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Determination of the Solution Structures of Conantokin-G and Conantokin-T by CD and NMR Spectroscopy*

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Determination of the Solution Structures of Conantokin-G and Conantokin-T by CD and NMR Spectroscopy* THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 272, No. 4, Issue of January 24, pp. 2291–2299, 1997 © 1997 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A. Determination of the Solution Structures of Conantokin-G and Conantokin-T by CD and NMR Spectroscopy* (Received for publication, June 18, 1996, and in revised form, September 5, 1996) Niels Skjærbæk, Katherine J. Nielsen, Richard J. Lewis, Paul Alewood‡, and David J. Craik§ From The Centre for Drug Design and Development, The University of Queensland, Brisbane, Qld 4072, Australia Conantokin-G and conantokin-T are two paralytic the very complex venom that these snails have developed for polypeptide toxins originally isolated from the venom of rapidly immobilizing their fast moving prey (3). Evidence indi- the fish-hunting cone snails of the genus Conus. Conan- cates that both con-G and con-T possess N-methyl-D-aspartate tokin-G and conantokin-T are the only naturally occur- receptor (NMDA receptor) antagonist activity, produced by a ring peptidic compounds which possess N-methyl-D-as- selective non-competitive antagonism of polyamine enhance- partate receptor antagonist activity, produced by a ment of NMDA receptor agonist activity (4–7). To date, these selective non-competitive antagonism of polyamine re- are the only naturally occurring peptides known to have this sponses. They are also structurally unusual in that they property. Con-G and con-T are also structurally unusual, in contain a disproportionately large number of acid labile that they lack the disulfide motifs commonly found in conotoxin post-translational g-carboxyglutamic acid (Gla) resi- peptides and instead contain a high proportion of the base- Downloaded from dues. Although no precise structural information has stable but acid-labile g-carboxyglutamic acid (Gla) residue (8, previously been published for these peptides, early 9). Gla residues were originally identified in the vitamin K-de- spectroscopic measurements have indicated that both pendent blood-clotting factors including prothrombin. Subse- conantokin-G and conantokin-T form a-helical struc- tures, although there is some debate whether the pres- quently they were found in other vertebrate proteins such as osteocalcin (calcium-binding protein) (10, 11) and have been ence of calcium ions is required for these peptides to http://www.jbc.org/ 21 adopt this fold. We now report a detailed structural implicated in Ca binding (12, 13). The discovery of Gla resi- study of synthetic conantokin-G and conantokin-T in a dues in several Conus peptides has established that this post- range of solution conditions using CD and 1H NMR spec- translational modification has a much wider phylogenetic dis- troscopy. The three-dimensional structures of conan- tribution than previously thought (14). tokin-T and conantokin-G were calculated from 1H Although no precise structural information has yet been NMR-derived distance and dihedral restraints. Both co- published for these peptides using either high field NMR or at UQ Library on October 19, 2016 nantokins were found to contain a mixture of a- and 310 x-ray techniques, previous CD spectroscopic measurements helix, that give rise to curved and straight helical con- have indicated that con-G and con-T are folded in a stable formers. Conantokin-G requires the presence of diva- a-helical conformation (7, 9). However, there is some dispute lent cations (Zn21,Ca21,Cu21,orMg21) to form a stable over whether these structures are stabilized by Ca21 (4, 7) and a-helix, while conantokin-T adopts a stable a-helical furthermore, whether Ca21 is essential to the biological func- structure in aqueous conditions, in the presence or ab- tion of the conantokin peptides (7). To investigate this intrigu- 21 21 21 21 sence of divalent cations (Zn ,Ca ,Cu ,orMg ). ing problem we have synthesized con-G and con-T using Boc chemistry and investigated their solution structures in detail 1 1 by CD and H NMR spectroscopy. The effects of the solution Conantokin-G (con-G) and conantokin-T (con-T) are highly environment including pH, monovalent cation (K1), divalent conserved polypeptides originally isolated from the venoms of cations (Zn21,Ca21,Cu21, and Mg21), EDTA, and TFE on the piscivorous cone snails Conus geographus (1) and Conus tulipa structures of con-G and con-T were monitored by CD spectros- (2), respectively. These polypeptides are just two components in copy. Both peptides were also examined under several condi- tions by 1H NMR spectroscopy and their three-dimensional * This work was supported in part by a research grant from the Alfred structures were calculated based on the NMR-derived Benzon Foundation, Østbanegade, Copenhagen, Denmark (to N. S.), a restraints. Generic Industry Research and Development grant from the Australian Department of Industry, Science and Technology, and AMRAD Corpo- EXPERIMENTAL PROCEDURES ration Ltd. The costs of publication of this article were defrayed in part Chemicals and Reagents by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 All coded Boc-L-amino acids and reagents used during chain assem- solely to indicate this fact. bly were peptide synthesis grade supplied by Auspep Ptd. Ltd. The atomic coordinates and structure factors (codes 1ONU and 1ONT) (Parkville, Victoria). Acetonitrile (HPLC grade) was purchased from have been deposited in the Protein Data Bank, Brookhaven National Laboratory Supply Pty. Ltd. (Coorparoo, Queensland). Deionized water Laboratory, Upton, NY. was used throughout and prepared by a Milli-Q water purification ‡ To whom correspondence should be addressed: Centre for Drug system (Millipore, Waters). Screw-cap glass peptide synthesis reaction Design and Development, The University of Queensland, Brisbane, Qld vessels (10 ml) with a sintered glass filter were obtained from Embel 4072, Australia. Tel.: 61-7-3365-1271; Fax: 61-7-3365-1990. Scientific Glassware (Queensland). An all-Kel-F apparatus (Peptide § Supported by an Australian Research Council Professorial Institute, Osaka, Japan) was used for HF cleavage. Argon, helium, and Fellowship. nitrogen were all ultrapure grade (CIG, Australia). 1 The abbreviations used are: con-G, conantokin-G; con-T, conan- tokin-T; NMR, nuclear magnetic resonance; MS, mass spectrometry; Peptide Synthesis DQF-COSY, double quantum filtered correlated spectroscopy; TOCSY, total correlated spectroscopy; NOESY, nuclear Overhauser enhance- The sequences of the two conantokins are: conantokin-G, Gly-Glu-G- ment spectroscopy; Boc, tert-butoxycarbonyl; HF, hydrogen fluoride; la-Gla-Leu-Gln-Gla-Asn-Gln-Gla-Leu-Ile-Arg-Gla-Lys-Ser-Asn-NH2; Gla, g-carboxyglutamic acid; TFE, 2,2,2-trifluoroethanol; NMDA, N- and conantokin-T, Gly-Glu-Gla-Gla-Tyr-Gln-Lys-Met-Leu-Gla-Asn- methyl-D-aspartate; RMSD, root mean square deviation; deg, degree. Leu-Arg-Gla-Ala-Glu-Val-Lys-Lys-Asn-Ala-NH2. The linear conanto- This paper is available on line at http://www-jbc.stanford.edu/jbc/ 2291 2292 Solution Structures of Conantokin-G and Conantokin-T kins were assembled manually using solid phase methodology with Boc Experimental estimates of helical contents were determined from the chemistry (15). Both peptides were assembled on p-methylbenzhy- [u]222 measurements. An estimate of helical content was determined drylamine-resinzHCl on a 0.2 mmol scale. The following side chain using an equation developed by Chakrabartty et al. (17) (240,000 protected amino acids were employed: Asn(xanthyl), Lys(2-chloroben- (1–2.5/n); n 5 number of residues), where 240,000 and 0 (degzcm2)/dmol zyloxycarbonyl), Glu(g-cyclohexyl), Arg(p-toluenesulfonyl), Ser(benzyl), are the values for 100% and 0% a-helix, respectively. This equation is Gln(xanthyl), and Gla(g-cyclohexyl)2. A 4-fold excess of Boc amino acids used only as a qualitative estimate of peptide helicity. was used based on the original substitution of p-methylbenzhy- NMR Spectroscopy drylamine-resinzHCl. Boc-L-Gla(g-cyclohexyl)2-OHzDCHA was coupled with a 1.25 mol excess. Boc-Gla(g-cyclohexyl)2 was synthesized as de- All NMR spectra were recorded on a Bruker ARX 500 spectrometer scribed previously (8). All amino acids were activated using 2-(1H- equipped with a z-gradient unit. Peptide concentrations were 2 mM. benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate Spectra of con-G were obtained in 95% H2O, 5% D2O with no CaCl2 (pH (0.5 M in dimethylformamide) and N,N-diisopropylethylamine and cou- 5.5), 1:1 (pH 5.5) and 5:1 (pH 3.5 and 5.5) ratios of CaCl2 to peptide, and pled for 10 min, except for Boc-Gla(g-cyclohexyl) which was coupled for 2 in 33% TFE. Spectra of con-T were obtained in 95% H2O, 5% D2O with 60 min. Boc protecting groups were removed using 100% trifluoroacetic no CaCl2 (pH 5.5) and a 5:1 (pH 5.5) ratio of CaCl2 to peptide, and in acid and the peptides were cleaved from the resin using HF. 33% TFE. 1H NMR spectra recorded were DQF-COSY (18), NOESY (19, Con-G—The peptide resin (415 mg, 94.3 mmol) was treated with HF 20) with mixing times of 100–300 ms and TOCSY (21) with a mixing (9 ml) in the presence of p-cresol (1 ml) at 25 °C for 2 h. Crude peptide time of 65 ms. All spectra were recorded at 280 K, except for some was precipitated with cold ether, washed with cold ether, then extracted additional spectra at 298 K. Spectra were run over 6024 Hz with 4 K into 20% acetic acid and lyophilized to yield 201 mg (86%) of crude data points, 400–600 free induction decays, 16–64 scans, and a recycle con-G. The crude peptide was purified on a conventional reverse-phase delay of 1 s (1.5 s for DQF-COSY).
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