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Guo J, Keegan RM, Rigden DJ, Erskine PT, Wood SP, Li S, Cooper JB. The X-ray structure of juvenile hormone diol kinase from the silkworm Bombyx mori. Acta Crystallogr F Struct Biol Commun 2021; 77:465-472. [PMID: 34866602 PMCID: PMC8647211 DOI: 10.1107/s2053230x21012012] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2021] [Accepted: 11/10/2021] [Indexed: 11/11/2022] Open
Abstract
Insect juvenile hormones (JHs) are a family of sesquiterpenoid molecules that are secreted into the haemolymph. JHs have multiple roles in insect development, metamorphosis and sexual maturation. A number of pesticides work by chemically mimicking JHs, thus preventing insects from developing and reproducing normally. The haemolymph levels of JH are governed by the rates of its biosynthesis and degradation. One enzyme involved in JH catabolism is JH diol kinase (JHDK), which uses ATP (or GTP) to phosphorylate JH diol to JH diol phosphate, which can be excreted. The X-ray structure of JHDK from the silkworm Bombyx mori has been determined at a resolution of 2.0 Å with an R factor of 19.0% and an Rfree of 24.8%. The structure possesses three EF-hand motifs which are occupied by calcium ions. This is in contrast to the recently reported structure of the JHDK-like-2 protein from B. mori (PDB entry 6kth), which possessed only one calcium ion. Since JHDK is known to be inhibited by calcium ions, it is likely that our structure represents the calcium-inhibited form of the enzyme. The electrostatic surface of the protein suggests a binding site for the triphosphate of ATP close to the N-terminal end of the molecule in a cavity between the N- and C-terminal domains. Superposition with a number of calcium-activated photoproteins suggests that there may be parallels between the binding of JH diol to JHDK and the binding of luciferin to aequorin.
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Affiliation(s)
- Jingxu Guo
- Division of Medicine, UCL, Gower Street, London WC1E 6BT, United Kingdom
- Department of Medicine, University of Cambridge School of Clinical Medicine, Cambridge CB2 0QQ, United Kingdom
| | - Ronan M. Keegan
- Scientific Computing Department, Science and Technologies Facilities Council, UK Research and Innovation, Didcot, United Kingdom
| | - Daniel J. Rigden
- Institute of Systems, Molecular and Integrative Biology, University of Liverpool, Biosciences Building, Crown Street, Liverpool L69 7BE, United Kingdom
| | - Peter T. Erskine
- Division of Medicine, UCL, Gower Street, London WC1E 6BT, United Kingdom
- Department of Biological Sciences, Birkbeck, University of London, Malet Street, Bloomsbury, London WC1E 7HX, United Kingdom
| | - Steve P. Wood
- Division of Medicine, UCL, Gower Street, London WC1E 6BT, United Kingdom
- Institute of Biomedical and Biomolecular Science, School of Biological Sciences, University of Portsmouth, King Henry Building, Portsmouth PO1 2DY, United Kingdom
| | - Sheng Li
- Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, 320 Yue Yang Road, Shanghai 200031, People’s Republic of China
| | - Jonathan B. Cooper
- Division of Medicine, UCL, Gower Street, London WC1E 6BT, United Kingdom
- Department of Biological Sciences, Birkbeck, University of London, Malet Street, Bloomsbury, London WC1E 7HX, United Kingdom
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Erskine PT, Fokas A, Muriithi C, Rehman H, Yates LA, Bowyer A, Findlow IS, Hagan R, Werner JM, Miles AJ, Wallace BA, Wells SA, Wood SP, Cooper JB. X-ray, spectroscopic and normal-mode dynamics of calexcitin: structure-function studies of a neuronal calcium-signalling protein. ACTA ACUST UNITED AC 2015; 71:615-31. [PMID: 25760610 DOI: 10.1107/s1399004714026704] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2014] [Accepted: 12/04/2014] [Indexed: 01/28/2023]
Abstract
The protein calexcitin was originally identified in molluscan photoreceptor neurons as a 20 kDa molecule which was up-regulated and phosphorylated following a Pavlovian conditioning protocol. Subsequent studies showed that calexcitin regulates the voltage-dependent potassium channel and the calcium-dependent potassium channel as well as causing the release of calcium ions from the endoplasmic reticulum (ER) by binding to the ryanodine receptor. A crystal structure of calexcitin from the squid Loligo pealei showed that the fold is similar to that of another signalling protein, calmodulin, the N- and C-terminal domains of which are known to separate upon calcium binding, allowing interactions with the target protein. Phosphorylation of calexcitin causes it to translocate to the cell membrane, where its effects on membrane excitability are exerted and, accordingly, L. pealei calexcitin contains two protein kinase C phosphorylation sites (Thr61 and Thr188). Thr-to-Asp mutations which mimic phosphorylation of the protein were introduced and crystal structures of the corresponding single and double mutants were determined, which suggest that the C-terminal phosphorylation site (Thr188) exerts the greatest effects on the protein structure. Extensive NMR studies were also conducted, which demonstrate that the wild-type protein predominantly adopts a more open conformation in solution than the crystallographic studies have indicated and, accordingly, normal-mode dynamic simulations suggest that it has considerably greater capacity for flexible motion than the X-ray studies had suggested. Like calmodulin, calexcitin consists of four EF-hand motifs, although only the first three EF-hands of calexcitin are involved in binding calcium ions; the C-terminal EF-hand lacks the appropriate amino acids. Hence, calexcitin possesses two functional EF-hands in close proximity in its N-terminal domain and one functional calcium site in its C-terminal domain. There is evidence that the protein has two markedly different affinities for calcium ions, the weaker of which is most likely to be associated with binding of calcium ions to the protein during neuronal excitation. In the current study, site-directed mutagenesis has been used to abolish each of the three calcium-binding sites of calexcitin, and these experiments suggest that it is the single calcium-binding site in the C-terminal domain of the protein which is likely to have a sensory role in the neuron.
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Affiliation(s)
- P T Erskine
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
| | - A Fokas
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
| | - C Muriithi
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
| | - H Rehman
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
| | - L A Yates
- Centre of Biological Sciences, University of Southampton, Southampton SO17 1BJ, England
| | - A Bowyer
- Centre of Biological Sciences, University of Southampton, Southampton SO17 1BJ, England
| | - I S Findlow
- Centre of Biological Sciences, University of Southampton, Southampton SO17 1BJ, England
| | - R Hagan
- Centre of Biological Sciences, University of Southampton, Southampton SO17 1BJ, England
| | - J M Werner
- Centre of Biological Sciences, University of Southampton, Southampton SO17 1BJ, England
| | - A J Miles
- Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, England
| | - B A Wallace
- Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, England
| | - S A Wells
- Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, England
| | - S P Wood
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
| | - J B Cooper
- Laboratory of Protein Crystallography, Centre for Amyloidosis and Acute Phase Proteins, UCL Division of Medicine (Royal Free Campus), Rowland Hill Street, London NW3 2PF, England
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3
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Erskine PT, Beaven GDE, Hagan R, Findlow IS, Werner JM, Wood SP, Vernon J, Giese KP, Fox G, Cooper JB. Structure of the Neuronal Protein Calexcitin Suggests a Mode of Interaction in Signalling Pathways of Learning and Memory. J Mol Biol 2006; 357:1536-47. [PMID: 16497326 DOI: 10.1016/j.jmb.2006.01.083] [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] [Received: 11/15/2005] [Revised: 01/22/2006] [Accepted: 01/23/2006] [Indexed: 10/25/2022]
Abstract
The three-dimensional structure of the neuronal calcium-sensor protein calexcitin from Loligo pealei has been determined by X-ray analysis at a resolution of 1.8A. Calexcitin is up-regulated following Pavlovian conditioning and has been shown to regulate potassium channels and the ryanodine receptor. Thus, calexcitin is implicated in neuronal excitation and plasticity. The overall structure is predominantly helical and compact with a pronounced hydrophobic core between the N and C-terminal domains of the molecule. The structure consists of four EF-hand motifs although only the first three EF hands are involved in binding calcium ions; the C-terminal EF-hand lacks the amino acids required for calcium binding. The overall structure is quite similar to that of the sarcoplasmic calcium-binding protein from Amphioxus although the sequence identity is very low at 31%. The structure shows that the two amino acids of calexcitin phosphorylated by protein kinase C are close to the domain interface in three dimensions and thus phosphorylation is likely to regulate the opening of the domains that is probably required for binding to target proteins. There is evidence that calexcitin is a GTPase and the residues, which have been implicated by mutagenesis in its GTPase activity, are in a short but highly conserved region of 3(10) helix close to the C terminus. This helix resides in a large loop that is partly sandwiched between the N and C-terminal domains suggesting that GTP binding may also require or may cause domain opening. The structure possesses a pronounced electropositive crevice in the vicinity of the 3(10) helix, that might provide an initial docking site for the triphosphate group of GTP. These findings elucidate a number of the reported functions of calexcitin with implications for neuronal signalling.
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Affiliation(s)
- P T Erskine
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, UK
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4
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Beaven GDE, Erskine PT, Wright JN, Mohammed F, Gill R, Wood SP, Vernon J, Giese KP, Cooper JB. Crystallization and preliminary X-ray diffraction analysis of calexcitin from Loligo pealei: a neuronal protein implicated in learning and memory. Acta Crystallogr Sect F Struct Biol Cryst Commun 2005; 61:879-81. [PMID: 16511184 PMCID: PMC1991304 DOI: 10.1107/s1744309105026758] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2005] [Accepted: 08/22/2005] [Indexed: 11/10/2022]
Abstract
The neuronal protein calexcitin from the long-finned squid Loligo pealei has been expressed in Escherichia coli and purified to homogeneity. Calexcitin is a 22 kDa calcium-binding protein that becomes up-regulated in invertebrates following Pavlovian conditioning and is likely to be involved in signal transduction events associated with learning and memory. Recombinant squid calexcitin has been crystallized using the hanging-drop vapour-diffusion technique in the orthorhombic space group P2(1)2(1)2(1). The unit-cell parameters of a = 46.6, b = 69.2, c = 134.8 A suggest that the crystals contain two monomers per asymmetric unit and have a solvent content of 49%. This crystal form diffracts X-rays to at least 1.8 A resolution and yields data of high quality using synchrotron radiation.
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Affiliation(s)
- G. D. E. Beaven
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - P. T. Erskine
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - J. N. Wright
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - F. Mohammed
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - R. Gill
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - S. P. Wood
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
| | - J. Vernon
- Wolfson Institute for Biomedical Research, University College London, Cruciform Building, Gower Street, London WC1E 6BT, England
| | - K. P. Giese
- Wolfson Institute for Biomedical Research, University College London, Cruciform Building, Gower Street, London WC1E 6BT, England
| | - J. B. Cooper
- School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England
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5
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Eyman M, Crispino M, Kaplan BB, Giuditta A. Squid photoreceptor terminals synthesize calexcitin, a learning related protein. Neurosci Lett 2003; 347:21-4. [PMID: 12865132 DOI: 10.1016/s0304-3940(03)00593-7] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Nerve endings of squid photoreceptor neurons generate large synaptosomes upon homogenization of the optic lobe. Using several independent methods, these presynaptic structures have been shown to synthesize a wealth of soluble, cytoskeletal and nuclear encoded mitochondrial proteins, and to account for essentially all the translation activity of the synaptosomal fraction. We are now presenting evidence that calexcitin, a learning related, Ca(2+)-binding protein of the B photoreceptors of Hermissenda crassicornis (a mollusk), is synthesized and subjected to post-translational modifications in the squid photoreceptor terminals. In view of the essential role of presynaptic protein synthesis in long-term memory formation in Aplysia, our data suggest that a comparable role may be played by calexcitin synthesized in the squid photoreceptor terminals.
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Affiliation(s)
- Maria Eyman
- Dipartimento di Fisiologia Generale e Ambientale, Università di Napoli "Federico II", Via Mezzocannone 8, 80134, Naples, Italy
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6
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Nelson TJ, Quattrone A, Kim J, Pacini A, Cesati V, Alkon DL. Calcium-regulated GTPase activity in the calcium-binding protein calexcitin. Comp Biochem Physiol B Biochem Mol Biol 2003; 135:627-38. [PMID: 12892754 DOI: 10.1016/s1096-4959(03)00144-1] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Calexcitin (CE) is a calcium-binding protein, closely related to sarcoplasmic calcium-binding proteins, that is involved in invertebrate learning and memory. Early reports indicated that both Hermissenda and squid CE also could bind GTP; however, the biochemical significance of GTP-binding and its relationship to calcium binding have remained unclear. Here, we report that the GTPase activity of CE is strongly regulated by calcium. CE possessed a P-loop-like structure near the C-terminal similar to the phosphate-binding regions in other GTP-binding proteins. Site-directed mutagenesis of this region showed that Gly(182), Phe(186) and Gly(187) are required for maximum affinity, suggesting that the GTP-binding motif is G-N-x-x-[FM]-G. CE cloned from Drosophila CNS possessed a similar C-terminal sequence and also bound and hydrolyzed GTP. GTPase activity in Drosophila CE was also strongly regulated by Ca(2+), exhibiting over 23-fold higher activity in the presence of 0.3 microM calcium. Analysis of the conserved protein motifs defines a new family of Ca(2+)-binding proteins representing the first example of proteins endowed with both EF-hand calcium binding domains and a C-terminal, P-loop-like GTP-binding motif. These results establish that, in the absence of calcium, both squid and Drosophila CE bind GTP at near-physiological concentrations and hydrolyze GTP at rates comparable to unactivated ras. Calcium functions to increase GTP-binding and GTPase activity in CE, similar to the effect of GTPase activating proteins in other low-MW GTP-binding proteins. CE may, therefore, act as a molecular interface between Ca(2+) cytosolic oscillations and the G protein-coupled signal transduction.
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Affiliation(s)
- Thomas J Nelson
- Blanchette Rockefeller Neurosciences Institute, 9601 Medical Center Drive, Rockville, MD 20850, USA.
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7
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Kawai R, Horikoshi T, Yasuoka T, Sakakibara M. In vitro conditioning induces morphological changes in Hermissenda type B photoreceptor. Neurosci Res 2002; 43:363-72. [PMID: 12135779 DOI: 10.1016/s0168-0102(02)00061-5] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Short- and long-term synaptic plasticity are considered to be cellular substrates of learning and memory. The mechanisms underlying synaptic plasticity especially with respect to morphology, however, are not known. In vitro conditioning in molluscan preparations is a well established form of short-term synaptic plasticity. Five paired presentations of light and vestibular stimulation to the isolated nervous system of Hermissenda results in an increase in excitability of the identified neuron, the type B photoreceptor, indicated by 2 measures, an increase in the input resistance and a cumulative depolarization after the cessation of light stimulus recorded from the cell soma. The terminal branches of type B photoreceptors iontophoretically injected with fluorescent dye were analyzed using computer-aided 3-dimensional reconstruction of images obtained using a confocal microscope under 'blind' conditions. The terminal branches contracted along the centro-lateral axis within an hour after conditioning, paralleling the increase in neuronal excitability. These data suggest that in vitro conditioning in Hermissenda is a form of short-term synaptic plasticity that involves changes in macromolecular synthesis.
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Affiliation(s)
- Ryo Kawai
- Graduate School of Science, Tokai University, Kita-Kaname, Hiratsuka 259-1292, Kanagawa, Japan
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8
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Gombos Z, Jeromin A, Mal TK, Chakrabartty A, Ikura M. Calexcitin B is a new member of the sarcoplasmic calcium-binding protein family. J Biol Chem 2001; 276:22529-36. [PMID: 11306567 DOI: 10.1074/jbc.m010508200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Calexcitin (CE) is a calcium sensor protein that has been implicated in associative learning. The CE gene was previously cloned from the long-finned squid, Loligo pealei, and the gene product was shown to bind GTP and modulate K(+) channels and ryanodine receptors in a Ca(2+)-dependent manner. We cloned a new gene from L. pealei, which encodes a CE-like protein, here named calexcitin B (CE(B)). CE(B) has 95% amino acid identity to the original form. Our sequence analyses indicate that CEs are homologous to the sarcoplasmic calcium-binding protein subfamily of the EF-hand superfamily. Far and near UV circular dichroism and nuclear magnetic resonance studies demonstrate that CE(B) binds Ca(2+) and undergoes a conformational change. CE(B) is phosphorylated by protein kinase C, but not by casein kinase II. CE(B) does not bind GTP. Western blot experiments using polyclonal antibodies generated against CE(B) showed that CE(B) is expressed in the L. pealei optic lobe. Taken together, the neuronal protein CE represents the first example of a Ca(2+) sensor in the sarcoplasmic calcium-binding protein family.
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Affiliation(s)
- Z Gombos
- Division of Molecular and Structural Biology, Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 2M9, Canada
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Nelson TJ, Zhao WQ, Yuan S, Favit A, Pozzo-Miller L, Alkon DL. Calexcitin interaction with neuronal ryanodine receptors. Biochem J 1999; 341 ( Pt 2):423-33. [PMID: 10393102 PMCID: PMC1220376 DOI: 10.1042/0264-6021:3410423] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Calexcitin (CE), a Ca2+- and GTP-binding protein, which is phosphorylated during memory consolidation, is shown here to co-purify with ryanodine receptors (RyRs) and bind to RyRs in a calcium-dependent manner. Nanomolar concentrations of CE released up to 46% of the 45Ca label from microsomes preloaded with 45CaCl2. This release was Ca2+-dependent and was blocked by antibodies against the RyR or CE, by the RyR inhibitor dantrolene, and by a seven-amino-acid peptide fragment corresponding to positions 4689-4697 of the RyR, but not by heparin, an Ins(1,4,5)P3-receptor antagonist. Anti-CE antibodies, in the absence of added CE, also blocked Ca2+ release elicited by ryanodine, suggesting that the CE and ryanodine binding sites were in relative proximity. Calcium imaging with bis-fura-2 after loading CE into hippocampal CA1 pyramidal cells in hippocampal slices revealed slow, local calcium transients independent of membrane depolarization. Calexcitin also released Ca2+ from liposomes into which purified RyR had been incorporated, indicating that CE binding can be a proximate cause of Ca2+ release. These results indicated that CE bound to RyRs and suggest that CE may be an endogenous modulator of the neuronal RyR.
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Affiliation(s)
- T J Nelson
- Laboratory of Adaptive Systems, National Institutes of Health, Bldg. 36, Room 4A-23, Bethesda, MD 20892, USA.
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Alkon DL. Molecular specificity of synaptic changes responsible for associative memory. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1999; 446:1-15. [PMID: 10079834 DOI: 10.1007/978-1-4615-4869-0_1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/11/2023]
Affiliation(s)
- D L Alkon
- Laboratory of Adaptive Systems, NINDS, NIH, Bethesda, Maryland, USA
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11
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Ascoli GA, Luu KX, Olds JL, Nelson TJ, Gusev PA, Bertucci C, Bramanti E, Raffaelli A, Salvadori P, Alkon DL. Secondary structure and Ca2+-induced conformational change of calexcitin, a learning-associated protein. J Biol Chem 1997; 272:24771-9. [PMID: 9312073 DOI: 10.1074/jbc.272.40.24771] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Calexcitin/cp20 is a low molecular weight GTP- and Ca2+-binding protein, which is phosphorylated by protein kinase C during associative learning, and reproduces many of the cellular effects of learning, such as the reduction of potassium currents in neurons. Here, the secondary structure of cloned squid calexcitin was determined by circular dichroism in aqueous solution and by Fourier transform infrared spectroscopy both in solution and on dried films. The results obtained with the two techniques are in agreement with each other and coincide with the secondary structure computed from the amino acid sequence. In solution, calexcitin is one-third in alpha-helix and one-fifth in beta-sheet. The conformation of the protein in solid state depends on the concentration of the starting solution, suggesting the occurrence of surface aggregation. The secondary structure also depends on the binding of calcium, which causes an increase in alpha-helix and a decrease in beta-sheet, as estimated by circular dichroism. The conformation of calexcitin is independent of ionic strength, and the calcium-induced structural transition is slightly inhibited by Mg2+ and low pH, while favored by high pH. The switch of calexcitin's secondary structure upon calcium binding, which was confirmed by intrinsic fluorescence spectroscopy and nondenaturing gel electrophoresis, is reversible and occurs in a physiologically meaningful range of Ca2+ concentration. The calcium-bound form is more globular than the apoprotein. Unlike other EF-hand proteins, calexcitin's overall lipophilicity is not affected by calcium binding, as assessed by hydrophobic liquid chromatography. Preliminary results from patch-clamp experiments indicated that calcium is necessary for calexcitin to inhibit potassium channels and thus to increase membrane excitability. Therefore the calcium-dependent conformational equilibrium of calexcitin could serve as a molecular switch for the short term modulation of neuronal activity following associative conditioning.
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Affiliation(s)
- G A Ascoli
- Laboratory of Adaptive Systems, NINDS, National Institutes of Health, Bethesda, Maryland 20892, USA.
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Suzuki N, Hirono M, Takagi H, Yoshioka T. Facilitation of Ca2+ action potential frequency by a small G protein Rab3A in rat pituitary GH3 cells. Biochem Biophys Res Commun 1997; 235:331-5. [PMID: 9199192 DOI: 10.1006/bbrc.1997.6781] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
GH3 pituitary cells have high tendency to exhibit spontaneous Ca2+ action potentials and their frequency (Ca2+ APF) is increased by treatment with thyrotropin-releasing hormone (TRH). Although spontaneous Ca2+ firing was thought to be significant for the induction of oscillations in cytosolic Ca2+ concentration ([Ca2+]i), little attempt to elucidate the mechanism has been done so far. We demonstrate here that spontaneous Ca2+ APF in GH3 cells was increased 1.5-3 fold, comparable to that for TRH, by injection of guanosine 5'-0-3-thiotriphosphate (GTPgammaS), rab3A effector domain peptide, and phorbol-dibutyrate (PDBu), whereas guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), H-rab5 peptide, ras peptide, and 4 alpha-phorbol did not. The enhancement of Ca2+ firing by rab3A effector domain peptide was blocked by a protein kinase C (PKC) inhibitor, PKC(19-36). The present study suggests that the spontaneous Ca2+APF may be controlled by small G protein phosphorylated by PKC.
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Affiliation(s)
- N Suzuki
- Department of Physiology, School of Medicine, Kitasato University, Sagamihara, Japan.
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Nelson TJ, Cavallaro S, Yi CL, McPhie D, Schreurs BG, Gusev PA, Favit A, Zohar O, Kim J, Beushausen S, Ascoli G, Olds J, Neve R, Alkon DL. Calexcitin: a signaling protein that binds calcium and GTP, inhibits potassium channels, and enhances membrane excitability. Proc Natl Acad Sci U S A 1996; 93:13808-13. [PMID: 8943017 PMCID: PMC19433 DOI: 10.1073/pnas.93.24.13808] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
A previously uncharacterized 22-kDa Ca(2+)-binding protein that also binds guanosine nucleotides was characterized, cloned, and analyzed by electrophysiological techniques. The cloned protein, calexcitin, contains two EF-hands and also has homology with GTP-binding proteins in the ADP ribosylation factor family. In addition to binding two molecules of Ca2+, calexcitin bound GTP and possessed GTPase activity. Calexictin is also a high affinity substrate for protein kinase C. Application of calexcitin to the inner surface of inside-out patches of human fibroblast membranes, in the presence of Ca2+ and the absence of endogenous Ca2+/calmodulin kinase type II or protein kinase C activity, reduced the mean open time and mean open probability of 115 +/- 6 pS K+ channels. Calexcitin thus appears to directly regulate K+ channels. When microinjected into molluscan neurons or rabbit cerebellar Purkinje cell dendrites, calexcitin was highly effective in enhancing membrane excitability. Because calexcitin translocates to the cell membrane after phosphorylation, calexcitin could serve as a Ca(2+)-activated signaling molecule that increases cellular excitability, which would in turn increase Ca2+ influx through the membrane. This is also the first known instance of a GTP-binding protein that binds Ca2+.
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Affiliation(s)
- T J Nelson
- Laboratory of Adaptive Systems, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA
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Kim CS, Han YF, Etcheberrigaray R, Nelson TJ, Olds JL, Yoshioka T, Alkon DL. Alzheimer and beta-amyloid-treated fibroblasts demonstrate a decrease in a memory-associated GTP-binding protein, Cp20. Proc Natl Acad Sci U S A 1995; 92:3060-4. [PMID: 7708775 PMCID: PMC42359 DOI: 10.1073/pnas.92.7.3060] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The two proteins most consistently identified in the brains of patients with Alzheimer disease (AD) have been beta-amyloid and tau, whose roles in the physiology or pathophysiology of brain cells are not fully understood. To identify other protein(s) involved in AD that have been implicated in physiological contexts, we undertook to analyze a specific memory-associated protein, Cp20, in fibroblasts from AD and control donors. Cp20, a GTP-binding protein that is a member of the ADP-ribosylation factor family, was significantly decreased in fibroblasts from AD patients. Normal control fibroblasts exposed to 10 nM beta-amyloid, the same concentration that induced AD-like K+ changes in control fibroblasts, showed a similar decrease in Cp20. Since it has been previously demonstrated that Cp20 is a potent regulator of K+ channels, these findings suggest that changes in this memory-associated protein may explain previously observed differences in AD K+ channels and suggest a pathophysiologic involvement linked to soluble beta-amyloid metabolism that could contribute to the characteristic memory loss of AD.
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Affiliation(s)
- C S Kim
- Laboratory of Adaptive Systems, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA
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