of these clones showed high homology (75.5 to 99.8% amino acid identity) with alpha and beta subunits of other plant G-proteins. Genomic Southern blot analysis showed that the amphidiploid tobacco genome possessed two major copies of both alpha and beta subunit genes and some minor homologous copies. Northern blot analysis showed that the transcript of alpha subunit gene was abundant in the root tissues, particularly in the hairy root tissues. In contrast, the level of expression of the beta subunit gene was equivalent in all the tissues studied. Possible function of tobacco G-protein was discussed.

Molecular characterization of cDNAs encoding G protein alpha and beta subunits and study of their temporal and spatial expression patterns in Nicotiana plumbaginifolia Viv

We have isolated cDNA sequences encoding alpha and beta subunits of potential G proteins from a cDNA library prepared from somatic embryos of Nicotiana plumbaginifolia Viv. at early developmental stages. The predicted NPGPA1 and NPGPB1 gene products are 75-98% identical to the known respective plant alpha and beta subunits. Southern hybridizations indicate that NPGPA1 is probably a single-copy gene, whereas at least two copies of NPGPB1 exist in the N. plumbaginifolia genome. Northern analyses reveal that both NPGPA1 and NPGPB1 mRNA are expressed in all embryogenic stages and plant tissues examined and their expression is obviously regulated by the plant hormone auxin. Immunohistological localization of NPGPalpha1 and NPGPbeta1 preferentially on plasma and endoplasmic reticulum membranes and their immunochemical detection exclusively in microsomal cell fractions implicate membrane association of both proteins. The temporal and spatial expression patterns of NPGPA1 and NPGPB1 show conformity as well as differences. This could account for not only cooperative, but also individual activities of both subunits during embryogenesis and plant development.

Increased nuclear traffic chaos in hyphae of Aspergillus nidulans: molecular characterization of apsB and in vivo observation of nuclear behaviour

Filamentous fungi are model microorganisms for studying nuclear migration in eukaryotic cells. Two genes, apsA and apsB (=anucleate primary sterigmata), were identified in Aspergillus nidulans that affect nuclear distribution in hyphae and specifically block conidiophore development at the metula stage when mutant. Here we describe the cloning, sequencing and molecular analysis of apsB. The gene encodes a 121 kDa coiled-coil, hydrophilic protein that was localized in the cytoplasm. No protein-protein interaction was detected between ApsB and ApsA, a membrane-associated, previously identified protein. An apsB null mutant was characterized by video epifluorescence microscopy using strains that express green fluorescent protein (GFP) in nuclei. With this novel approach, we have discovered a new mutant phenotype and have found that nuclei display an increased chaotic movement in older hyphal compartments that results in clustering and an uneven distribution of these organelles. These results suggest a regulatory role of ApsB in nuclear migration.

The coiled-coil region of the G protein beta subunit. Mutational analysis of Ggamma and effector interactions

The beta and gamma subunits of the heterotrimeric G proteins remain tightly associated throughout the signaling cycle as the betagamma dimer interacts with Galpha, receptors, and effectors. A coiled-coil structure involving alpha-helical segments at the N termini of the beta and gamma subunits contributes to the dimerization interface and has been implicated in effector signaling in yeast. Scanning mutagenesis of the coiled-coil region of the mammalian beta1 subunit was performed to examine the effect of point mutations on betagamma assembly and effector signaling in COS cell cotransfection assays. In addition to the E10K mutation described previously, mutations A11E, L14E, and I18E in beta1 were found to block betagamma association, as evidenced by the failure of the Gbeta mutants to undergo cytosolic translocation with cotransfected nonisoprenylated Ggamma. Although none of 14 beta1 point mutations prevented the betagamma-dependent activation of the c-Jun N-terminal kinase (JNK) effector pathway, the D20K point mutation enhanced JNK but not phospholipase C-beta2 activation. These findings implicate the coiled-coil region of Gbeta in JNK signaling, provide further evidence that the structural features of the betagamma complex mediating effector regulation may differ among effectors, and identify single codons in the mammalian beta subunit where mutation might yield a phenotype of defective signal transduction.

Predicting coiled-coil regions in proteins

The past several years have seen significant advances in our ability to recognize coiled coils from protein sequences and model their structures. New methods include a detection program based on pairwise residue correlations, a program that distinguishes two-stranded from three-stranded coiled coils and a routine for modelling the coordinates of the core residues in coiled coils. Several widely noted predictions, among them those for heterotrimeric G proteins and for cartilage oligomeric matrix protein, have been confirmed by crystal structures, and several new predictions have been made, including a model for the still hypothetical right-handed coiled coil.

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+.

The G protein beta subunit Gpb1 of Schizosaccharomyces pombe is a negative regulator of sexual development

A Schizosaccharomyces pombe homolog of mammalian genes encoding G protein beta subunits, gpb1+, was cloned by the polymerase chain reaction using primer pairs that correspond to sequences conserved in several G beta genes of other species followed by screening of genomic and cDNA libraries. The gpb1 gene encodes 317 amino acids that show 47% homology with human G beta 1 and G beta 2 and 40% homology with Saccharomyces cerevisiae G beta protein. Disruption of the gpb1 gene indicated that this gene is not required for vegetative cell growth. However, gpb1-disrupted haploid cells mated and sporulated faster than wild-type cells, both in sporulation (MEA) and in complex medium (YE): when examined 23 h after transfer to sporulation medium, 35% of gpb1-disrupted haploid pairs had undergone conjugation and sporulation, whereas only 3-5% of wild-type haploid pairs had done so. Overexpression of the gpb1 gene suppressed this facilitated conjugation and sporulation phenotype of gpb1-disrupted cells but did not cause any obvious effect in wild-type cells. Co-disruption of one of the two S. pombe G alpha-subunit genes, gpa2, in the gpb1-disrupted cells did not change the accelerated conjugation and sporulation phenotype of the gpb1- cells. However, co-disruption of the ras1 gene abolished the gpb1- phenotype. These results suggest that Gpb1 is a negative regulator of conjugation and sporulation that apparently works upstream of Ras1 function in S. pombe. The possible relationship of Gpb1 to two previously identified, putative G alpha proteins of S. pombe is discussed.

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.

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.

Regulatory GTPases

The past year has witnessed a tremendous increase in our understanding of the structures and interactions of the GTPases. The highlights include crystal structures of G alpha subunits, as well as the first complex between a GTPase (Rap1A) and an effector molecule (c-Raf1 Ras-binding domain). In the field of elongation factors (EFs), three very important structures have been determined: EF-G, the ternary complex of EF-Tu.GTP with aminoacyl-tRNA, and the EF-Tu.EF-Ts complex.

Identification of a three-amino-acid region in G protein gamma 1 as a determinant of selective beta gamma heterodimerization

Guanine-nucleotide-binding protein beta and gamma subunits belong to large protein families encompassing at least five and ten members, respectively, from mammalian cells. The formation of stable beta gamma heterodimers is a selective process determined by the primary sequences of both the beta and gamma subunit. For example, gamma 2 dimerizes with both beta 1 and beta 2, gamma 1 with beta 1, but not with beta 2. To identify the structural elements of gamma subunits relevant to the selectivity of beta gamma dimerization, we have used the baculovirus-insect cell-expression system to produce chimeric beta and gamma subunits and have studied their dimerization using an assay based on the ability of isoprenylation-resistant gamma subunit mutants to draw beta subunits into the cytosol and including sucrose density gradient analysis of soluble recombinant beta gamma dimers. The results show that replacement of three consecutive residues of gamma 1, Cys36-Cys37-Glu38, by the corresponding residues of gamma 2, Ala33-Ala34-Ala35, suffices to render the mutant gamma 1 subunit capable of forming heterodimers with beta 2. The ability of mutant gamma 1 subunits to dimerize with beta 2 does not correlate with the probability of the mutated region to participate in coiled-coil structures. The tripeptide region identified here as a critical determinant of the selectivity of beta gamma dimer formation is distinct from, but partially overlaps with, the region reported by Lee et al. [Lee, C., Murakami, T. & Simonds, W. F. (1995) J. Biol. Chem. 270, 8779-8784]. The results of this study, therefore, not only extend the region of gamma 1 selecting between beta 1 and beta 2 to the five-residue sequence between Cys36 and Phe40, but also argue against the notion that the hydrophobic terminal residue of this motif represents the key determinant of selective beta gamma interaction.

Predicting oligomerization states of coiled coils

An algorithm based on the profile method was developed that faithfully distinguishes between the amino acid sequences of dimeric and trimeric coiled coils. Normalized sequence profiles derived from nonhomologous, two- and three-stranded, coiled-coil sequences with unambiguous registers were used to assign dimer and trimer propensities to test sequences. The difference between the dimer and trimer profile scores accurately reflected the preferred oligomerization state. The method relied on two strategies that may be generally applicable to profile calculations--profile values of solvent-exposed residues and of amino acids that were underrepresented in the data-base were given zero weight. Differences between the dimer and trimer profiles revealed sequence patterns that match and extend experimental studies of oligomer specification.

The cytoplasmic tyrosine kinase FER is associated with the catenin-like substrate pp120 and is activated by growth factors

The FER gene encodes a cytoplasmic tyrosine kinase with a single SH2 domain and an extensive amino terminus. In order to understand the cellular function of the FER kinase, we analyzed the effect of growth factor stimulation on the phosphorylation and activity of FER. Stimulation of A431 cells and 3T3 fibroblasts with epidermal growth factor or platelet-derived growth factor results in the phosphorylation of FER and two associated polypeptides. The associated polypeptides were shown to be the epidermal growth factor receptor or the platelet-derived growth factor receptor and a previously identified target, pp120. Since pp120 had previously been shown to interact with components of the cadherin-catenin complex, these results implicate FER in the regulation of cell-cell interactions. The physical association of FER with pp120 was found to be constitutive and was mediated by a 400-amino-acid sequence in the amino terminus of FER. Analyses of that sequence revealed that it has the ability to form coiled coils and that it oligomerizes in vitro. The identification of a coiled coil sequence in the FER kinase and the demonstration that the sequence mediates association with a potential substrate suggest a novel mechanism for signal transduction by cytoplasmic tyrosine kinases.

Lack of association of G-protein beta 2- and gamma 2-subunit N-terminal fragments provides evidence against the coiled-coil model of subunit-beta gamma assembly

The association between peptides from bovine G-protein beta 2- and gamma 2-subunits was studied by CD spectroscopy and cross-linking. Both peptides had approximately 25% stable alpha-helical structure at 25 degrees C, but there was no increase on mixing subunits as expected for coiled-coil formation. Also, disulphide cross-linking gave more beta 2 beta 2 homodimer than beta 2 gamma 2 heterodimer. These data do not support the proposed N-terminal coiled-coil model of beta gamma-subunit association.

The G protein gamma subunit. Requirements for dimerization with beta subunits

Guanine nucleotide-binding protein beta and gamma subunits form a tightly bound complex that can only be separated by denaturation. Assembly of beta and gamma subunits is a complicated process. The beta 1 and gamma 2 subunits can be synthesized in vitro in rabbit reticulocyte lysate and then assembled into dimers, but beta 1 cannot form beta gamma dimers when synthesized in a wheat germ extract. In contrast, gamma 2 translated in either system can dimerize with beta 1, suggesting that dimerization-competent gamma 2 can be synthesized without the aid of specific chaperonins or other cofactors. Dimerization-competent gamma 2 in solution forms an asymmetric particle with a Stokes radius of about 21 +/- 0.4 A (n = 4), s20,w of 0.9 S (range 0.8-1.0 S, n = 2), and frictional ratio of 1.57 (assuming no hydration). To define the part of gamma 2 that is needed for native beta gamma dimer formation, a series of N- and C-terminal truncations were generated, synthesized in vitro, and incubated with beta 1. Dimerization was assessed by stabilization of beta 1 to tryptic proteolysis. Truncation of up to 13 amino acids at the C terminus did not affect dimerization with beta 1, whereas removal of 27 amino acids prevented it. Therefore, a region between residues 45 and 59 of gamma 2 is important for dimerization. Truncation of 15 amino acids from the N terminus greatly diminished the formation of beta gamma dimers, while removal of 25 amino acids entirely blocked it. Thus, another region important for forming native beta gamma is near the N terminus. Extension of the N terminus by 12 amino acids that include the influenza virus hemagglutinin epitope did not prevent beta gamma dimerization. Furthermore, in intact 35S-labeled COS cells, epitope-tagged gamma 2 coimmunoprecipitates with beta and alpha subunits. The N-terminal epitope tag must lie at the surface of the heterotrimer since it prevents neither heterotrimer formation nor access of the antibody.

Identification of a discrete region of the G protein gamma subunit conferring selectivity in beta gamma complex formation

The identification of multiple G protein beta and gamma subunit subtypes suggests a potential diversity of beta gamma heterodimers, which may contribute to the specificity of signal transduction between receptors and effectors. The assembly of beta and gamma subtypes is selective. For example, gamma 1 can assemble with beta 1 but not with beta 2, whereas gamma 2 assembles with both beta isoforms. To identify the structural features of the beta and gamma subunits governing selectivity in heterodimer assembly, a series of nonisoprenylated chimeras of gamma 1 and gamma 2 was constructed, and their interaction with beta 1 and beta 2 was assessed by their ability to direct beta expression to the cytosol in cotransfected COS cells. All of the gamma 1/gamma 2 chimeras were capable of interacting with beta 1 as judged by the cotransfection assay. Chimeras containing gamma 2 sequence near the middle of the molecule between two conserved sequence motifs were capable of interacting as well with beta 2. Among 12 divergent residues in this region, it was found that replacement of three consecutive amino acids in gamma 1, Glu-Glu-Phe (residues 38-40), with the three corresponding amino acids of gamma 2, Ala-Asp-Leu (residues 35-37), conferred the ability to assemble with beta 2. The reciprocal chimera containing Glu-Glu-Phe in the context of gamma 2 failed to assemble with beta 2. The last residue of this triplet is occupied by a leucine in all known mammalian gamma subunits except gamma 1 and appears to be a key determinant of the ability of a gamma subunit to assemble with beta 2. This locus maps to a region of predicted alpha-helical structure in the gamma subunit, likely to represent a point of physical contact with the beta subunit.

The small G-protein ARF1GDP binds to the Gt beta gamma subunit of transducin, but not to Gt alpha GDP-Gt beta gamma

AlF4- activates heterotrimeric G-proteins G alpha subunits but not small GDP/GTP-binding proteins like ARF1. On retinal membranes containing holotransducin (Gt alpha GDP-Gt beta gamma) and incubated with ARFGDP, AlF4- induced Gt alpha GDP-AlF4 release and ARFGDP binding, probably to the remaining membrane-attached Gt beta gamma. On phospholipid vesicles reconstituted with Gt beta gamma, ARFGDP bound in proportion to Gt beta gamma, and was released upon subsequent Gt alpha GDP addition. Thus ARFGDP competes with Gt alpha GDP for binding to Gt beta gamma, probably through a conserved motif in the 'alpha 2 helix' of Gt alpha and ARF. This motif is found in the C-terminal helix of PH domains that bind to G beta gamma.

NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration

During a study of the genetics of nuclear migration in the filamentous fungus Aspergillus nidulans, we cloned a gene, nudF, which is required for nuclear migration during vegetative growth as well as development. The NUDF protein level is controlled by another protein NUDC, and extra copies of the nudF gene can suppress the nudC3 mutation. nudF encodes a protein with 42% sequence identity to the human LIS-1 (Miller-Dieker lissencephaly-1) gene, which is required for proper neuronal migration during brain development. This strong similarity suggests that the LIS-1 gene product may have a function similar to that of NUDF and supports previous findings to suggest that nuclear migration may play a role in neuronal migration.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Specificity of G protein alpha-gamma subunit interactions. N-terminal 15 amino acids of gamma subunit specifies interaction with alpha subunit

The existence of multiple alpha, beta, and gamma subunits raises questions regarding the assembly of particular G proteins. Based on the results of a previous study (Rahmatullah, M., and Robishaw, J. D. (1994) J. Biol. Chem. 269, 3574-3580), we hypothesized that the assembly of G proteins may be determined by the interactions of the more structurally diverse alpha and gamma subunits. This hypothesis was confirmed in the present study by showing striking differences in the abilities of the gamma 1 and gamma 2 subunits to interact with the alpha o subunit. Chimeras of the gamma 1 and gamma 2 subunits were used to delineate which region is responsible. Support for the importance of the N-terminal region of the gamma subunit comes from our observations that 1) the gamma 2 subunit and the gamma 211 chimera bound strongly to the alpha o-agarose matrix, but the gamma 1 subunit and the gamma 112 chimera bound weakly, if at all; 2) an N-terminal peptide made to the gamma 2 subunit blocked the binding of the gamma 211 chimera to the alpha o-agarose matrix; 3) both the gamma 211 chimera and the N-terminal peptide were able to partially protect the alpha o subunit against tryptic cleavage; and 4) the gamma 211 chimera, but not the gamma 112 chimera, supported ADP-ribosylation of the alpha o subunit.

Aspergillus nidulans apsA (anucleate primary sterigmata) encodes a coiled-coil protein required for nuclear positioning and completion of asexual development

Many fungi are capable of growing by polarized cellular extension to form hyphae or by isotropic expansion to form buds. Aspergillus nidulans anucleate primary sterigmata (apsA) mutants are defective in nuclear distribution in both hyphae and in specialized, multicellular reproductive structures, called conidiophores. apsA mutations have a negligible effect on hyphal growth, unlike another class of nuclear distribution (nud) mutants. By contrast, they almost completely block entry of nuclei into primary buds, or sterigmata (bud nucleation), produced during development of conidiophores. Failure of the primary sterigmata to become nucleated results in developmental arrest and a failure to activate the transcriptional program associated with downstream developmental steps. However, occasionally in mutants a nucleus enters a primary bud and this event relieves the developmental blockage. Thus, there is a stringent developmental requirement for apsA function, but only at the stage of primary bud formation. apsA encodes a 183-kD coiled-coil protein with similarity to Saccharomyces cerevisiae NUM1p, required for nuclear migration in the budding process.

Roles of lipid modifications of transducin subunits in their GDP-dependent association and membrane binding

Transducin is an unusually soluble and dissociable heterotrimeric G-protein, although its T alpha and T beta gamma subunits are N-acylated and farnesylated, respectively. These lipid modifications have been suggested to contribute directly to the GDP-dependent T alpha-T beta gamma association, through specific lipid recognition sites on both protein subunits. We studied the dependence of subunit association on their bound lipids and on the presence of different lipidic environments. Association of native N-acylated (nT alpha) or acyl-free recombinant (rT alpha) T alpha with farnesylated and carboxymethylated (fcT beta gamma), farnesylated (fT beta gamma), or farnesyl-free (dfT beta gamma) T beta gamma was analyzed by gradient centrifugation and gel filtration in the presence of detergent or phospholipid-cholate micelles and by cosedimentation with phospholipid vesicles. Without detergent, nT alpha GDP and fcT beta gamma associate only weakly in solution. The loss of T alpha acyl or T beta gamma farnesyl residues induces total dissociation. With detergent or lipids, isolated fcT beta gamma binds tightly to micelles or vesicles, while dfT beta gamma does not; nT alpha GDP binds weakly, while deacylated rT alpha GDP does not bind at all; and nT alpha GDP binds cooperatively with fcT beta gamma, while rT alpha GDP does not. Thus (i) the T alpha acyl chain binds weakly, whereas the T beta gamma farnesyl chain binds strongly to membrane lipids; (ii) there is no evidence for binding of the T alpha acyl chain to a polypeptide site in T beta gamma, nor for binding of the T beta gamma farnesyl chain to a polypeptidic site in T alpha, but the T alpha acyl chain seems to bind cooperatively with the T beta gamma farnesyl chain in the membrane lipids; (iii) the insertion of the two protein-attached lipids into the same membrane could contribute to the association of both subunits by favoring collision coupling of the properly oriented protein moieties on the membrane surface.

A G-protein beta subunit that is expressed in the central nervous system of the mollusc Lymnaea stagnalis identified through cDNA cloning

We have cloned cDNA encoding a G-protein beta subunit from the central nervous system (CNS) of the mollusc Lymnaea stagnalis. The deduced protein is very homologous to other metazoan beta subunits. Thus, the Lymnaea CNS can be used as a model system to study beta gamma subunits in their native setting since its large neurons can be manipulated and studied relatively easily in vivo.

G proteins: critical control points for transmembrane signals

Heterotrimeric GTP-binding proteins (G proteins) that are made up of alpha and beta gamma subunits couple many kinds of cell-surface receptors to intracellular effector enzymes or ion channels. Every cell contains several types of receptors, G proteins, and effectors. The specificity with which G protein subunits interact with receptors and effectors defines the range of responses a cell is able to make to an external signal. Thus, the G proteins act as a critical control point that determines whether a signal spreads through several pathways or is focused to a single pathway. In this review, I will summarize some features of the structure and function of mammalian G protein subunits, discuss the role of both alpha and beta gamma subunits in regulation of effectors, the role of the beta gamma subunit in macromolecular assembly, and the mechanisms that might make some responses extremely specific and others rather diffuse.

New roles for G-protein beta gamma-dimers in transmembrane signalling

When a membrane-bound receptor acts on a G protein, the GTP-binding or G alpha subunit dissociates from the G beta gamma dimer. Until recently, the G alpha subunit alone was thought to act on the enzymes and ion channels controlled by these proteins. Newer evidence indicates that the G beta gamma dimer also plays a major part in signal transmission, enhancing the complexity of the possible interactions between the G proteins and their targets.

The N-terminal coiled-coil domain of beta is essential for gamma association: a model for G-protein beta gamma subunit interaction

We have identified the N terminus of the beta subunit as an essential domain for G-protein beta gamma assembly. A C-terminal fragment, beta 1-(130-340), fails to bind gamma unless coexpressed with the complementary N-terminal fragment, beta 1-(1-129). Deletion of the N-terminal 33 residues of beta 1, a region identified by computer algorithm to favor coiled-coil formation, abolishes gamma 2 association. On the basis of these findings, we propose a coiled-coil model of beta gamma interaction and refine this by computer-assisted molecular modeling. The model is tested by further mutagenesis: reversing the charge of residues in beta 1 that are hypothesized to be involved in interhelical salt bridges precludes gamma association. Insertions in the coiled-coil region, which disrupt the proposed hydrophobic interface, prevent gamma association. This structural basis for beta gamma dimerization provides a starting point for the design of beta and gamma mutants that can be used to map regions in beta gamma critical for interactions with the alpha subunit, receptors, and effectors.