Regulator of G-protein signaling 3 (RGS3) inhibits Gbeta1gamma 2-induced inositol phosphate production, mitogen-activated protein kinase activation, and Akt activation

Regulator of G-protein signaling 3 (RGS3) enhances the intrinsic rate at which Galpha(i) and Galpha(q) hydrolyze GTP to GDP, thereby limiting the duration in which GTP-Galpha(i) and GTP-Galpha(q) can activate effectors. Since GDP-Galpha subunits rapidly combine with free Gbetagamma subunits to reform inactive heterotrimeric G-proteins, RGS3 and other RGS proteins may also reduce the amount of Gbetagamma subunits available for effector interactions. Although RGS6, RGS7, and RGS11 bind Gbeta(5) in the absence of a Ggamma subunit, RGS proteins are not known to directly influence Gbetagamma signaling. Here we show that RGS3 binds Gbeta(1)gamma(2) subunits and limits their ability to trigger the production of inositol phosphates and the activation of Akt and mitogen-activated protein kinase. Co-expression of RGS3 with Gbeta(1)gamma(2) inhibits Gbeta(1)gamma(2)-induced inositol phosphate production and Akt activation in COS-7 cells and mitogen-activated protein kinase activation in HEK 293 cells. The inhibition of Gbeta(1)gamma(2) signaling does not require an intact RGS domain but depends upon two regions in RGS3 located between acids 313 and 390 and between 391 and 458. Several other RGS proteins do not affect Gbeta(1)gamma(2) signaling in these assays. Consistent with the in vivo results, RGS3 inhibits Gbetagamma-mediated activation of phospholipase Cbeta in vitro. Thus, RGS3 may limit Gbetagamma signaling not only by virtue of its GTPase-activating protein activity for Galpha subunits, but also by directly interfering with the activation of effectors.

RGS2 regulates signal transduction in olfactory neurons by attenuating activation of adenylyl cyclase III

The heterotrimeric G-protein Gs couples cell-surface receptors to the activation of adenylyl cyclases and cyclic AMP production (reviewed in refs 1, 2). RGS proteins, which act as GTPase-activating proteins (GAPs) for the G-protein alpha-subunits alpha(i) and alpha(q), lack such activity for alpha(s) (refs 3-6). But several RGS proteins inhibit cAMP production by Gs-linked receptors. Here we report that RGS2 reduces cAMP production by odorant-stimulated olfactory epithelium membranes, in which the alpha(s) family member alpha(olf) links odorant receptors to adenylyl cyclase activation. Unexpectedly, RGS2 reduces odorant-elicited cAMP production, not by acting on alpha(olf) but by inhibiting the activity of adenylyl cyclase type III, the predominant adenylyl cyclase isoform in olfactory neurons. Furthermore, whole-cell voltage clamp recordings of odorant-stimulated olfactory neurons indicate that endogenous RGS2 negatively regulates odorant-evoked intracellular signalling. These results reveal a mechanism for controlling the activities of adenylyl cyclases, which probably contributes to the ability of olfactory neurons to discriminate odours.

Molecular basis for P-site inhibition of adenylyl cyclase

P-site inhibitors are adenosine and adenine nucleotide analogues that inhibit adenylyl cyclase, the effector enzyme that catalyzes the synthesis of cyclic AMP from ATP. Some of these inhibitors may represent physiological regulators of adenylyl cyclase, and the most potent may ultimately serve as useful therapeutic agents. Described here are crystal structures of the catalytic core of adenylyl cyclase complexed with two such P-site inhibitors, 2'-deoxyadenosine 3'-monophosphate (2'-d-3'-AMP) and 2',5'-dideoxyadenosine 3'-triphosphate (2',5'-dd-3'-ATP). Both inhibitors bind in the active site yet exhibit non- or uncompetitive patterns of inhibition. While most P-site inhibitors require pyrophosphate (PP(i)) as a coinhibitor, 2',5'-dd-3'-ATP is a potent inhibitor by itself. The crystal structure reveals that this inhibitor exhibits two binding modes: one with the nucleoside moiety bound to the nucleoside binding pocket of the enzyme and the other with the beta and gamma phosphates bound to the pyrophosphate site of the 2'-d-3'-AMP.PP(i) complex. A single metal binding site is observed in the complex with 2'-d-3'-AMP, whereas two are observed in the complex with 2', 5'-dd-3'-ATP. Even though P-site inhibitors are typically 10 times more potent in the presence of Mn(2+), the electron density maps reveal no inherent preference of either metal site for Mn(2+) over Mg(2+). 2',5'-dd-3'-ATP binds to the catalytic core of adenylyl cyclase with a K(d) of 2.4 microM in the presence of Mg(2+) and 0.2 microM in the presence of Mn(2+). Pyrophosphate does not compete with 2',5'-dd-3'-ATP and enhances inhibition.

RGS3 is a GTPase-activating protein for g(ialpha) and g(qalpha) and a potent inhibitor of signaling by GTPase-deficient forms of g(qalpha) and g(11alpha)

Many Regulators of G protein Signaling (RGS) proteins accelerate the intrinsic GTPase activity of G(ialpha) and G(qalpha)-subunits [i.e., behave as GTPase-activating proteins (GAPs)] and several act as G(qalpha)-effector antagonists. RGS3, a structurally distinct RGS member with a unique N-terminal domain and a C-terminal RGS domain, and an N-terminally truncated version of RGS3 (RGS3CT) both stimulated the GTPase activity of G(ialpha) (except G(zalpha)) and G(qalpha) but not that of G(salpha) or G(12alpha). RGS3 and RGS3CT had G(qalpha) GAP activity similar to that of RGS4. RGS3 impaired signaling through G(q)-linked receptors, although RGS3CT invariably inhibited better than did full-length RGS3. RGS3 potently inhibited G(qalpha)Q209L- and G(11alpha)Q209L-mediated activation of a cAMP-response element-binding protein reporter gene and G(qalpha)Q209L induced inositol phosphate production, suggesting that RGS3 efficiently blocks G(qalpha) from activating its downstream effector phospholipase C-beta. Whereas RGS2 and to a lesser extent RGS10 also inhibited signaling by these GTPase-deficient G proteins, other RGS proteins including RGS4 did not. Mutation of residues in RGS3 similar to those required for RGS4 G(ialpha) GAP activity, as well as several residues N terminal to its RGS domain impaired RGS3 function. A greater percentage of RGS3CT localized at the cell membrane than the full-length version, potentially explaining why RGS3CT blocked signaling better than did full-length RGS3. Thus, RGS3 can impair Gi- (but not Gz-) and Gq-mediated signaling in hematopoietic and other cell types by acting as a GAP for G(ialpha) and G(qalpha) subfamily members and as a potent G(qalpha) subfamily effector antagonist.

Slow modal gating of single G protein-activated K+ channels expressed in Xenopus oocytes

The slow kinetics of G protein-activated K+ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (KACh), were activated by a near-saturating dose of G protein betagamma subunits (Gbetagamma; 20 nM). The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac KACh. GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, Po. The unexpectedly low Po (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent approximately 90 % of the time in the long closed states. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the Gbetagamma-activated channels cycled between a low-Po mode (Po of about 0.0034) and a bursting mode characterized by an approximately 30-fold higher Po and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of Gbetagamma. The GTPgammaS-activated Galphai1 subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-Po mode. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.

Lys-Ala mutations of type I adenylyl cyclase result in altered susceptibility to inhibition by adenine nucleoside 3'-polyphosphates

Native and recombinant wild type and mutant forms of type I adenylyl cyclase, expressed in fall army worm ovarian cells (Sf9) cells, with mutations Lys-923-Ala, Lys-921-Ala, and Lys-350-Ala, retained the characteristic noncompetitive inhibition by adenine nucleoside 3'-polyphosphates, but exhibited substantially different sensitivities to inhibition by them. The type I K923A enzyme resulted in increased IC(50) values, e.g., >100-fold for 2'-deoxyadenosine-3'-monophosphate, but the shift diminished as the number of 3'-phosphates increased. The K921A mutation increased IC(50) values approximately 5-fold for all adenine nucleosides tested, whereas the K350A mutation increased IC(50) values approximately 6- to 8-fold for all adenine nucleosides tested except 2'-deoxyadenosine-3'-diphosphate, which was increased >/=2-fold. The data suggest that 3'-phosphates sufficiently increase binding affinity of these ligands to compensate for the reduced coordination of the adenine moiety induced by the K923A mutation. Moreover, the altered structures induced by both K350A and K921A mutations impair ligand binding in general, but paradoxically those resulting from the K350A change minimally affected nucleoside 3'-diphosphate binding, implying that selective changes in ligand binding can be induced by this site-specific mutation.

Covalent labeling of adenylyl cyclase cytosolic domains with gamma-methylimidazole-2',5'-dideoxy-[gamma-(32)P]3'-ATP and the mechanism for P-site-mediated inhibition

A truncated first cytosolic domain of type V adenylyl cyclase (VC(1)) and a truncated second cytosolic domain of type II adenylyl cyclase (IIC(2)) were used alone and in the readily reversible complex (VC(1).IIC(2)) to evaluate interactions with each other and with reversible and irreversible P-site ligands. Enzyme activity was used to assess formation and dissolution of VC(1).IIC(2). The data suggest that binding of 2',5'-dideoxy-3'-ATP to VC(1) and IIC(2) prevented formation of VC(1).IIC(2) and that 2',5'-dideoxy-3'-ATP dissociation occurred slowly. To enable configuration specific cross-linking to the catalytic site, 2',5'-dideoxyadenosine 3'-[gamma-(1-methylimidazole)-triphosphate] (gamma-MetIm-2', 5'-dd-3'-ATP) and 2',5'-dd-adenosine 3'-(gamma-azidoanilido)-triphosphate (gamma-azidoanilido-2', 5'-dd-3'-ATP) were synthesized, the former also as its gamma-(32)P-labeled analog. gamma-Azidoanilido-2',5'-dd-3'-ATP exhibited an inhibitory potency comparable with that of 2', 5'-dd-3'-ATP. gamma-MetIm-2',5'-dd-[gamma-(32)P]3'-ATP labeled the individual VC(1) and IIC(2) domains comparably and covalently to approximately 20% within 1 h. Formation of VC(1).IIC(2) resulted in reduced labeling of VC(1) but enhanced labeling of IIC(2). The data imply that formation of the catalytically active VC(1).IIC(2) complex affects the interaction of each domain with the 2', 5'-dd-3'-ATP, the binding of which also affects the interaction between the two cytosolic domains, leading to a pseudo-irreversible inhibition.

Regulation of L-type Ca2+ channels in rabbit portal vein by G protein alphas and betagamma subunits

1. The effect of purified G protein subunits alphas and betagamma on L-type Ca2+ channels in vascular smooth muscle and the possible pathways involved were investigated using freshly isolated smooth muscle cells from rabbit portal vein and the whole-cell patch clamp technique. 2. Cells dialysed with either Galphas or Gbetagamma exhibited significant increases in peak Ba2+ current (IBa) density (148 % and 131 %, respectively) compared with control cells. The combination of Galphas and Gbetagamma further increased peak IBa density (181 %). Inactive Galphas and Gbetagamma did not have any effect on Ca2+ channels. 3. The stimulatory effect of Galphas on peak IBa was entirely abolished by the protein kinase A inhibitor Rp-8-Br-cAMPS, or the adenylyl cyclase inhibitor SQ 22536. On the other hand, the stimulatory response of Ca2+ channels to Gbetagamma was not affected by the protein kinase A inhibitors Rp-8-Br-cAMPS and KT 5720, or by the Ca2+-dependent protein kinase C inhibitor bisindolylmaleimide 1, but was completely blocked by the protein kinase C inhibitor calphostin C. Pretreatment of cells with phorbol 12-myristate 13-acetate for over 18 h prevented the stimulatory effect of Gbetagamma on peak IBa. In addition, acute application of phorbol 12,13-dibutyrate enhanced peak IBa density in control cells, which could be entirely blocked by calphostin C. 4. These data indicate that enhancement of Ba2+ currents by Galphas and Gbetagamma can be attributed to increased activity of protein kinase A and protein kinase C, respectively. No direct membrane-delimited pathway for Ca2+ channel regulation by activated Gs proteins could be detected in vascular smooth muscle cells.

The interactions of adenylate cyclases with P-site inhibitors

Recent kinetic, binding and crystallographic studies using P-site inhibitors of mammalian adenylate bases provide new insights into the catalytic mechanism of these highly regulated enzymes. Here, Carmen Dessauer and colleagues discuss the conformational states of adenylate cyclase, the structural determinants of inhibitor binding and the potential uses of these inhibitors as pharmacological agents.

Identification of a Gialpha binding site on type V adenylyl cyclase

The stimulatory G protein alpha subunit Gsalpha binds within a cleft in adenylyl cyclase formed by the alpha1-alpha2 and alpha3-beta4 loops of the C2 domain. The pseudosymmetry of the C1 and C2 domains of adenylyl cyclase suggests that the homologous inhibitory alpha subunit Gialpha could bind to the analogous cleft within C1. We demonstrate that myristoylated guanosine 5'-3-O-(thio)triphosphate-Gialpha1 forms a stable complex with the C1 (but not the C2) domain of type V adenylyl cyclase. Mutagenesis of the membrane-bound enzyme identified residues whose alteration either increased or substantially decreased the IC50 for inhibition by Gialpha1. These mutations suggest binding of Gialpha within the cleft formed by the alpha2 and alpha3 helices of C1, analogous to the Gsalpha binding site in C2. Adenylyl cyclase activity reconstituted by mixture of the C1 and C2 domains of type V adenylyl cyclase was also inhibited by Gialpha. The C1b domain of the type V enzyme contributed to affinity for Gialpha, but the source of C2 had little effect. Mutations in this soluble system faithfully reflected the phenotypes observed with the membrane-bound enzyme. The pseudosymmetrical structure of adenylyl cyclase permits bidirectional regulation of activity by homologous G protein alpha subunits.

The catalytic mechanism of mammalian adenylyl cyclase. Equilibrium binding and kinetic analysis of P-site inhibition

The mechanism of P-site inhibition of adenylyl cyclase has been probed by equilibrium binding measurements using 2'-[3H]deoxyadenosine, a P-site inhibitor, and by kinetic analysis of both the forward and reverse reactions (i.e. cyclic AMP and ATP synthesis, respectively). There is one binding site for 2'-deoxyadenosine per C1/C2 heterodimer; the Kd is 40 +/- 3 microM. Binding is observed only in the presence of one of the products of the adenylyl cyclase reaction, pyrophosphate (PPi). A substrate analog, Ap(CH2)pp (alpha,beta-methylene adenosine 5'-triphosphate), and cyclic AMP compete for the P-site in the presence of PPi, but P-site analogs do not compete for substrate binding (in the absence of PPi). Kinetic analysis indicates that release of products from the enzyme is random. These facts permit formulation of a model for the adenylyl cyclase reaction, for which we provide substantial kinetic support. We propose that P-site analogs act as dead-end inhibitors of product release, stabilizing an enzyme-product (E-PPi) complex by binding at the active site. Although product release is random, cyclic AMP dissociates from the enzyme preferentially. Release of PPi is slow and partially rate-limiting.

Interaction of Gsalpha with the cytosolic domains of mammalian adenylyl cyclase

Forskolin- and Gsalpha-stimulated adenylyl cyclase activity is observed after mixture of two independently-synthesized approximately 25-kDa cytosolic fragments derived from mammalian adenylyl cyclases (native Mr approximately 120,000). The C1a domain from type V adenylyl cyclase (VC1) and the C2 domain from type II adenylyl cyclase (IIC2) can both be expressed in large quantities and purified to homogeneity. When mixed, their maximally stimulated specific activity, 150 micromol/min/mg protein, substantially exceeds values observed previously with the intact enzyme. A soluble, high-affinity complex containing one molecule each of VC1, IIC2, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-Gsalpha is responsible for the observed enzymatic activity and can be isolated. In addition, GTPgammaS-Gsalpha interacts with homodimers of IIC2 to form a heterodimeric complex (one molecule each of Gsalpha and IIC2) but not detectably with homodimers of VC1. Nevertheless, Gsalpha can be cross-linked to VC1 in the activated heterotrimeric complex of VC1, IIC2, and Gsalpha, indicating its proximity to both components of the enzyme that are required for efficient catalysis. These results and those in the accompanying report (Dessauer, C. W., Scully, T. T., and Gilman, A. G. (1997) J. Biol. Chem. 272, 22272-22277) suggest that activators of adenylyl cyclase facilitate formation of a single, high-activity catalytic site at the interface between C1 and C2.

Interactions of forskolin and ATP with the cytosolic domains of mammalian adenylyl cyclase

Fragments of the two cytoplasmic domains of mammalian adenylyl cyclases can be synthesized independently (and abundantly) as soluble proteins; Gsalpha- and forskolin-stimulated enzymatic activity is restored upon their mixture. We have utilized this system to characterize the interactions of adenylyl cyclase with forskolin and its substrate, ATP. In the presence of Gsalpha, adenylyl cyclase is activated in response to occupation of only one forskolin-binding site. A single binding site for forskolin was identified by equilibrium dialysis; its Kd (0.1 microM) corresponds to the EC50 for enzyme activation. The affinity of forskolin for adenylyl cyclase is greatly reduced in the absence of Gsalpha ( approximately 40 microM). Binding of forskolin to the individual cytoplasmic domains of the enzyme was not detected. A single binding site for the ATP analog, alpha,beta-methylene ATP (Ap(CH2)pp), was also detected by equilibrium dialysis. Such binding was not observed with the individual domains. Binding of Ap(CH2)pp was unaffected by P-site inhibitors of adenylyl cyclase. A modified P-loop sequence located near the carboxyl terminus of adenylyl cyclase has been implicated in ATP binding. Mutation of the conserved, non-glycine residues within this region caused no significant changes in the Km for ATP or the Ki for Ap(CH2)pp. It thus seems unlikely that this region is part of the active site. However, a mutation in the C1 domain (E518A) causes a 10-fold decrease in the binding affinity for Ap(CH2)pp. This residue and the active site of the enzyme may lie at the interface between the two cytosolic domains.

Visualizing signal transduction: receptors, G-proteins, and adenylate cyclases

1. The first glimpses of heterotrimeric G-proteins came with the discoveries of the ubiquitous adenylate cyclase activator, Gs, and the specialized retinal cGMP phosphodiesterase activator, Gi or transducin. The model that evolved for regulation of adenylate cyclase activity by G-proteins soon proved to be a general paradigm for a large number of signalling pathways. Although many different G-proteins interact with a diverse array of receptors and effectors, each is composed of a guaninenucleotide-binding alpha-subunit and a tightly associated complex of a beta- and a gamma-subunit. 2. Receptors catalyse the activation of G-proteins by promoting exchange of GDP for GTP, while G-proteins catalyse their own deactivation as a result of their intrinsic GTPase activity. Crystallographic analysis has described several of the various conformational states that G-proteins undergo as they are activated and deactivated and has provided great insight into the kinetic models of G-protein-mediated signal transduction. 3. The regulation of adenylate cyclase has proven to be intriguing and complex. Gsx activates all forms of mammalian adenylate cyclase; other G-proteins (Gi, Go and Gz) inhibit certain isoforms of the enzyme. The discovery of new isoforms of adenylate cyclase has revealed synergistic and conditional mechanisms of regulation. These include activation or inhibition by the G-protein beta gamma-subunit complex, activation by Ca(2+)-calmodulin, and phosphorylation by protein kinases. The large number of receptors, G-proteins and adenylate cyclases provides a complex signalling network that integrates and interprets a multitude of convergent inputs.

Purification and characterization of a soluble form of mammalian adenylyl cyclase

An engineered, soluble form of mammalian adenylyl cyclase has been expressed in Escherichia coli and purified by three chromatographic steps. The enzyme utilizes one molecule of ATP to synthesize one molecule of cyclic AMP and pyrophosphate at a maximal specific activity of 12.8 micromol/min/mg, corresponding to a turnover number of 720 min-1. Although devoid of membrane spans, the enzyme displays all of the regulatory properties that are common to mammalian adenylyl cyclases. It is activated synergistically by Gsalpha and forskolin and is inhibited by adenosine (P-site) analogs with kinetic patterns that are identical to those displayed by the native enzymes. The purified enzyme is also inhibited directly by the G protein betagamma subunit complex. After adenovirus-mediated expression in adenylyl cyclase-deficient HC-1 cells, the enzyme can be stimulated synergistically by Gs-coupled receptors and forskolin.

Interaction of the two cytosolic domains of mammalian adenylyl cyclase

Adenylyl cyclase activity can be reconstituted by simple mixture of the two cytosolic domains of the enzyme after their independent synthesis in Escherichia coli. We have synthesized and purified the C1a domain of type I adenylyl cyclase and the C2 domain of the type II enzyme to assess their interactions with each other and with the activators Gsalpha and forskolin. In the absence of an activator, the fragments associate with low affinity and display low catalytic activity. This basal activity can be stimulated more than 100-fold by either forskolin or activated Gsalpha. Further, the addition of these activators increases the apparent affinity of the fragments for each other. Stimulation of catalysis by Gsalpha and forskolin is synergistic. These data suggest a model wherein either Gsalpha or forskolin enhances association of the other activator with adenylyl cyclase, as well as facilitating the interaction between the C1 and C2 domains of the enzyme.

Inhibition of an inwardly rectifying K+ channel by G-protein alpha-subunits

Cholinergic muscarinic, serotonergic, opioid and several other G-protein-coupled neurotransmitter receptors activate inwardly rectifying K+ channels of the GIRK family, slowing the heartbeat and decreasing the excitability of neuronal cells. Inhibitory modulation of GIRKs by G-protein-coupled receptors may have important implications in cardiac and brain physiology. Previously G alpha and G beta gamma subunits of heterotrimeric G proteins have both been implicated in channel opening, but recent studies attribute this role primarily to the G beta gamma dimer that activates GIRKs in a membrane-delimited fashion, probably by direct binding to the channel protein. We report here that free GTP gamma S-activated G alpha i 1, but not G alpha i 2 or G alpha i 3, potently inhibits G beta 1 gamma 2-induced GIRK activity in excised membrane patches of Xenopus oocytes expressing GIRK1. High-affinity but partial inhibition is produced by G alpha s-GTP gamma S. G alpha i 1-GTP gamma S also inhibits G beta 1 gamma 2-activated GIRK in atrial myocytes. Antagonistic interactions between G alpha and G beta gamma may be among the mechanisms determining specificity of G protein coupling to GIRKs.

Complexity and diversity of mammalian adenylyl cyclases

Molecular cloning has permitted identification of several novel isoforms of mammalian adenylyl cyclase; these proteins now comprise a family of at least 10. All of the membrane-bound enzymes are activated by the alpha subunit of G alpha, a receptor-regulated, heterotrimeric guanine nucleotide-binding protein, and by the diterpene forskolin. Certain cyclases are also activated by Ca(2+)-calmodulin, while some are inhibited by the alpha subunits of the three Gi proteins. The discovery of new isoforms has also revealed unanticipated mechanisms of regulation, including activation or inhibition by the G-protein beta gamma subunit complex, inhibition by G(o) alpha, inhibition by Ca2+, and phosphorylation by protein kinases C and A. The effects of activators are often highly synergistic or conditional, suggesting function of these enyzmes as coincidence detectors. The plethora of receptors, G proteins, and adenylyl cyclases permits assembly of very complex signaling systems with a wide variety of integrative characteristics.

Time resolved kinetics of direct G beta 1 gamma 2 interactions with the carboxyl terminus of Kir3.4 inward rectifier K+ channel subunits

The direct interaction of recombinant G beta 1 gamma 2 proteins with the carboxyl terminal domain of a G protein-gated inward rectifier K channel subunit, Kir3.4 (GIRK4), was measured in real time using biosensor chip technology. The carboxyl terminus of Kir3.4 (a.a. 186-419) was expressed in bacteria as a glutathione-S-transferase (GST) fusion protein, GST-Kir3. 4ct. GST-Kir3.4ct was immobilized to the surface of a biosensor chip by high affinity binding of the GST domain to a covalently attached anti-GST antibody. The association and dissociation rates of G beta 1 gamma 2 dimers with the immobilized Kir3.4ct domain were temporally resolved as a change in refractive index detected by surface plasmon resonance. Specific binding of G beta 1 gamma 2 dimers to Kir3.4ct was characterized by a dissociation rate (kd) of approximately 0.003 s-1. Association kinetics were dominated by a concentration-independent component (time constant approximately 50 s) which complicates models of binding and may indicate conformational changes during binding of G beta 1 gamma 2 to Kir3.4ct. The estimated equilibrium dissociation binding constant (Kd) was approximately 800 nM. These studies demonstrate that G beta gamma dimers interact directly with the Kir3.4 channel subunit, and suggest interesting details in the interaction with the major cytosolic carboxyl terminal domain. The slow G beta 1 gamma 2 dissociation rate measured on the sensor chip is similar in magnitude to a slow component of channel deactivation measured electrophysiologically in Xenopus oocytes expressing Kir3.1/3.4 multimeric channels and a G protein-coupled receptor. Biosensor-based experiments such as those described here will complement electrophysiological studies on the molecular basis of G protein interactions with Kir channels and other ion channel proteins.

Identification of a chaperonin binding site in a chloroplast precursor protein

Although chaperonin-assisted protein folding has been studied in vitro by a number of investigators, the feature(s) of the unfolded polypeptide that are recognized and bound by chaperonins is not known. We have addressed this question using the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pS) as a substrate for GroEL. The protein was expressed in Escherichia coli as a C-terminal fusion to protein A. Protein A-pS and any associated cellular proteins were then purified by affinity chromatography. GroEL could be eluted from the fusion protein by incubation with ATP and either GroES or casein, consistent with results of in vitro folding assays. At least half of the transit sequence of pS is required to maintain this high stoichiometry of chaperonin binding. Using deletion mutagenesis from the C terminus of pS, we defined the smallest truncation of pS, PAxpS90T, that binds GroEL with high avidity (dissociation constant = 53 nM). A series of site-specific mutations targeting the C-terminal 15-20 amino acids of PAxpS90T was constructed and analyzed for the ability to bind GroEL. Our results show that complex formation between GroEL and pS is not dependent upon any single feature such as overall hydrophobicity, a net positive charge, or secondary structure but may be dependent upon a combination of these features.