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Frederick JM, Hanke-Gogokhia C, Ying G, Baehr W. Diffuse or hitch a ride: how photoreceptor lipidated proteins get from here to there. Biol Chem 2019; 401:573-584. [DOI: 10.1515/hsz-2019-0375] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2019] [Accepted: 12/02/2019] [Indexed: 12/23/2022]
Abstract
Abstract
Photoreceptors are polarized neurons, with specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment (OS) where vision begins, an inner segment (IS) where protein synthesis occurs and a synaptic terminal for signal transmission to second-order neurons. The OS is a large, modified primary cilium attached to the IS by a slender connecting cilium (CC), the equivalent of the transition zone (TZ). Daily renewal of ~10% of the OS requires massive protein biosynthesis in the IS with reliable transport and targeting pathways. Transport of lipidated (‘sticky’) proteins depends on solubilization factors, phosphodiesterase δ (PDEδ) and uncoordinated protein-119 (UNC119), and the cargo dispensation factor (CDF), Arf-like protein 3-guanosine triphosphate (ARL3-GTP). As PDE6 and transducin still reside prominently in the OS of PDEδ and UNC119 germline knockout mice, respectively, we propose the existence of an alternate trafficking pathway, whereby lipidated proteins migrate in rhodopsin-containing vesicles of the secretory pathway.
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Affiliation(s)
- Jeanne M. Frederick
- Department of Ophthalmology and Visual Sciences , University of Utah Health Science Center , 65 Mario Capecchi Drive , Salt Lake City , UT 84132 , USA
| | - Christin Hanke-Gogokhia
- Department of Ophthalmology and Visual Sciences , University of Utah Health Science Center , 65 Mario Capecchi Drive , Salt Lake City , UT 84132 , USA
| | - Guoxin Ying
- Department of Ophthalmology and Visual Sciences , University of Utah Health Science Center , 65 Mario Capecchi Drive , Salt Lake City , UT 84132 , USA
| | - Wolfgang Baehr
- Department of Ophthalmology and Visual Sciences , University of Utah Health Science Center , 65 Mario Capecchi Drive , Salt Lake City , UT 84132 , USA
- Department of Neurobiology and Anatomy , University of Utah , Salt Lake City , UT 84112 , USA
- Department of Biology , University of Utah , Salt Lake City , UT 84132 , USA
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2
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Muntean BS, Patil DN, Madoux F, Fossetta J, Scampavia L, Spicer TP, Martemyanov KA. A High-Throughput Time-Resolved Fluorescence Energy Transfer Assay to Screen for Modulators of RGS7/Gβ5/R7BP Complex. Assay Drug Dev Technol 2019; 16:150-161. [PMID: 29658790 DOI: 10.1089/adt.2017.839] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023] Open
Abstract
G protein-coupled receptors (GPCRs) are excellent drug targets exploited by majority of the Food and Drug Administration-approved medications, but when modulated, are often accompanied by significant adverse effects. Targeting of other elements in GPCR pathways for improved safety and efficacy is thus an unmet need. The strength of GPCR signaling is tightly regulated by regulators of G protein signaling (RGS) proteins, making them attractive drug targets. We focused on a prominent RGS complex in the brain consisting of RGS7 and its binding partners Gβ5 and R7BP. These complexes play critical roles in regulating multiple GPCRs and essential physiological processes, yet no small molecule modulators are currently available to modify its function. In this study, we report a novel high-throughput approach to screen for small molecule modulators of the intramolecular transitions in the RGS7/Gβ5/R7BP complex known to be involved in its allosteric regulation. We developed a time-resolved fluorescence energy transfer-based in vitro assay that utilizes full-length recombinant proteins and shows consistency, excellent assay statistics, and high level of sensitivity. We demonstrated the potential of this approach by screening two compound libraries (LOPAC 1280 and MicroSource Spectrum). This study confirms the feasibility of the chosen strategy for identifying small molecule modulators of RGS7/Gβ5/R7BP complex for impacting signaling downstream of the GPCRs.
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Affiliation(s)
- Brian S Muntean
- 1 Department of Neuroscience, The Scripps Research Institute , Jupiter, Florida
| | - Dipak N Patil
- 1 Department of Neuroscience, The Scripps Research Institute , Jupiter, Florida
| | - Franck Madoux
- 2 Department of Molecular Medicine, The Scripps Research Institute , Jupiter, Florida
| | | | - Louis Scampavia
- 2 Department of Molecular Medicine, The Scripps Research Institute , Jupiter, Florida
| | - Timothy P Spicer
- 2 Department of Molecular Medicine, The Scripps Research Institute , Jupiter, Florida
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3
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Petrukhin OV, Orlova TG, Nezvetsky AR, Orlov NY. Modeling of phototransduction processes in the photoreceptor disk membranes by the Monte Carlo method. Biophysics (Nagoya-shi) 2016. [DOI: 10.1134/s000635091606021x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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4
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Orlandi C, Xie K, Masuho I, Fajardo-Serrano A, Lujan R, Martemyanov KA. Orphan Receptor GPR158 Is an Allosteric Modulator of RGS7 Catalytic Activity with an Essential Role in Dictating Its Expression and Localization in the Brain. J Biol Chem 2015; 290:13622-39. [PMID: 25792749 DOI: 10.1074/jbc.m115.645374] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2015] [Indexed: 11/06/2022] Open
Abstract
Regulators of G protein signaling control the duration and extent of signaling via G protein-coupled receptor (GPCR) pathways by accelerating the GTP hydrolysis on G protein α subunits thereby promoting termination of GPCR signaling. A member of this family, RGS7, plays a critical role in the nervous system where it regulates multiple neurotransmitter GPCRs that mediate vision, memory, and the action of addictive drugs. Previous studies have established that in vivo RGS7 forms mutually exclusive complexes with the membrane protein RGS7-binding protein or the orphan receptor GPR158. In this study, we examine the impact of GPR158 on RGS7 in the brain. We report that knock-out of GPR158 in mice results in marked post-transcriptional destabilization of RGS7 and substantial loss of its association with membranes in several brain regions. We further identified the RGS7-binding site in the C terminus of GPR158 and found that it shares significant homology with the RGS7-binding protein. The proximal portion of the GPR158 C terminus additionally contained a conserved sequence that was capable of enhancing RGS7 GTPase-activating protein activity in solution by an allosteric mechanism acting in conjunction with the regulators of the G protein signaling-binding domain. The distal portion of the GPR158 C terminus contained several phosphodiesterase E γ-like motifs and selectively recruited G proteins in their activated state. The results of this study establish GPR158 as an essential regulator of RGS7 in the native nervous system with a critical role in controlling its expression, membrane localization, and catalytic activity.
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Affiliation(s)
- Cesare Orlandi
- From the Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida 33458 and
| | - Keqiang Xie
- From the Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida 33458 and
| | - Ikuo Masuho
- From the Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida 33458 and
| | - Ana Fajardo-Serrano
- the Instituto de Investigación en Descapacidades Neuronales (IDINE), Departamento de Ciencias Médicas, Facultad de Medicina, Universidad de Castilla-La Mancha, 02006 Albacete, Spain
| | - Rafael Lujan
- the Instituto de Investigación en Descapacidades Neuronales (IDINE), Departamento de Ciencias Médicas, Facultad de Medicina, Universidad de Castilla-La Mancha, 02006 Albacete, Spain
| | - Kirill A Martemyanov
- From the Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida 33458 and
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5
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Petrukhin OV, Orlova TG, Nezvetsky AR, Orlov NY. Activation of bovine retinal rod outer segment cGMP-specific phosphodiesterase by the transducin-GTP complex in a physiologically significant range of free calcium ion concentrations. Biophysics (Nagoya-shi) 2014. [DOI: 10.1134/s0006350914050200] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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6
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Loss of retinoschisin (RS1) cell surface protein in maturing mouse rod photoreceptors elevates the luminance threshold for light-driven translocation of transducin but not arrestin. J Neurosci 2012; 32:13010-21. [PMID: 22993419 DOI: 10.1523/jneurosci.1913-12.2012] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Loss of retinoschisin (RS1) in Rs1 knock-out (Rs1-KO) retina produces a post-photoreceptor phenotype similar to X-linked retinoschisis in young males. However, Rs1 is expressed strongly in photoreceptors, and Rs1-KO mice have early reduction in the electroretinogram a-wave. We examined light-activated transducin and arrestin translocation in young Rs1-KO mice as a marker for functional abnormalities in maturing rod photoreceptors. We found a progressive reduction in luminance threshold for transducin translocation in wild-type (WT) retinas between postnatal days P18 and P60. At P21, the threshold in Rs1-KO retinas was 10-fold higher than WT, but it decreased to <2.5-fold higher by P60. Light-activated arrestin translocation and re-translocation of transducin in the dark were not affected. Rs1-KO rod outer segment (ROS) length was significantly shorter than WT at P21 but was comparable with WT at P60. These findings suggested a delay in the structural and functional maturation of Rs1-KO ROS. Consistent with this, transcription factors CRX and NRL, which are fundamental to maturation of rod protein expression, were reduced in ROS of Rs1-KO mice at P21 but not at P60. Expression of transducin was 15-30% lower in P21 Rs1-KO ROS and transducin GTPase hydrolysis was nearly twofold faster, reflecting a 1.7- to 2.5-fold increase in RGS9 (regulator of G-protein signaling) level. Transduction protein expression and activity levels were similar to WT at P60. Transducin translocation threshold elevation indicates photoreceptor functional abnormalities in young Rs1-KO mice. Rapid reduction in threshold coupled with age-related changes in transduction protein levels and transcription factor expression are consistent with delayed maturation of Rs1-KO photoreceptors.
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7
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Tachibanaki S, Yonetsu SI, Fukaya S, Koshitani Y, Kawamura S. Low activation and fast inactivation of transducin in carp cones. J Biol Chem 2012; 287:41186-94. [PMID: 23045532 DOI: 10.1074/jbc.m112.403717] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Cone photoreceptors show lower light sensitivity and briefer light responses than rod photoreceptors. The light detection signal in these cells is amplified through a phototransduction cascade. The first step of amplification in the cascade is the activation of a GTP-binding protein, transducin (Tr), by light-activated visual pigment (R*). We quantified transducin activation by measuring the binding of GTPγS in purified carp rod and cone membrane preparations with the use of a rapid quench apparatus and found that transducin activation by an R* molecule is ∼5 times less efficient in cones than in rods. Transducin activation terminated in less than 1 s in cones, more quickly than in rods. The rate of GTP hydrolysis in Tr*, and thus the rate of Tr* inactivation, was ∼25 times higher in cones than in rods. This faster inactivation of Tr* ensures briefer light responses in cones. The expression level of RGS9 was found to be ∼20 times higher in cones than in rods, which explains higher GTP hydrolytic activity and, thus, faster Tr* inactivation in cones than in rods. Although carp rods and cones express rod- or cone-versions of visual pigment and transducin, these molecules themselves do not seem to induce the differences significantly in the transducin activation and Tr* inactivation in rods and cones. Instead, the differences seem to be brought about in a rod or cone cell-type specific manner.
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Affiliation(s)
- Shuji Tachibanaki
- Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan
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8
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Constantine R, Zhang H, Gerstner CD, Frederick JM, Baehr W. Uncoordinated (UNC)119: coordinating the trafficking of myristoylated proteins. Vision Res 2012; 75:26-32. [PMID: 23000199 DOI: 10.1016/j.visres.2012.08.012] [Citation(s) in RCA: 52] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2012] [Revised: 08/16/2012] [Accepted: 08/17/2012] [Indexed: 01/06/2023]
Abstract
The mechanism by which myristoylated proteins are targeted to specific subcellular membrane compartments is poorly understood. Two novel acyl-binding proteins, UNC119A and UNC119B, have been shown recently to function as chaperones/co-factors in the transport of myristoylated G protein α-subunits and src-type tyrosine kinases. UNC119 polypeptides feature an immunoglobulin-like β-sandwich fold that forms a hydrophobic pocket capable of binding lauroyl (C12) and myristoyl (C14) side chains. UNC119A in rod photoreceptors facilitates the transfer of transducin α subunits (Tα) from inner segment to outer segment membranes by forming an intermediate diffusible UNC119-Tα complex. Similar complexes are formed in other sensory neurons, as the G proteins ODR-3 and GPA-13 in Caenorhabditis elegans unc-119 mutants traffic inappropriately. UNC119B knockdown in IMCD3 cells prevents trafficking ofmyristoylated nephrocystin-3 (NPHP3), a protein associated with nephronophthisis, to cilia. Further, UNC119A was shown to transport myristoylated src-type tyrosine kinases to cell membranes and to affect T-cell receptor (TCR) and interleukin-5 receptor (IL-5R) activities. These interactions establish UNC119 polypeptides as novel lipid-binding chaperones with specificity for a diverse subset of myristoylated proteins.
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Affiliation(s)
- Ryan Constantine
- Graduate Program in Neuroscience, University of Utah Health Science Center, Salt Lake City, UT 84132, USA
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9
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Zhang XJ, Gao XZ, Yao W, Cote RH. Functional mapping of interacting regions of the photoreceptor phosphodiesterase (PDE6) γ-subunit with PDE6 catalytic dimer, transducin, and regulator of G-protein signaling9-1 (RGS9-1). J Biol Chem 2012; 287:26312-20. [PMID: 22665478 DOI: 10.1074/jbc.m112.377333] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cGMP phosphodiesterase (PDE6) involved in visual transduction in photoreceptor cells contains two inhibitory γ-subunits (Pγ) which bind to the catalytic core (Pαβ) to inhibit catalysis and stimulate cGMP binding to the GAF domains of Pαβ. During visual excitation, interaction of activated transducin with Pγ relieves inhibition. Pγ also participates in a complex with RGS9-1 and other proteins to accelerate the GTPase activity of activated transducin. We studied the structural determinants for these important functions of Pγ. First, we identified two important sites in the middle region of Pγ (amino acids 27-38 and 52-54) that significantly stabilize the overall binding affinity of Pγ with Pαβ. The ability of Pγ to stimulate noncatalytic cGMP binding to the GAF domains of PDE6 has been localized to amino acids 27-30 of Pγ. Transducin activation of PDE6 catalysis critically depends on the presence of Ile54 in the glycine-rich region of Pγ in order to relieve inhibition of catalysis. The central glycine-rich region of Pγ is also required for transducin to increase cGMP exchange at the GAF domains. Finally, Thr-65 and/or Val-66 of Pγ are critical residues for Pγ to stimulate GTPase activity of transducin in a complex with RGS9-1. We propose that the glycine-rich region of Pγ is a primary docking site for PDE6-interacting proteins involved in the activation/inactivation pathways of visual transduction. This functional mapping of Pγ with its binding partners demonstrates the remarkable versatility of this multifunctional protein and its central role in regulating the activation and lifetime of visual transduction.
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Affiliation(s)
- Xiu-Jun Zhang
- Department of Molecular, Cellular, and Biomedical Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA
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10
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Zhang H, Constantine R, Vorobiev S, Chen Y, Seetharaman J, Huang YJ, Xiao R, Montelione GT, Gerstner CD, Davis MW, Inana G, Whitby FG, Jorgensen EM, Hill CP, Tong L, Baehr W. UNC119 is required for G protein trafficking in sensory neurons. Nat Neurosci 2011; 14:874-80. [PMID: 21642972 DOI: 10.1038/nn.2835] [Citation(s) in RCA: 133] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2011] [Indexed: 12/20/2022]
Abstract
UNC119 is widely expressed among vertebrates and other phyla. We found that UNC119 recognized the acylated N terminus of the rod photoreceptor transducin α (Tα) subunit and Caenorhabditis elegans G proteins ODR-3 and GPA-13. The crystal structure of human UNC119 at 1.95-Å resolution revealed an immunoglobulin-like β-sandwich fold. Pulldowns and isothermal titration calorimetry revealed a tight interaction between UNC119 and acylated Gα peptides. The structure of co-crystals of UNC119 with an acylated Tα N-terminal peptide at 2.0 Å revealed that the lipid chain is buried deeply into UNC119's hydrophobic cavity. UNC119 bound Tα-GTP, inhibiting its GTPase activity, thereby providing a stable UNC119-Tα-GTP complex capable of diffusing from the inner segment back to the outer segment after light-induced translocation. UNC119 deletion in both mouse and C. elegans led to G protein mislocalization. Thus, UNC119 is a Gα subunit cofactor essential for G protein trafficking in sensory cilia.
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Affiliation(s)
- Houbin Zhang
- Department of Ophthalmology, University of Utah Health Science Center, Salt Lake City, Utah, USA
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11
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Membrane anchoring subunits specify selective regulation of RGS9·Gbeta5 GAP complex in photoreceptor neurons. J Neurosci 2010; 30:13784-93. [PMID: 20943919 DOI: 10.1523/jneurosci.1191-10.2010] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
The RGS9·Gβ5 complex is the key regulator of neuronal G-protein signaling and shows remarkable selectivity of subunit composition. In retinal photoreceptors, RGS9·Gβ5 is bound to the membrane anchor R9AP and the complex regulates visual signaling. In the basal ganglia neurons, RGS9·Gβ5 is instead associated with a homologous protein, R7BP, and regulates reward circuit. Switching this selective subunit composition of the complex in rod photoreceptors allowed us to study the molecular underpinning of signaling specificity in diverse G-protein pathways. We have found that both membrane anchoring subunits play a conserved role in regulating protein levels of RGS9·Gβ5 and enhancing the ability of RGS·Gβ5 complexes to stimulate GTPase activity of G proteins. However, notable differences exist in the subcellular targeting of alternatively configured complexes. Unlike R9AP, which relies on passive targeting mechanisms for the delivery to the outer segments of the photoreceptors, R7BP is excluded from this location and is instead specifically targeted to the plasma membrane. R7BP-containing complexes could be rerouted to the outer segments, where they are capable of regulating the phototransduction cascade by the active targeting signals derived from rhodopsin. These findings illustrate the diversity of the G-protein signaling regulation by RGS·Gβ5 complexes achieved by differential recruitment of the membrane anchors.
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12
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Masuho I, Celver J, Kovoor A, Martemyanov KA. Membrane anchor R9AP potentiates GTPase-accelerating protein activity of RGS11 x Gbeta5 complex and accelerates inactivation of the mGluR6-G(o) signaling. J Biol Chem 2009; 285:4781-7. [PMID: 20007977 DOI: 10.1074/jbc.m109.058511] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The R7 subfamily of RGS proteins critically regulates neuronal G protein-signaling pathways that are essential for vision, nociception, motor coordination, and reward processing. A member of the R7 RGS family, RGS11, is a GTPase-accelerating protein specifically expressed in retinal ON-bipolar cells where it forms complexes with the atypical G protein beta subunit, Gbeta(5), and transmembrane protein R9AP. Association with R9AP has been shown to be critical for the proteolytic stability of the complex in the retina. In this study we report that R9AP can in addition stimulate the GTPase-accelerating protein activity of the RGS11 x Gbeta(5) complex at Galpha(o). Single turnover GTPase assays reveal that R9AP co-localizes RGS11 x Gbeta(5) and Galpha(o) on the membrane and allosterically potentiates the GTPase-accelerating function of RGS11 x Gbeta(5). Reconstitution of mGluR6-Galpha(o) signaling in Xenopus oocytes indicates that RGS11 x Gbeta(5)-mediated GTPase acceleration in this system requires co-expression of R9AP. The results provide new insight into the regulation of mGluR6-Galpha(o) signaling by the RGS11 x Gbeta(5) x R9AP complex and establish R9AP as a general GTPase-accelerating protein activity regulator of R7 RGS complexes.
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Affiliation(s)
- Ikuo Masuho
- Department of Pharmacology, University of Minnesota, Minneapolis, Minnesota 55455, USA
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13
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Hanson SM, Van Eps N, Francis DJ, Altenbach C, Vishnivetskiy SA, Arshavsky VY, Klug CS, Hubbell WL, Gurevich VV. Structure and function of the visual arrestin oligomer. EMBO J 2007; 26:1726-36. [PMID: 17332750 PMCID: PMC1829381 DOI: 10.1038/sj.emboj.7601614] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2006] [Accepted: 01/26/2007] [Indexed: 01/24/2023] Open
Abstract
A distinguishing feature of rod arrestin is its ability to form oligomers at physiological concentrations. Using visible light scattering, we show that rod arrestin forms tetramers in a cooperative manner in solution. To investigate the structure of the tetramer, a nitroxide side chain (R1) was introduced at 18 different positions. The effects of R1 on oligomer formation, EPR spectra, and inter-spin distance measurements all show that the structures of the solution and crystal tetramers are different. Inter-subunit distance measurements revealed that only arrestin monomer binds to light-activated phosphorhodopsin, whereas both monomer and tetramer bind microtubules, which may serve as a default arrestin partner in dark-adapted photoreceptors. Thus, the tetramer likely serves as a 'storage' form of arrestin, increasing the arrestin-binding capacity of microtubules while readily dissociating to supply active monomer when it is needed to quench rhodopsin signaling.
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Affiliation(s)
- Susan M Hanson
- Department of Pharmacology, Vanderbilt University, Nashville, TN, USA
| | - Ned Van Eps
- Jules Stein Eye Institute and Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA, USA
| | - Derek J Francis
- Department of Biophysics, Medical College of Wisconsin, Milwaukee, WI, USA
| | - Christian Altenbach
- Jules Stein Eye Institute and Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA, USA
| | | | - Vadim Y Arshavsky
- Albert Eye Research Institute, Duke University Medical Center, Durham, NC, USA
| | - Candice S Klug
- Department of Biophysics, Medical College of Wisconsin, Milwaukee, WI, USA
- Department of Biophysics, Medical College of Wisconsin, Milwaukee, WI 53226, USA; Tel.: +1 414 456 4015; Fax: +1 414 456 6512; E-mail:
| | - Wayne L Hubbell
- Jules Stein Eye Institute and Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA, USA
- Jules Stein Eye Institute and Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA 90095, USA; Tel.: +1 310 206 8830; Fax: +1 310 794 2144; E-mail:
| | - Vsevolod V Gurevich
- Department of Pharmacology, Vanderbilt University, Nashville, TN, USA
- Department of Pharmacology, Vanderbilt University, Nashville, TN 37232, USA. Tel.: +1 615 322 7070; Fax: +1 615 343 6532; E-mail:
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14
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Grant JE, Guo LW, Vestling MM, Martemyanov KA, Arshavsky VY, Ruoho AE. The N terminus of GTP gamma S-activated transducin alpha-subunit interacts with the C terminus of the cGMP phosphodiesterase gamma-subunit. J Biol Chem 2006; 281:6194-202. [PMID: 16407279 DOI: 10.1074/jbc.m509511200] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Dynamic regulation of G-protein signaling in the phototransduction cascade ensures the high temporal resolution of vision. In a key step, the activated alpha-subunit of transducin (Galphat-GTP) activates the cGMP phosphodiesterase (PDE) by binding the inhibitory gamma-subunit (PDEgamma). Significant progress in understanding the interaction between Galphat and PDEgamma was achieved by solving the crystal structure of the PDEgamma C-terminal peptide bound to Galphat in the transition state for GTP hydrolysis (Slep, K. C., Kercher, M. A., He, W., Cowan, C. W., Wensel, T. G., and Sigler, P. B. (2001) Nature 409, 1071-1077). However, some of the structural elements of each molecule were absent in the crystal structure. We have probed the binding surface between the PDEgamma C terminus and activated Galphat bound to guanosine 5'-O-(3-thio)-triphosphate (GTPgammaS) using a series of full-length PDEgamma photoprobes generated by intein-mediated expressed protein ligation. For each of seven PDEgamma photoprobe species, expressed protein ligation allowed one benzoyl-L-phenylalaine substitution at selected hydrophobic C-terminal positions, and the addition of a biotin affinity tag at the extreme C terminus. We have detected photocross-linking from several PDEgamma C-terminal positions to the Galphat-GTPgammaS N terminus, particularly from PDEgamma residue 73. The overall percentage of cross-linking to the Galphat-GTPgammaSN terminus was analyzed using a far Western method for examining Galphat-GTPgammaS proteolytic digestion patterns. Furthermore, mass spectrometric analysis of cross-links to Galphat from a benzoyl-phenylalanine replacement at PDEgamma position 86 localized the region of photoinsertion to Galphat N-terminal residues Galphat-(22-26). This novel Galphat/PDEgamma interaction suggests that the transducin N terminus plays an active role in signal transduction.
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Affiliation(s)
- Jennifer E Grant
- Department of Pharmacology, University of Wisconsin Medical School, Madison, 53706, USA
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15
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Abstract
The experimental strategies developed in kinetic studies of interactions between RGS9 isoforms with G proteins of the Gi subfamily provide a useful framework for conducting similar studies with essentially any regulator of G-protein signaling (RGS) protein-G-protein pair. This article describes two major kinetic approaches used in the studies of RGS9 isoforms: single turnover and multiple turnover GTPase assays. We also describe pull-down assays as a method complementary to the kinetic assays. The discussion of the strengths and limitations of each individual assay emphasizes the importance of combining multiple experimental approaches in order to obtain comprehensive and internally consistent information regarding the mechanisms of RGS protein action.
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Affiliation(s)
- Kirill A Martemyanov
- Howe Laboratory of Ophthalmology, Harvard Medical School and Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA
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16
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Krapivinsky G, Medina I, Krapivinsky L, Gapon S, Clapham DE. SynGAP-MUPP1-CaMKII Synaptic Complexes Regulate p38 MAP Kinase Activity and NMDA Receptor- Dependent Synaptic AMPA Receptor Potentiation. Neuron 2004; 43:563-74. [PMID: 15312654 DOI: 10.1016/j.neuron.2004.08.003] [Citation(s) in RCA: 214] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2004] [Revised: 06/01/2004] [Accepted: 07/28/2004] [Indexed: 01/07/2023]
Abstract
The synapse contains densely localized and interacting proteins that enable it to adapt to changing inputs. We describe a Ca2+-sensitive protein complex involved in the regulation of AMPA receptor synaptic plasticity. The complex is comprised of MUPPI, a multi-PDZ domain-containing protein; SynGAP, a synaptic GTPase-activating protein; and the Ca2+/calmodulin-dependent kinase CaMKII. In synapses of hippocampal neurons, SynGAP and CaMKII are brought together by direct physical interaction with the PDZ domains of MUPP1, and in this complex, SynGAP is phosphorylated. Ca2+CaM binding to CaMKII dissociates it from the MUPP1 complex, and Ca2+ entering via the NMDAR drives the dephosphorylation of SynGAP. Specific peptide-induced SynGAP dissociation from the MUPP1-CaMKII complex results in SynGAP dephosphorylation accompanied by P38 MAPK inactivation, potentiation of synaptic AMPA responses, and an increase in the number of AMPAR-containing clusters in hippocampal neuron synapses. siRNA-mediated SynGAP knockdown confirmed these results. These data implicate SynGAP in NMDAR- and CaMKII-dependent regulation of AMPAR trafficking.
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Affiliation(s)
- Grigory Krapivinsky
- Howard Hughes Medical Institute, Children's Hospital, 1309 Enders Building, 320 Longwood Avenue, Boston, Massachusetts 02115, USA
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17
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Hu G, Wensel TG. Characterization of R9AP, a membrane anchor for the photoreceptor GTPase-accelerating protein, RGS9-1. Methods Enzymol 2004; 390:178-96. [PMID: 15488178 DOI: 10.1016/s0076-6879(04)90012-2] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The proper recovery of photoreceptor light responses requires timely inactivation of the G-protein transducin (Gt) by GTP hydrolysis. It is now well established that the GTPase-accelerating protein (GAP) RGS9-1 plays an important role in determining the recovery kinetics of photoresponses. RGS9-1 has been found to be anchored to photoreceptor disk membranes by a novel photoreceptor protein, R9AP. R9AP has a single transmembrane domain at its C-terminal region. Membrane tethering by R9AP enhances RGS9-1 GAP activity in vitro and has been hypothesized to be important for the regulation of RGS9-1 function in vivo. In addition, R9AP shows structural similarity to the SNARE complex protein syntaxin and has been shown to be required for the correct targeting and localization of the RGS9-1 protein in photoreceptors. Therefore, R9AP may have additional functions other than that in the phototransduction pathway. This article presents methods and protocols developed for the functional characterization of R9AP in phototransduction, including the immunoprecipitation of the endogenous protein, the expression and purification of recombinant proteins, the reconstitution of proteoliposomes, and assays for its interaction with RGS9-1 and its effects on RGS9-1 GAP activity. These methods may also be applied to the study of R9AP function in other pathways or other cell types or to the studies of other membrane proteins that are structurally similar to R9AP.
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Affiliation(s)
- Guang Hu
- Brigham and Women's Hospital, Department of Medicine, Boston, Massachusetts 02115, USA
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18
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Hu G, Zhang Z, Wensel TG. Activation of RGS9-1GTPase acceleration by its membrane anchor, R9AP. J Biol Chem 2003; 278:14550-4. [PMID: 12560335 DOI: 10.1074/jbc.m212046200] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The GTPase-accelerating protein (GAP) complex RGS9-1.G beta(5) plays an important role in the kinetics of light responses by accelerating the GTP hydrolysis of G alpha(t) in vertebrate photoreceptors. Much, but not all, of this complex is tethered to disk membranes by the transmembrane protein R9AP. To determine the effect of the R9AP membrane complex on GAP activity, we purified recombinant R9AP and reconstituted it into lipid vesicles along with the photon receptor rhodopsin. Full-length RGS9-1.G beta(5) bound to R9AP-containing vesicles with high affinity (K(d) < 10 nm), but constructs lacking the DEP (dishevelled/EGL-10/pleckstrin) domain bound with much lower affinity, and binding of those lacking the entire N-terminal domain (i.e. the dishevelled/EGL-10/pleckstrin domain plus intervening domain) was not detectable. Formation of the membrane-bound complex with R9AP increased RGS9-1 GAP activity by a factor of 4. Vesicle titrations revealed that on the time scale of phototransduction, the entire reaction sequence from GTP uptake to GAP-catalyzed hydrolysis is a membrane-delimited process, and exchange of G alpha(t) between membrane surfaces is much slower than hydrolysis. Because in rod cells different pools exist of RGS9-1.G beta(5) that are either associated with R9AP or not, regulation of the association between R9AP and RGS9-1.G beta(5) represents a potential mechanism for the regulation of recovery kinetics.
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Affiliation(s)
- Guang Hu
- Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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19
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Martemyanov KA, Arshavsky VY. Noncatalytic domains of RGS9-1.Gbeta 5L play a decisive role in establishing its substrate specificity. J Biol Chem 2002; 277:32843-8. [PMID: 12093815 DOI: 10.1074/jbc.m205170200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The complex between the photoreceptor-specific regulator of G protein signaling (RGS) protein, RGS9-1, and type 5 G protein beta-subunit, Gbeta5L, regulates the duration of the cellular response to light by stimulating the GTPase activity of G protein, transducin. An important property of RGS9-1.Gbeta5L is that it interacts specifically with transducin bound to its effector, cGMP phosphodiesterase, rather than with transducin alone. The minimal structure within the RGS9-1.Gbeta5L complex capable of activating transducin GTPase is the catalytic domain of RGS9. This domain itself is also able to discriminate between free and effector-bound transducin but to a lesser degree than RGS9-1.Gbeta5L. The goal of this study was to determine whether other, noncatalytic domains of RGS9-1.Gbeta5L enhance the intrinsic specificity of the catalytic domain or whether they set the specificity of RGS9-1.Gbeta5L regardless of the specificity of its catalytic domain. We found that a double L353E/R360P amino acid substitution reversed the specificity of the recombinant catalytic domain but did not reverse the specificity of RGS9-1.Gbeta5L. However, the degree of discrimination between free and effector-bound transducin was reduced. Therefore, noncatalytic domains of RGS9-1.Gbeta5L play a decisive role in establishing its substrate specificity, yet the high degree of this specificity observed under physiological conditions requires an additional contribution from the catalytic domain.
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Affiliation(s)
- Kirill A Martemyanov
- Department of Ophthalmology, Harvard Medical School and the Massachusetts Eye and Ear Infirmary, Boston Massachusetts 02114, USA
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20
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Lishko PV, Martemyanov KA, Hopp JA, Arshavsky VY. Specific binding of RGS9-Gbeta 5L to protein anchor in photoreceptor membranes greatly enhances its catalytic activity. J Biol Chem 2002; 277:24376-81. [PMID: 12006596 DOI: 10.1074/jbc.m203237200] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The complex between the short splice variant of the ninth member of the RGS protein family and the long splice variant of type 5 G protein beta subunit (RGS9-Gbeta5L) plays a critical role in regulating the duration of the light response in vertebrate photoreceptors by activating the GTPase activity of the photoreceptor-specific G protein, transducin. RGS9-Gbeta5L is tightly associated with the membranes of photoreceptor outer segments; however, the nature of this association remains unknown. Here we demonstrate that rod outer segment membranes contain a limited number of sites for high affinity RGS9-Gbeta5L binding, which are highly sensitive to proteolysis. In membranes isolated from bovine rod outer segments, all of these sites are occupied by the endogenous RGS9-Gbeta5L, which prevents the binding of exogenous recombinant RGS9-Gbeta5L to these sites. However, treating membranes with urea or high pH buffers causes either removal or denaturation of the endogenous RGS9-Gbeta5L, allowing for high affinity binding of recombinant RGS9-Gbeta5L to these sites. This binding results in a striking approximately 70-fold increase in the RGS9-Gbeta5L ability to activate transducin GTPase. The DEP (disheveled/EGL-10/pleckstrin) domain of RGS9 plays a crucial role in the RGS9-Gbeta5L membrane attachment, as evident from the analysis of membrane-binding properties of deletion mutants lacking either N- or C-terminal parts of the RGS9 molecule. Our data indicate that specific association of RGS9-Gbeta5L with photoreceptor disc membranes serves not only as a means of targeting it to an appropriate subcellular compartment but also serves as an important determinant of its catalytic activity.
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Affiliation(s)
- Polina V Lishko
- Department of Ophthalmology, Harvard Medical School and the Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA
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21
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Affiliation(s)
- Wei He
- Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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22
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Lichtarge O, Sowa ME, Philippi A. Evolutionary traces of functional surfaces along G protein signaling pathway. Methods Enzymol 2002; 344:536-56. [PMID: 11771409 DOI: 10.1016/s0076-6879(02)44739-8] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Olivier Lichtarge
- Department of Molecular and Human Genetics, Program in Structural and Computational Biology and Molecular Biophysics, Baylor Human Genome Sequencing Center, Program in Developmental Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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23
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Skiba NP, Martemyanov KA, Elfenbein A, Hopp JA, Bohm A, Simonds WF, Arshavsky VY. RGS9-G beta 5 substrate selectivity in photoreceptors. Opposing effects of constituent domains yield high affinity of RGS interaction with the G protein-effector complex. J Biol Chem 2001; 276:37365-72. [PMID: 11495924 DOI: 10.1074/jbc.m106431200] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
RGS proteins regulate the duration of G protein signaling by increasing the rate of GTP hydrolysis on G protein alpha subunits. The complex of RGS9 with type 5 G protein beta subunit (G beta 5) is abundant in photoreceptors, where it stimulates the GTPase activity of transducin. An important functional feature of RGS9-G beta 5 is its ability to activate transducin GTPase much more efficiently after transducin binds to its effector, cGMP phosphodiesterase. Here we show that different domains of RGS9-G beta 5 make opposite contributions toward this selectivity. G beta 5 bound to the G protein gamma subunit-like domain of RGS9 acts to reduce RGS9 affinity for transducin, whereas other structures restore this affinity specifically for the transducin-phosphodiesterase complex. We suggest that this mechanism may serve as a general principle conferring specificity of RGS protein action.
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Affiliation(s)
- N P Skiba
- Department of Ophthalmology, Harvard Medical School, Boston, Massachusetts 02114, USA
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24
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He W, Lu L, Zhang X, El-Hodiri HM, Chen CK, Slep KC, Simon MI, Jamrich M, Wensel TG. Modules in the photoreceptor RGS9-1.Gbeta 5L GTPase-accelerating protein complex control effector coupling, GTPase acceleration, protein folding, and stability. J Biol Chem 2000; 275:37093-100. [PMID: 10978345 DOI: 10.1074/jbc.m006982200] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
RGS (regulators of G protein signaling) proteins regulate G protein signaling by accelerating GTP hydrolysis, but little is known about regulation of GTPase-accelerating protein (GAP) activities or roles of domains and subunits outside the catalytic cores. RGS9-1 is the GAP required for rapid recovery of light responses in vertebrate photoreceptors and the only mammalian RGS protein with a defined physiological function. It belongs to an RGS subfamily whose members have multiple domains, including G(gamma)-like domains that bind G(beta)(5) proteins. Members of this subfamily play important roles in neuronal signaling. Within the GAP complex organized around the RGS domain of RGS9-1, we have identified a functional role for the G(gamma)-like-G(beta)(5L) complex in regulation of GAP activity by an effector subunit, cGMP phosphodiesterase gamma and in protein folding and stability of RGS9-1. The C-terminal domain of RGS9-1 also plays a major role in conferring effector stimulation. The sequence of the RGS domain determines whether the sign of the effector effect will be positive or negative. These roles were observed in vitro using full-length proteins or fragments for RGS9-1, RGS7, G(beta)(5S), and G(beta)(5L). The dependence of RGS9-1 on G(beta)(5) co-expression for folding, stability, and function has been confirmed in vivo using transgenic Xenopus laevis. These results reveal how multiple domains and regulatory polypeptides work together to fine tune G(talpha) inactivation.
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Affiliation(s)
- W He
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology and the Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas 77030, USA
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25
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Skiba NP, Hopp JA, Arshavsky VY. The effector enzyme regulates the duration of G protein signaling in vertebrate photoreceptors by increasing the affinity between transducin and RGS protein. J Biol Chem 2000; 275:32716-20. [PMID: 10973941 DOI: 10.1074/jbc.c000413200] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The photoreceptor-specific G protein transducin acts as a molecular switch, stimulating the activity of its downstream effector in its GTP-bound form and inactivating the effector upon GTP hydrolysis. This activity makes the rate of transducin GTPase an essential factor in determining the duration of photoresponse in vertebrate rods and cones. In photoreceptors, the slow intrinsic rate of transducin GTPase is accelerated by the complex of the ninth member of the regulators of G protein signaling family with the long splice variant of type 5 G protein beta subunit (RGS9.Gbeta5L). However, physiologically rapid GTPase is observed only when transducin forms a complex with its effector, the gamma subunit of cGMP phosphodiesterase (PDEgamma). In this study, we addressed the mechanism by which PDEgamma regulates the rate of transducin GTPase. We found that RGS9.Gbeta5L alone has a significant ability to activate transducin GTPase, but its affinity for transducin is low. PDEgamma acts by enhancing the affinity between activated transducin and RGS9.Gbeta5L by more than 15-fold, which is evident both from kinetic measurements of transducin GTPase rate and from protein binding assays with immobilized transducin. Furthermore, our data indicate that a single RGS9.Gbeta5L molecule is capable of accelerating the GTPase activity of approximately 100 transducin molecules/s. This rate is faster than the rates reported previously for any RGS protein and is sufficient for timely photoreceptor recovery in both rod and cone photoreceptors.
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Affiliation(s)
- N P Skiba
- Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston Massachusetts 02114, USA
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26
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Sowa ME, He W, Wensel TG, Lichtarge O. A regulator of G protein signaling interaction surface linked to effector specificity. Proc Natl Acad Sci U S A 2000; 97:1483-8. [PMID: 10677488 PMCID: PMC26460 DOI: 10.1073/pnas.030409597] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Proteins of the regulator of G protein signaling (RGS) family accelerate GTP hydrolysis by the alpha subunits (G(alpha)) of G proteins, leading to rapid recovery of signaling cascades. Many different RGS proteins can accelerate GTP hydrolysis by an individual G(alpha), and GTP hydrolysis rates of different G(alpha)s can be enhanced by the same RGS protein. Consequently, the mechanisms for specificity in RGS regulation and the residues involved remain unclear. Using the evolutionary trace (ET) method, we have identified a cluster of residues in the RGS domain that includes the RGS-G(alpha) binding interface and extends to include additional functionally important residues on the surface. One of these is within helix alpha3, two are in alpha5, and three are in the loop connecting alpha5 and alpha6. A cluster of surface residues on G(alpha) previously identified by ET, and composed predominantly of residues from the switch III region and helix alpha3, is spatially contiguous with the ET-identified residues in the RGS domain. This cluster includes residues proposed to interact with the gamma subunit of G(talpha)'s effector, cGMP phosphodiesterase (PDEgamma). The proximity of these clusters suggests that they form part of an interface between the effector and the RGS-G(alpha) complex. Sequence variations in these residues correlate with PDEgamma effects on GTPase acceleration. Because ET identifies residues important for all members of a protein family, these residues likely form a general site for regulation of G protein-coupled signaling cascades, possibly by means of effector interactions.
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Affiliation(s)
- M E Sowa
- Verna and Marrs McLean Department of Biochemistry, Structural and Computational Biology and Molecular Biophysics Program, Baylor College of Medicine, Houston, TX 77030, USA
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27
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Chen CK, Burns ME, He W, Wensel TG, Baylor DA, Simon MI. Slowed recovery of rod photoresponse in mice lacking the GTPase accelerating protein RGS9-1. Nature 2000; 403:557-60. [PMID: 10676965 DOI: 10.1038/35000601] [Citation(s) in RCA: 373] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Timely deactivation of the alpha-subunit of the rod G-protein transducin (Galphat) is essential for the temporal resolution of rod vision. Regulators of G-protein signalling (RGS) proteins accelerate hydrolysis of GTP by the alpha-subunits of heterotrimeric G proteins in vitro. Several retinal RGS proteins can act in vitro as GTPase accelerating proteins (GAP) for Galphat. Recent reconstitution experiments indicate that one of these, RGS9-1, may account for much of the Galphat GAP activity in rod outer segments (ROS). Here we report that ROS membranes from mice lacking RGS9-1 hydrolyse GTP more slowly than ROS membranes from control mice. The Gbeta5-L protein that forms a complex with RGS9-1 was absent from RGS9-/- retinas, although Gbeta5-L messenger RNA was still present. The flash responses of RGS9-/- rods rose normally, but recovered much more slowly than normal. We conclude that RGS9-1, probably in a complex with Gbeta5-L, is essential for acceleration of hydrolysis of GTP by Galphat and for normal recovery of the photoresponse.
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Affiliation(s)
- C K Chen
- Division of Biology, California Institute of Technology, Pasadena 91125, USA
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