Dominant negative mutants of transducin-alpha that block activated receptor

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins, composed of Gα, Gβ, and Gγ subunits, are a class of signal transduction complexes with broad roles in human health and agriculturally relevant plant physiological and developmental traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. We sought to develop improved methods for heterologous expression and rapid purification of Gα subunits. Using GPA1, the sole canonical Gα subunit of the model plant species, Arabidopsis thaliana, we observed that, compared to conventional purification methods, rapid StrepII-tag mediated purification facilitates isolation of protein with increased GTP binding and hydrolysis activities. Human GNAI1 purified using our approach also displayed the expected binding and hydrolysis activities, indicating our protocol is applicable to mammalian Gα subunits, potentially including those for which purification of enzymatically active protein has been historically problematic. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate that the inherent instability of GPA1 is a function of the interaction between the Ras and helical domains. Additionally, we found that GPA1-GNAO1 domain swaps partially uncouple the instability from the rapid nucleotide binding kinetics displayed by GPA1.

Gαo is a major determinant of cAMP signaling in the pathophysiology of movement disorders

The G protein alpha subunit o (Gαo) is one of the most abundant proteins in the nervous system, and pathogenic mutations in its gene (GNAO1) cause movement disorder. However, the function of Gαo is ill defined mechanistically. Here, we show that Gαo dictates neuromodulatory responsiveness of striatal neurons and is required for movement control. Using in vivo optical sensors and enzymatic assays, we determine that Gαo provides a separate transduction channel that modulates coupling of both inhibitory and stimulatory dopamine receptors to the cyclic AMP (cAMP)-generating enzyme adenylyl cyclase. Through a combination of cell-based assays and rodent models, we demonstrate that GNAO1-associated mutations alter Gαo function in a neuron-type-specific fashion via a combination of a dominant-negative and loss-of-function mechanisms. Overall, our findings suggest that Gαo and its pathological variants function in specific circuits to regulate neuromodulatory signals essential for executing motor programs.

Muntean et al. describe biochemical, cellular, and physiological mechanisms by which the heterotrimeric G protein subunit Gαo controls neuromodulatory signaling in the striatum and elucidate mechanisms by which Gαo mutations compromise movements in GNAO1 disorder.

Nucleotide exchange-dependent and nucleotide exchange-independent functions of plant heterotrimeric GTP-binding proteins

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile, guanine nucleotide-dependent, molecular on-off switches. In animals and fungi, the exchange of GDP for GTP on Gα controls G protein activation and is crucial for normal cellular responses to diverse extracellular signals. The model plant Arabidopsis thaliana has a single canonical Gα subunit, AtGPA1. We found that, in planta, the constitutively active, GTP-bound AtGPA1(Q222L) mutant and the nucleotide-free AtGPA1(S52C) mutant interacted with Gβγ1 and Gβγ2 dimers with similar affinities, suggesting that G protein heterotrimer formation occurred independently of nucleotide exchange. In contrast, AtGPA1(Q222L) had a greater affinity than that of AtGPA1(S52C) for Gβγ3, suggesting that the GTP-bound conformation of AtGPA1(Q222L) is distinct and tightly associated with Gβγ3. Functional analysis of transgenic lines expressing either AtGPA1(S52C) or AtGPA1(Q222L) in the gpa1-null mutant background revealed various mutant phenotypes that were complemented by either AtGPA1(S52C) or AtGPA1(Q222L). We conclude that, in addition to the canonical GDP-GTP exchange-dependent mechanism, plant G proteins can function independently of nucleotide exchange.

The N54-αs Mutant Has Decreased Affinity for βγ and Suggests a Mechanism for Coupling Heterotrimeric G Protein Nucleotide Exchange with Subunit Dissociation

Ser54 of G_sα binds guanine nucleotide and Mg²⁺ as part of a conserved sequence motif in GTP binding proteins. Mutating the homologous residue in small and heterotrimeric G proteins generates dominant-negative proteins, but by protein-specific mechanisms. For α_i/o, this results from persistent binding of α to βγ, whereas for small GTP binding proteins and α_s this results from persistent binding to guanine nucleotide exchange factor or receptor. This work examined the role of βγ interactions in mediating the properties of the Ser54-like mutants of Gα subunits. Unexpectedly, WT-α_s or N54-α_s coexpressed with α_1B-adrenergic receptor in human embryonic kidney 293 cells decreased receptor stimulation of IP3 production by a cAMP-independent mechanism, but WT-α_s was more effective than the mutant. One explanation for this result would be that α_s, like Ser47 α_i/o, blocks receptor activation by sequestering βγ; implying that N54-α_S has reduced affinity for βγ since it was less effective at blocking IP3 production. This possibility was more directly supported by the observation that WT-α_s was more effective than the mutant in inhibiting βγ activation of phospholipase Cβ2. Further, in vitro synthesized N54-α_s bound biotinylated-βγ with lower apparent affinity than did WT-α_s The Cys54 mutation also decreased βγ binding but less effectively than N54-α_s Substitution of the conserved Ser in α_o with Cys or Asn increased βγ binding, with the Cys mutant being more effective. This suggests that Ser54 of α_s is involved in coupling changes in nucleotide binding with altered subunit interactions, and has important implications for how receptors activate G proteins.

The G alpha subunit Gα8 inhibits proliferation, promotes adhesion and regulates cell differentiation

Heterotrimeric G protein-mediated signal transduction plays a pivotal role in both vegetative and developmental stages in the eukaryote Dictyostelium discoideum. Here we describe novel functions of the G protein alpha subunit Gα8 during vegetative and development stages. Gα8 is expressed at low levels during vegetative growth. Loss of Gα8 promotes cell proliferation, whereas excess Gα8 expression dramatically inhibits growth and induces aberrant cytokinesis on substrates in a Gβ-dependent manner. Overexpression of Gα8 also leads to increased cell-cell cohesion and cell-substrate adhesion. We demonstrate that the increased cell-cell cohesion is mainly caused by induced CadA expression, and the induced cell-substrate adhesion is responsible for the cytokinesis defects. However, the expression of several putative constitutively active mutants of Gα8 does not augment the phenotypes caused by intact Gα8. Gα8 is strongly induced after starvation, and loss of Gα8 results in decreased expression of certain adhesion molecules including CsA and tgrC1. Interestingly, Gα8 is preferentially distributed in the upper and lower cup of the fruiting body. Lack of Gα8 decreases the expression of the specific marker of the anterior-like cells, suggesting that Gα8 is required for anterior-like cell differentiation.

A dominant-negative Galpha mutant that traps a stable rhodopsin-Galpha-GTP-betagamma complex

Residues comprising the guanine nucleotide-binding sites of the α subunits of heterotrimeric (large) G-proteins (Gα subunits), as well as the Ras-related (small) G-proteins, are highly conserved. This is especially the case for the phosphate-binding loop (P-loop) where both Gα subunits and Ras-related G-proteins have a conserved serine or threonine residue. Substitutions for this residue in Ras and related (small) G-proteins yield nucleotide-depleted, dominant-negative mutants. Here we have examined the consequences of changing the conserved serine residue in the P-loop to asparagine, within a chimeric Gα subunit (designated αT*) that is mainly comprised of the α subunit of the retinal G-protein transducin and a limited region from the α subunit of Gi1. The αT*(S43N) mutant exhibits a significantly higher rate of intrinsic GDP-GTP exchange compared with wild-type αT*, with light-activated rhodopsin (R*) causing only a moderate increase in the kinetics of nucleotide exchange on αT*(S43N). The αT*(S43N) mutant, when bound to either GDP or GTP, was able to significantly slow the rate of R*-catalyzed GDP-GTP exchange on wild-type αT*. Thus, GTP-bound αT*(S43N), as well as the GDP-bound mutant, is capable of forming a stable complex with R*. αT*(S43N) activated the cGMP phosphodiesterase (PDE) with a dose-response similar to wild-type αT*. Activation of the PDE by αT*(S43N) was unaffected if either R* or β1γ1 alone was present, whereas it was inhibited when R* and the β1γ1 subunit were added together. Overall, our studies suggest that the S43N substitution on αT* stabilizes an intermediate on the G-protein activation pathway consisting of an activated G-protein-coupled receptor, a GTP-bound Gα subunit, and the β1γ1 complex.

Light-dependent compartmentalization of transducin in rod photoreceptors

Three major visual signaling proteins, transducin, arrestin, and recoverin undergo bidirectional translocations between the outer segment and inner compartments of rod photoreceptors in a light-dependent manner. The light-dependent translocation of proteins is believed to contribute to adaptation and neuroprotection of photoreceptor cells. The potential physiological significance and mechanisms of light-controlled protein translocations are at the center of current discussion. In this paper, I outline the latest advances in understanding the mechanisms of bidirectional translocation of transducin and determinants of its steady-state distribution in dark- and light-adapted photoreceptor cells.

Mechanisms of dominant negative G-protein α subunits

G-protein-coupled receptors (GPCRs) represent the largest class of membrane proteins and are the targets of 25-50% of drugs currently on the market. Dominant negative mutant Galpha subunits of heterotrimeric G-proteins have been extensively utilized to delineate G-protein signaling pathways and represent a promising new tool to study GPCR-dependent signaling in the CNS. There are different regions in various types of Galpha subunits in which mutations can give rise to a dominant negative phenotype. Such a mutant Galpha would compete with wild-type Galpha for binding to other proteins involved in the G-protein cycle and either block or reduce the response caused by wild-type Galpha. To date, there are three different mechanisms described for dominant negative Galpha subunits: sequestration of the Gbetagamma subunits, sequestration of the activated GPCR by the heterotrimeric complex, and sequestration of the activated GPCR by nucleotide-free Galpha. This review focuses on the development of dominant negative Galpha subunits, the different mechanisms used by various mutant Galpha subunits, and potential structural changes underlying the dominant negative effects.