Silver spoons and other personal reflections.

The intent is to tell a story-hopefully one that is at various times serious, light-hearted, or provocative-that describes my life in biomedical science, especially focusing on the 50 years from 1961 (as a college senior) to the present.

Targeted knockdown of G protein subunits selectively prevents receptor-mediated modulation of effectors and reveals complex changes in non-targeted signaling proteins.

Heterotrimeric G protein signaling specificity has been attributed to select combinations of Galpha, beta, and gamma subunits, their interactions with other signaling proteins, and their localization in the cell. With few exceptions, the G protein subunit combinations that exist in vivo and the significance of these specific combinations are largely unknown. We have begun to approach these problems in HeLa cells by: 1) determining the concentrations of Galpha and Gbeta subunits; 2) examining receptor-dependent activities of two effector systems (adenylyl cyclase and phospholipase Cbeta); and 3) systematically silencing each of the Galpha and Gbeta subunits by using small interfering RNA while quantifying resultant changes in effector function and the concentrations of other relevant proteins in the network. HeLa cells express equimolar amounts of total Galpha and Gbeta subunits. The most prevalent Galpha proteins were one member of each Galpha subfamily (Galpha(s), Galpha(i3), Galpha(11), and Galpha(13)). We substantially abrogated expression of most of the Galpha and Gbeta proteins expressed in these cells, singly and some in combinations. As expected, agonist-dependent activation of adenylyl cyclase or phospholipase Cbeta was specifically eliminated following the silencing of Galpha(s) or Galpha(q/11), respectively. We also confirmed that Gbeta subunits are necessary for stable accumulation of Galpha proteins in vivo. Gbeta subunits demonstrated little isoform specificity for receptor-dependent modulation of effector activity. We observed compensatory changes in G protein accumulation following silencing of individual genes, as well as an apparent reciprocal relationship between the expression of certain Galpha(q) and Galpha(i) subfamily members. These findings provide a foundation for understanding the mechanisms that regulate the adaptability and remarkable resilience of G protein signaling networks.

Gialpha and Gbeta subunits both define selectivity of G protein activation by alpha2-adrenergic receptors.

Previous studies of the specificity of receptor interactions with G protein subunits in living cells have relied on measurements of second messengers or other downstream responses. We have examined the selectivity of interactions between alpha2-adrenergic receptors (alpha2R) and various combinations of Gialpha and Gbeta subunit isoforms by measuring changes in FRET between Gialpha-yellow fluorescent protein and cyan fluorescent protein-Gbeta chimeras in HeLa cells. All combinations of Gialpha1, -2, or -3 with Gbeta1, -2, or -4 were activated to some degree by endogenous alpha2Rs as judged by agonist-dependent decreases in FRET. The degree of G protein activation is determined by the combination of Gialpha and Gbeta subunits rather than by the identity of an individual subunit. RT-PCR analysis and small interfering RNA knockdown of alpha2R subtypes, followed by quantification of radiolabeled antagonist binding, demonstrated that HeLa cells express alpha2a- and alpha2b-adrenergic receptor isoforms in a 2:1 ratio. Increasing receptor number by overexpression of the alpha2aR subtype minimized the differences among coupling preferences for Gialpha and Gbeta isoforms. The molecular properties of each Gialpha, Gbeta, and alpha2-adrenergic receptor subtype influence signaling efficiency for the alpha2-adrenergic receptor-mediated signaling pathway.

Resistance to inhibitors of cholinesterase 8A catalyzes release of Galphai-GTP and nuclear mitotic apparatus protein (NuMA) from NuMA/LGN/Galphai-GDP complexes.

Resistance to inhibitors of cholinesterase (Ric) 8A is a guanine nucleotide exchange factor that activates certain G protein alpha-subunits. Genetic studies in Caenorhabditis elegans and Drosophila melanogaster have placed RIC-8 in a previously uncharacterized G protein signaling pathway that regulates centrosome movements during cell division. Components of this pathway include G protein subunits of the Galphai class, GPR or GoLoco domain-containing proteins, RGS (regulator of G protein signaling) proteins, and accessory factors. These proteins interact to regulate microtubule pulling forces during mitotic movement of chromosomes. It is unclear how the GTP-binding and hydrolysis cycle of Galphai functions in the context of this pathway. In mammals, the GoLoco domain-containing protein LGN (GPSM2), the LGN- and microtubule-binding nuclear mitotic apparatus protein (NuMA), and Galphai regulate a similar process. We find that mammalian Ric-8A dissociates Galphai-GDP/LGN/NuMA complexes catalytically, releasing activated Galphai-GTP in vitro. Ric-8A-stimulated activation of Galphai caused concomitant liberation of NuMA from LGN. We conclude that Ric-8A efficiently utilizes GoLoco/Galphai-GDP complexes as substrates in vitro and suggest that Ric-8A-stimulated release of Galphai-GTP and/or NuMA regulates the microtubule pulling forces on centrosomes during cell division.

Purification and functional analysis of Ric-8A: a guanine nucleotide exchange factor for G-protein alpha subunits.

Ric-8A (synembryn) has been shown to accelerate the in vitro guanine nucleotide exchange activities of most G-protein alpha subunits (with the exception of Galphas). Methods are presented in this article for the purification of Ric-8A and functional analysis of the effects Ric-8A has on G-protein alpha subunit guanine nucleotide-binding activities. The use of Ric-8A to prepare GTPgammaS-Galpha and nucleotide-free Galpha (in complex with Ric-8A) is described.

Allosteric determinants in guanine nucleotide-binding proteins.

Members of the G protein superfamily contain nucleotide-dependent switches that dictate the specificity of their interactions with binding partners. Using a sequence-based method termed statistical coupling analysis (SCA), we have attempted to identify the allosteric core of these proteins, the network of amino acid residues that couples the domains responsible for nucleotide binding and protein-protein interactions. One-third of the 38 residues identified by SCA were mutated in the G protein Gs alpha, and the interactions of guanosine 5'-3-O-(thio)triphosphate- and GDP-bound mutant proteins were tested with both adenylyl cyclase (preferential binding to GTP-Gs alpha) and the G protein beta gamma subunit complex (preferential binding to GDP-Gs alpha). A two-state allosteric model predicts that mutation of residues that control the equilibrium between GDP- and GTP-bound conformations of the protein will cause the ratio of affinities of these species for adenylyl cyclase and G beta gamma to vary in a reciprocal fashion. Observed results were consistent with this prediction. The network of residues identified by the SCA appears to comprise a core allosteric mechanism conferring nucleotide-dependent switching; the specific features of different G protein family members are built on this core.

TAO (thousand-and-one amino acid) protein kinases mediate signaling from carbachol to p38 mitogen-activated protein kinase and ternary complex factors.

The TAO (for thousand-and-one amino acids) protein kinases activate p38 mitogen-activated protein (MAP) kinase cascades in vitro and in cells by phosphorylating the MAP/ERK kinases (MEKs) 3 and 6. We found that TAO2 activity was increased by carbachol and that carbachol and the heterotrimeric G protein Galphao could activate p38 in 293 cells. Using dominant interfering kinase mutants, we found that MEKs 3 and 6 and TAOs were required for p38 activation by carbachol or the constitutively active mutant GalphaoQ205L. To explore events downstream of TAOs, the effects of TAO2 on ternary complex factors (TCFs) were investigated. Transfection studies demonstrated that TAO2 stimulates phosphorylation of the TCF Elk1 on the major activating site, Ser383, and that TAO2 stimulates transactivation of Elk1 and the related TCF, Sap1. Reporter activity was reduced by the p38-selective inhibitor SB203580. Taken together, these studies suggest that TAO protein kinases relay signals from carbachol through heterotrimeric G proteins to the p38 MAP kinase, which then activates TCFs in the nucleus.

Mammalian Ric-8A (synembryn) is a heterotrimeric Galpha protein guanine nucleotide exchange factor.

The activation of heterotrimeric G proteins is accomplished primarily by the guanine nucleotide exchange activity of ligand-bound G protein-coupled receptors. The existence of nonreceptor guanine nucleotide exchange factors for G proteins has also been postulated. Yeast two-hybrid screens with Galpha(o) and Galpha(s) as baits were performed to identify binding partners of these proteins. Two mammalian homologs of the Caenorhabditis elegans protein Ric-8 were identified in these screens: Ric-8A (Ric-8/synembryn) and Ric-8B. Purification and biochemical characterization of recombinant Ric-8A revealed that it is a potent guanine nucleotide exchange factor for a subset of Galpha proteins including Galpha(q), Galpha(i1), and Galpha(o), but not Galpha(s). The mechanism of Ric-8A-mediated guanine nucleotide exchange was elucidated. Ric-8A interacts with GDP-bound Galpha proteins, stimulates release of GDP, and forms a stable nucleotide-free transition state complex with the Galpha protein; this complex dissociates upon binding of GTP to Galpha.

Overview of the Alliance for Cellular Signaling.

The Alliance for Cellular Signaling is a large-scale collaboration designed to answer global questions about signalling networks. Pathways will be studied intensively in two cells--B lymphocytes (the cells of the immune system) and cardiac myocytes--to facilitate quantitative modelling. One goal is to catalyse complementary research in individual laboratories; to facilitate this, all alliance data are freely available for use by the entire research community.

Characterization of Saccharomyces cerevisiae acyl-protein thioesterase 1, the enzyme responsible for G protein alpha subunit deacylation in vivo.

Thioacylation is a reversible lipid modification of proteins that plays a role in the regulation of signal transduction. Acyl-protein thioesterase 1 (APT1) was identified as an enzyme capable of deacylating some thioacylated proteins in vitro. Saccharomyces cerevisiae open reading frame YLR118c encodes an enzyme homologous to Rattus norvegicus APT1. We demonstrate that the catalytic activity of the protein encoded by the yeast open reading frame is similar to that of rat APT1, and we designate the protein S. cerevisiae Apt1p. Yeasts bearing a disruption of the APT1 gene lack significant biochemically detectable acyl-protein thioesterase activity. They also fail to deacylate Gpa1p, the yeast G alpha subunit, in metabolic radiolabeling studies. We conclude that native APT1 is the enzyme responsible for G alpha subunit deacylation in S. cerevisiae and presumably other eukaryotes as well.

Assay of RGS protein activity in vitro using purified components.

Single-turnover and steady-state GTPase assays are an effective means to identify and characterize interactions between RGS and G alpha proteins in vitro. The advantage of the single turnover GTPase assay is that it permits simple and rapid assessment of RGS protein activity toward a putative G alpha-GTP substrate. Moreover, once an interaction between an RGS protein and a G alpha-GTP subunit has been identified, the single-turnover assay can be used to determine Michaelis-Menten constants and/or KI values for other competing G alpha substrates. A disadvantage of the single-turnover assay is that a negative result does not preclude the possibility of an interaction between given RGS and G alpha proteins in vivo. Inappropriate reaction conditions or the presence (or absence) of appropriate posttranslational modifications may result in small or undetectable increases in RGS protein-dependent GTPase activity. In these cases it may be tempting to examine RGS protein activity using steady-state GTPase assays in phospholipid vesicles reconstituted with receptors and heterotrimetric G proteins. The advantage to monitoring steady-state GTPase activity in reconstituted proteoliposomes is that ligand-dependent activation of the receptor facilitates GDP dissociation, such that effects of RGS proteins can be observed; multiple cycles of GTP binding and hydrolysis then amplify the GTPase signal. Additionally, the presence of the phospholipid membrane can increase the local RGS protein concentration approximately 10(4)-fold, permitting observation of interactions that are weak in solution. The primary disadvantage of the reconstituted system is the requirement for receptor purification, a technically demanding undertaking in comparison to the purification of G alpha, G beta gamma, and most RGS proteins.

Expression, purification, and assay of cytosolic (catalytic) domains of membrane-bound mammalian adenylyl cyclases.

The identification and isolation of the soluble catalytic domains of adenylyl cyclase have provided investigators with useful reagents for the study of these enzymes. They have permitted detailed mechanistic investigation of the actions of forskolin, Gs alpha, and the inhibitory G protein, Gi alpha. They have served as critical reagents for the development of plausible models of the catalytic mechanism of the enzyme. They have enabled X-ray crystallographic analysis of adenylyl cyclase; this technique was not approachable with the small quantities of the membrane-bound enzyme available previously. The information obtained by using the soluble domains of adenylyl cyclase has provided templates for description of the behavior of many forms of purine nucleotide cyclases from a variety of species. We now appreciate both adenylyl cyclases and guanylyl cyclases as dimeric enzymes with a 2-fold symmetrical domain arrangement (or pseudosymmetrical in the case of heterodimerization). The active sites are located at the interface between the two domains, both of which contribute binding surfaces.