A mutation in the heterotrimeric stimulatory guanine nucleotide binding protein alpha-subunit with impaired receptor-mediated activation because of elevated GTPase activity

Visualizing the impact of disease-associated mutations on G protein-nucleotide interactions

Activation of G proteins stimulates ubiquitous intracellular signaling cascades essential for life processes. Under normal physiological conditions, nucleotide exchange is initiated upon the formation of complexes between a G protein and G protein-coupled receptor (GPCR), which facilitates exchange of bound GDP for GTP, subsequently dissociating the trimeric G protein into its Gα and Gβγ subunits. However, single point mutations in Gα circumvent nucleotide exchange regulated by GPCR-G protein interactions, leading to either loss-of-function or constitutive gain-of-function. Mutations in several Gα subtypes are closely linked to the development of multiple diseases, including several intractable cancers. We leveraged an integrative spectroscopic and computational approach to investigate the mechanisms by which seven of the most frequently observed clinically-relevant mutations in the α subunit of the stimulatory G protein result in functional changes. Variable temperature circular dichroism (CD) spectroscopy showed a bimodal distribution of thermal melting temperatures across all GαS variants. Modeling from molecular dynamics (MD) simulations established a correlation between observed thermal melting temperatures and structural changes caused by the mutations. Concurrently, saturation-transfer difference NMR (STD-NMR) highlighted variations in the interactions of GαS variants with bound nucleotides. MD simulations indicated that changes in local interactions within the nucleotide-binding pocket did not consistently align with global structural changes. This collective evidence suggests a multifaceted energy landscape, wherein each mutation may introduce distinct perturbations to the nucleotide-binding site and protein-protein interaction sites. Consequently, it underscores the importance of tailoring therapeutic strategies to address the unique challenges posed by individual mutations.

Fibrous Dysplasia of Temporal Bone

Fibrous dysplasia of temporal bone is a rare benign bone disease. With the advance in imaging, more cases are likely to get reported and present with less disease severity. Temporal Bone Fibrous dysplasia most commonly affects External Auditory canal resulting in External auditory canal stenosis and Conductive Hearing Loss. Many patients are asymptomatic and rarely require intervention. However, we present 3 different clinical scenarios of Temporal Bone Fibrous Dysplasia and elaborate on clinical course that lead to varied managements for each of them.

Molecular mechanism of acetoacetyl-CoA enhanced kinetics for increased bioplastic production from Cupriavidus necator 428

Polyhydroxyalkanoates are gaining importance due to their biodegradable nature and close analogy to plastics. Polyhydroxybutyrate (PHB) is the most widely used bioplastic from polyalkanoate family, which is produced by a legion of bacterial species via phbCAB operon encoding β-ketothiolase (PhaA), NADPH-dependent acetoacetyl-coenzyme A (acetoacetyl-CoA) reductase (PhaB) and polyhydroxyalkanoate synthase (PhaC). Augmentation in the activity of these enzymes is promising for increased PHB production which is achieved by enzyme engineering strategies including non-structural and structural approaches. Our study is deployed on directed evolution-based experimentally reported mutants of PhaB enzyme with increased efficiency due to impact on critical structural factors. We have analyzed and compared the native PhaB with two of its variants Q47L and T173S in complex with their cofactor i.e. NADPH as well as the substrate i.e. acetoacetyl-CoA, via long range molecular dynamics simulations. Interaction profile, MMPBSA, essential dynamics, and free energy landscape analysis revealed that the enzyme efficiency is critically affected by cofactor interactions. It was also observed that mutants have higher equilibrium constant with lesser but optimal affinity for substrate and cofactor than the wild type, which might be the reason for increased efficiency of the mutants via enhanced substrate and cofactor exchange rate. Our study provides insights into the cofactor and substrate binding affinities to PhaB enzyme at atomistic level, which will facilitate designing of highly efficient PhaB enzymes for increased PHB production. Communicated by Ramaswamy H. Sarma.

Disease-Causing Mutations in the G Protein Gαs Subvert the Roles of GDP and GTP

The single most frequent cancer-causing mutation across all heterotrimeric G proteins is R201C in Gαs. The current model explaining the gain-of-function activity of the R201 mutations is through the loss of GTPase activity and resulting inability to switch off to the GDP state. Here, we find that the R201C mutation can bypass the need for GTP binding by directly activating GDP-bound Gαs through stabilization of an intramolecular hydrogen bond network. Having found that a gain-of-function mutation can convert GDP into an activator, we postulated that a reciprocal mutation might disrupt the normal role of GTP. Indeed, we found R228C, a loss-of-function mutation in Gαs that causes pseudohypoparathyroidism type 1a (PHP-Ia), compromised the adenylyl cyclase-activating activity of Gαs bound to a non-hydrolyzable GTP analog. These findings show that disease-causing mutations in Gαs can subvert the canonical roles of GDP and GTP, providing new insights into the regulation mechanism of G proteins.

Cyclic 3',5'-adenosine monophosphate (cAMP) signaling in the anterior pituitary gland in health and disease

The cyclic 3',5'-adenosine monophosphate (cAMP) was the first among the so-called "second messengers" to be described. It is conserved in most organisms and functions as a signal transducer by mediating the intracellular effects of multiple hormones and neurotransmitters. In this review, we first delineate how different members of the cAMP pathway ensure its correct compartmentalization and activity, mediate the terminal intracellular effects, and allow the crosstalk with other signaling pathways. We then focus on the pituitary gland, where cAMP exerts a crucial function by controlling the responsiveness of the cells to hypothalamic hormones, neurotransmitters and peripheral factors. We discuss the most relevant physiological functions mediated by cAMP in the different pituitary cell types, and summarize the defects affecting this pathway that have been reported in the literature. We finally discuss how a deregulated cAMP pathway is involved in the pathogenesis of pituitary disorders and how it affects the response to therapy.

Structure and dynamics of GPCR signaling complexes

G-protein-coupled receptors (GPCRs) relay numerous extracellular signals by triggering intracellular signaling through coupling with G proteins and arrestins. Recent breakthroughs in the structural determination of GPCRs and GPCR-transducer complexes represent important steps toward deciphering GPCR signal transduction at a molecular level. A full understanding of the molecular basis of GPCR-mediated signaling requires elucidation of the dynamics of receptors and their transducer complexes as well as their energy landscapes and conformational transition rates. Here, we summarize current insights into the structural plasticity of GPCR-G-protein and GPCR-arrestin complexes that underlies the regulation of the receptor's intracellular signaling profile.

Biarsenical ligands bind to endogenous G-protein α-subunits and enable allosteric sensing of nucleotide binding

BACKGROUND

Heterotrimeric G-proteins relay extracellular signals to intracellular effector proteins. Multiple methods have been developed to monitor their activity; including labeled nucleotides and biosensors based on genetically engineered G-proteins. Here we describe a method for monitoring unlabeled nucleotide binding to endogenous G-proteins α-subunits in a homogeneous assay based on the interaction of 4',5'-bis(1,2,3-dithioarsolan-2-yl)-2',7'-difluorofluorescein (F2FlAsH) with G-protein α-subunits.

RESULTS

The biarsenic fluorescent ligand F2FlAsH binds to various wild-type G-protein α-subunits (αi1, αi2, αi3, αslong, αsshort, αolf, αq, α13) via high affinity As-cysteine interactions. This allosteric label enables real time monitoring of the nucleotide bound states of α-subunits via changes in fluorescence anisotropy and intensity of their F2FlAsH-complexes. We have found that different α-subunits displayed different signal amplitudes when interacting with F2FlAsH, being more sensitive to nucleotide binding to αi, αs, αolf and αq than to α13. Addition of nucleotides to F2FlAsH-labeled α-subunits caused concentration-dependent effects on their fluorescence anisotropy. pEC50 values of studied nucleotides depended on the subtype of the α-subunit and were from 5.7 to 8.2 for GTPγS, from 5.4 to 8.1 for GppNHp and from 4.8 to 8.2 for GDP and lastly up to 5.9 for GMP. While GDP and GMP increased the fluorescence anisotropy of F2FlAsH complexes with αi-subunits, they had the opposite effect on the other αβγM complexes studied.

CONCLUSIONS

Biarsenical ligands interact allosterically with endogenous G-protein α-subunits in a nucleotide-sensitive manner, so the presence or absence of guanine nucleotides has an effect on the fluorescence anisotropy, intensity and lifetime of F2FlAsH-G-protein complexes.

Molecular basis of a novel oncogenic mutation in GNAO1

Heterotrimeric G proteins are molecular switches that control signal transduction, and their dysregulation can promote oncogenesis. Somatic mutations in GNAS, GNAI2 and GNAQ genes induce oncogenesis by rendering Gα subunits constitutively activated. Recently the first somatic mutation, arginine(243) → histidine (R243H) in the GNAO1 (Gαo) gene was identified in breast carcinomas and shown to promote oncogenic transformation when introduced into cells. Here, we provide the molecular basis for the oncogenic properties of the Gαo R243H mutant. Using limited proteolysis assays, nucleotide-binding assays, and single-turnover and steady-state GTPase assays, we demonstrate that the oncogenic R234H mutation renders Gαo constitutively active by accelerating the rate of nucleotide exchange; however, this mutation does not affect Gαo's ability to become deactivated by GTPase-activating proteins (GAPs) or by its intrinsic GTPase activity. This mechanism differs from that of previously reported oncogenic mutations that impair GTPase activity and GAP sensitivity without affecting nucleotide exchange. The constitutively active Gαo R243H mutant also enhances Src-STAT3 signaling in NIH-3T3 cells, a pathway previously shown to be directly triggered by active Gαo proteins to promote cellular transformation. Based on structural analyses, we propose that the enhanced rate of nucleotide exchange in Gαo R243H results from loss of the highly conserved electrostatic interaction of R243 with E43, located in the in the P-loop that represents the binding site for the α- and β-phosphates of the nucleotide. We conclude that the novel R234H mutation imparts oncogenic properties to Gαo by accelerating nucleotide exchange and rendering it constitutively active, thereby enhancing signaling pathways, for example, src-STAT3, responsible for neoplastic transformation.

A structural determinant that renders G alpha(i) sensitive to activation by GIV/girdin is required to promote cell migration

Although several non-receptor activators of heterotrimeric G proteins have been identified, the structural features of G proteins that determine their interaction with such activators and the subsequent biological effects are poorly understood. Here we investigated the structural determinants in G alpha(i3) necessary for its regulation by GIV/girdin, a guanine-nucleotide exchange factor (GEF) that activates G alpha(i) subunits. Using G protein activity and in vitro pulldown assays we demonstrate that G alpha(i3) is a better substrate for GIV than the highly homologous G alpha(o). We identified Trp-258 in the G alpha(i) subunit as a novel structural determinant for GIV binding by comparing GIV binding to G alpha(i3)/G alpha(o) chimeras. Mutation of Trp-258 to the corresponding Phe in G alpha(o) decreased GIV binding in vitro and in cultured cells but did not perturb interaction with other G alpha-binding partners, i.e. G betagamma, AGS3 (a guanine nucleotide dissociation inhibitor), GAIP/RGS19 (a GTPase-activating protein), and LPAR1 (a G protein-coupled receptor). Activation of G alpha(i3) by GIV was also dramatically reduced when Trp-258 was replaced with Tyr, Leu, Ser, His, Asp, or Ala, highlighting that Trp is required for maximal activation. Moreover, when mutant G alpha(i3) W258F was expressed in HeLa cells they failed to undergo cell migration and to enhance Akt signaling after growth factor or G protein-coupled receptor stimulation. Thus activation of G alpha(i3) by GIV is essential for biological functions associated with G alpha(i3) activation. In conclusion, we have discovered a novel structural determinant on G alpha(i) that plays a key role in defining the selectivity and efficiency of the GEF activity of GIV on G alpha(i) and that represents an attractive target site for designing small molecules to disrupt the G alpha(i)-GIV interface for therapeutic purposes.

Outward bulging of the right parietal bone in connection with fibrous dysplasia in an infant: a case report

Background

Fibrous dysplasia (FD) is a developmental disease of bone in which there is replacement of normal spongiosa and filling of the medullary cavity of affected bones by an abnormal fibrous tissue that contains trabeculae of poorly calcified primitive bone formed by osseous metaplasia. Fibrous dysplasia is a common benign bone disease existing in monostotic and polyostotic forms. It is sometimes associated with aneurysmal bone cysts, and it is a component of McCune-Albright and Mazabraud syndromes.

Case presentation

We describe here a 4-months old Austrian infant who presented with a hard bulging painless mass of (5 x 3 cm) of the right parietal bone. Radiographs showed a large irregular osteolytic lesion. T1-weighted MR image showed significant expansile lesion associated with a dense zone of calcification in the diploic space. To the best of our knowledge this is the first clinical report of an infant with early presentation of monostotic fibrous dysplasia of the right parietal bone.

Conclusion

Fibrous dysplasia of the skull is a painless progressively expanding destructive bone swellings produce cosmetic deformities. The clinical course may be unpredictable, with sudden appearance of symptoms, some of which can be important and irreversible. In our present patient, the possibility that an early surgical correction might positively interfere with the natural history of the lesion has to be evaluated by taking into account the obvious difficulties that will be encountered in reconstructing the skull after a wide excision of the pathologic bone.

How do receptors activate G proteins?

Heterotrimeric G proteins couple the activation of heptahelical receptors at the cell surface to the intracellular signaling cascades that mediate the physiological responses to extracellular stimuli. G proteins are molecular switches that are activated by receptor-catalyzed GTP for GDP exchange on the G protein alpha subunit, which is the rate-limiting step in the activation of all downstream signaling. Despite the important biological role of the receptor-G protein interaction, relatively little is known about the structure of the complex and how it leads to nucleotide exchange. This chapter will describe what is known about receptor and G protein structure and outline a strategy for assembling the current data into improved models for the receptor-G protein complex that will hopefully answer the question as to how receptors flip the G protein switch.

Heterotrimeric G protein activation by G-protein-coupled receptors

Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP-GDP exchange on the G protein alpha-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

Mechanism of the receptor-catalyzed activation of heterotrimeric G proteins

Heptahelical receptors activate intracellular signaling pathways by catalyzing GTP for GDP exchange on the heterotrimeric G protein alpha subunit (G alpha). Despite the crucial role of this process in cell signaling, little is known about the mechanism of G protein activation. Here we explore the structural basis for receptor-mediated GDP release using electron paramagnetic resonance spectroscopy. Binding to the activated receptor (R*) causes an apparent rigid-body movement of the alpha5 helix of G alpha that would perturb GDP binding at the beta6-alpha5 loop. This movement was not observed when a flexible loop was inserted between the alpha5 helix and the R*-binding C terminus, which uncouples R* binding from nucleotide exchange, suggesting that this movement is necessary for GDP release. These data provide the first direct observation of R*-mediated conformational changes in G proteins and define the structural basis for GDP release from G alpha.

Can a GDP-liganded G-protein be active?

The replacement of GDP bound to the alpha-subunit of a G-protein by GTP is generally considered a crucial step in the activation of effectors by a G-protein. New data by Uğur et al. (2005) (p. 720) raise the possibility that for the heterotrimeric G-protein Gs, GDP-liganded Gs is able to activate the effector adenylyl cyclase as potently and effectively as when Gs is in its GTP bound form. We summarize here the evidence that GTP is necessary for effector activation by G-proteins and discuss potential implications and limitations of data to the contrary.

Guanine nucleotide exchange-independent activation of Gs protein by beta2-adrenoceptor

beta2-adrenoceptor-mediated activation of Gs and adenylyl cyclase or other receptor-mediated G protein activations is believed to occur by receptor-catalyzed replacement of GDP with GTP on the alpha-subunit of the G protein. Here we showed that a beta2-adrenoceptor-Gs system, heterologously expressed in cyc- or human embryonic kidney (HEK)-293 cells, can be activated in the presence of GDP or its phosphorylation-resistant analog, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS). The potency and maximal ability of GDP to activate Gs and adenylyl cyclase were identical to those of GTP. GDP-mediated activation of adenylyl cyclase, similar to that mediated by GTP, was concentration-dependent, required high magnesium concentrations, was inhibited by inverse agonists, and was correlated with the efficacy of receptor ligands used to stimulate the receptor. UDP did not block the GDP-mediated activation, although it completely blocked the formation of a small amount of GTP ( approximately 5% GDP) from GDP. Moreover, the activation of Gs in the presence of GDP was insensitive to cholera toxin treatment of the cells, whereas that observed in the presence of GTP was amplified by the treatment, which showed that the activation observed in the presence of GDP was not mediated by GTP. Therefore, we concluded that GDP itself could mediate beta-adrenoceptor-induced activation of Gs-adenylyl cyclase system as much as GTP. We discuss the results in the context of the current paradigm of receptor-mediated G protein activation and propose an additional mode of activation for beta2-adrenoceptor-G(s) adenylyl cyclase system where nucleotide exchange is not necessary and GDP and GTP play identical roles in receptor-induced Gs protein activation.

Simultaneous stimulation of GABA and beta adrenergic receptors stabilizes isotypes of activated adenylyl cyclase heterocomplex

Background

We investigated how the synthesis of cAMP, stimulated by isoproterenol acting through β-adrenoreceptors and Gs, is strongly amplified by simultaneous incubation with baclofen. Baclofen is an agonist of δ-aminobutyric acid type B receptors [GABA_B], known to inhibit adenylyl cyclase via Gi. Because these agents have opposite effects on cAMP levels, the unexpected increase in cAMP synthesis when they are applied simultaneously has been intensively investigated. From previous reports, it appears that cyclase type II contributes most significantly to this phenomenon.

Results

We found that simultaneous application of isoproterenol and baclofen specifically influences the association/dissociation of molecules involved in the induction and termination of cyclase activity. Beta/gamma from [GABA]B receptor-coupled Gi has a higher affinity for adenylyl cyclase isoform(s) when these isoforms are co-associated with Gs. Our data also suggest that, when beta/gamma and Gαs are associated with adenylyl cyclase isoform(s), beta/gamma from [GABA]B receptor-coupled Gi retards the GTPase activity of Gαs from adrenergic receptor. These reciprocal regulations of subunits of the adenylyl cyclase complex might be responsible for the drastic increase of cAMP synthesis in response to the simultaneous signals.

Conclusions

Simultaneous signals arriving at a particular synapse converge on molecular detectors of coincidence and trigger specific biochemical events. We hypothesize that this phenomenon comes from the complex molecular architectures involved, including scaffolding proteins that make reciprocal interactions between associated molecules possible. The biochemistry of simultaneous signaling is addressed as a key to synaptic function.

Gs(alpha) mutations and imprinting defects in human disease

Gs is the ubiquitously expressed heterotrimeric G protein that couples receptors to the effector enzyme adenylyl cyclase and is required for receptor-stimulated intracellular cAMP generation. Activated receptors promote the exchange of GTP for GDP on the Gs alpha-subunit (Gs(alpha)), resulting in Gs activation; an intrinsic GTPase activity of Gs(alpha) deactivates Gs by hydrolyzing bound GTP to GDP. Mutations of Gs(alpha) residues involved in the GTPase reaction that lead to constitutive activation are present in endocrine tumors, fibrous dysplasia of bone, and McCune-Albright syndrome. Heterozygous loss-of-function mutations lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, and skeletal defects, and are sometimes associated with progressive osseous heteroplasia. Maternal transmission of Gs(alpha) mutations leads to AHO plus resistance to several hormones (e.g., parathyroid hormone) that activate Gs in their target tissues (pseudohypoparathyroidism type IA), while paternal transmission leads only to the AHO phenotype (pseudopseudohypoparathyroidism). Studies in both mice and humans demonstrate that Gs(alpha) is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues and biallelically expressed in most other tissues. This likely explains why multihormone resistance occurs only when Gs(alpha) mutations are inherited maternally. The Gs(alpha) gene GNAS1 has at least four alternative promoters and first exons, leading to the production of alternative gene products including Gs(alpha), XL alphas (a novel Gs(alpha) isoform expressed only from the paternal allele), and NESP55 (a chromogranin-like protein expressed only from the maternal allele). The fourth alternative promoter and first exon (exon 1A) located just upstream of the Gs(alpha) promoter is normally methylated on the maternal allele and is transcriptionally active on the paternal allele. In patients with parathyroid hormone resistance but without AHO (pseudohypoparathyroidism type IB), the exon 1A promoter region is unmethylated and transcriptionally active on both alleles. This GNAS1 imprinting defect is predicted to decrease Gs(alpha) expression in tissues where Gs(alpha) is normally imprinted and therefore to lead to renal parathyroid hormone resistance.

Albright's hereditary osteodystrophy associated with cerebellar pilocytic astrocytoma: coincidence or genetic relationship?

Albright's hereditary osteodystrophy (AHO) is a rare inherited disease characterized by skeletal abnormalities, short stature, and, in some cases, resistance to parathyroid hormone, resulting in pseudohypoparathyroidism (PHP). Heterozygous inactivating mutations of the GNAS1 gene are responsible for reduced activity of the alpha subunit of the Gs protein (G(Salpha)), a protein that mediates hormone signal transduction across cell membranes. G(salpha) is also known to have oncogenic potentials, leading to the development of human pituitary tumors and Leydig cell tumors. Here, we report the 1st case, a 3.5-year-old girl, with classic AHO phenotype and PHP type 1A associated with a cerebellar pilocytic astrocytoma. Coincidence or genetic relationships of both diseases are discussed according to molecular findings and current literature.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

The function of interdomain interactions in controlling nucleotide exchange rates in transducin

The intramolecular contacts in heterotrimeric G proteins that determine the rates of basal and receptor-stimulated nucleotide exchange are not fully understood. The alpha subunit of heterotrimeric G proteins consists of two domains: a Ras-like domain with structural homology to the monomeric G protein Ras and a helical domain comprised of six alpha-helices. The bound nucleotide lies in a deep cleft between the two domains. Exchange of the bound nucleotide may involve opening of this cleft. Thus interactions between the domains may affect the rate of nucleotide exchange in G proteins. We have tested this hypothesis in the alpha subunit of the rod cell G protein transducin (Galpha(t)). Site-directed mutations were prepared in a series of residues located at the interdomain interface. The proteins were expressed in vitro in a reticulocyte lysate system. The rates of basal and rhodopsin-catalyzed nucleotide exchange were determined using a trypsin digestion assay specifically adapted for kinetic measurements. Charge-altering substitutions of two residues at the interdomain interface, Lys(273) and Lys(276), increased basal nucleotide exchange rates modestly (5-10-fold). However, we found no evidence that interactions spanning the two domains in Galpha(t) significantly affected either basal or rhodopsin-catalyzed nucleotide exchange rates. These results suggest that opening of the interdomain cleft is not an energetic barrier to nucleotide exchange in Galpha(t). Experiments with Galpha(i1) suggest by comparison that the organization and function of the interdomain region differ among various G protein subtypes.

Delineation of the structural basis for the activation properties of the dopamine D1 receptor subtypes

To delineate the structural determinants involved in the constitutive activation of the D1 receptor subtypes, we have constructed chimeras between the D1A and D1B receptors. These chimeras harbored a cognate domain corresponding to transmembrane regions 6 and 7 as well as the third extracellular loop (EL3) and cytoplasmic tail, a domain referred herein to as the terminal receptor locus (TRL). A chimeric D1A receptor harboring the D1B-TRL (chimera 1) displays an increased affinity for dopamine that is indistinguishable from the wild-type D1B receptor. Likewise, a chimeric D1B receptor containing the D1A-TRL cassette (chimera 2) binds dopamine with a reduced affinity that is highly reminiscent of the dopamine affinity for the wild-type D1A receptor. Furthermore, we show that the agonist independent activity of chimera 1 is identical to the wild-type D1B receptor whereas the chimera 2 displays a low agonist independent activity that is indistinguishable from the wild-type D1A receptor. Dopamine potencies for the wild-type D1A and D1B receptor were recapitulated in cells expressing the chimera 2 or chimera 1, respectively. However, the differences observed in agonist-mediated maximal activation of adenylyl cyclase elicited by the D1A and D1B receptors remain unchanged in cells expressing the chimeric receptors. To gain further mechanistic insights into the structural determinants of the TRL involved in the activation properties of the D1 receptor subtypes, we have engineered two additional chimeric D1 receptors that contain the EL3 region of their respective cognate wild-type counterparts (hD1A-EL3B and hD1B-EL3A). In marked contrast to chimera 1 and 2, dopamine affinity and constitutive activation were partially modulated by the exchange of the EL3. Meanwhile, hD1A-EL3B and hD1B-EL3A mutant receptors display a full switch in the agonist-mediated maximal activation, which is reminiscent of their cognate wild-type counterparts. Overall, our studies suggest a fundamental role for the TRL in shaping the intramolecular interactions implicated in the constitutive activation and coupling properties of the dopamine D1 receptor subtypes.