Solution Structures and Dynamics of Conotoxins and Small MutS Related Domain from Helicobacter Pylori MutS2

Abstract

The work presented in this thesis describes the determination of structures of peptides and proteins at atomic resolution. Nuclear Magnetic Resonance (NMR) spectroscopy was used as the principal method of investigation. The thesis is divided into three parts. Part I of the thesis consists of chapters 1 to 4, and deals with structural studies of two novel conotoxins. Part II of the thesis consists of chapter 5 and deals with structural studies of Small MutS Related (Smr) domain from Helicobacter pylori MutS2. Part III of the thesis consists of Appendices A to D. Appendix A describes implementation of a novel pulse sequence for determination of disulfide connectivity using long-range 13 C–13 C scalar couplings across disulfide bonds. Appendices B, C and D contain supplementary infor- mation (acquisition parameters and chemical shifts) for the structural studies presented in parts I and II of the thesis. Part I: Structural studies of novel conotoxins from Conus monile Chapter 1 gives a brief overview of the conotoxins and their structural studies. The first half of the chapter describes biosynthesis, classification schemes, nomenclature, com- monly observed post-translational modifications and applications of conotoxins. The latter half of this chapter summarizes the challenges involved in the structural studies of conotoxins in light of the recent developments in integrated transcriptomic and venomic studies of conotoxins. The key homonuclear and heteronuclear NMR experiments that are employed for structural studies of conotoxins are summarized. Emphasis was laid on describing the spectral features and the structural information that can be gleaned from these experiments. Finally, the current mass spectrometric and NMR methods available for determination of disulfide connectivity are discussed Chapter 2 describes sample preparation and preliminary biophysical characteriza- tion of a conotoxin Mo3964 that contains a hitherto uncharacterized cysteine framework (C–CC–C–C–C). The sequence of Mo3964 was identified at the nucleic acid level as a cDNA clone. Analysis of the signal sequence revealed that the toxin belongs to the M-superfamily, while the cysteine framework bears more resemblance to O- and K- super- family of conotoxins. Structural studies were initiated to determine the disulfide connec- tivity, tertiary structure and biological activity. The gene corresponding to the mature toxin sequence was cloned in a bacterial expression vector pET21a(+) as a C-terminal tag to the cytochrome b5 fusion protein host system. The fusion protein was obtained by recombinant expression using the bacterial expression host E. coli BL21(DE3) and the mature toxin was obtained by either enzymatic or chemical cleavage of the fusion protein followed by size exclusion chromatography and reverse phase HPLC. Proton 1D NMR spectra of the purified peptide exhibited sharp lines and good spec- tral dispersion indicating that molecule was well folded. Formation of disulfide bonds in the mature toxin was ascertained by high resolution mass spectra of intact and chemically modified Mo3964. The peptide toxin exhibited remarkable stability to chemical denatu- ration and proteolytic digestion. Spectroscopic studies clearly showed that Mo3964 pos- sesses a very stable and well defined structure as long as its disulfide bonds are intact. Analytical size exclusion chromatography and Multi Angle Light Scattering (MALS) studies showed that Mo3964 exists in solution as monomer albeit with a non-globular structure. Electrophysiological studies showed that Mo3964 inhibits outward potassium currents in rat Dorsal Root Ganglion (DRG) neurons and increases the reversal potential of rat voltage gated sodium channel rNav 1.2 stably expressed on Chinese Hamster Ovary (CHO) cells at peptide concentrations as low as 10 nM. Chapter 3 describes the determination of disulfide connectivity and tertiary stricture of Mo3964. Initial attempts to determine disulfide connectivity using direct fragmenta- tion of the intact peptide in the mass spectrometer failed due to the relatively large size of the molecule and its resistance to endoproteases. Partial reduction alkylation based methods failed as the first stage of partial reduction gave rise to a mixture of various single disulfide bond reduced species which could not be separated from each other. Subsequently, information about the disulfide connectivity was obtained using a method that does not necessitate separation of such a mixture of single disulfide bond reduced species. This method involves partial reduction, cyanylation of the reduced cysteines and alkali mediated cleavage of the peptide backbone on the N-terminus of cyanylated cysteines. Structural studies were carried out using homonuclear and heteronuclear NMR meth- ods. The hydrogen bond network and hence topology of the molecule was determined with high accuracy using the long-range HNCO-COSY experiment that correlates hydrogen- bond donor-acceptor pairs. This experiment utilizes the three bond heteronuclear scalar coupling, i.e., the h3JN C O′ coupling across the hydrogen bonds. All these restraints proved crucial to the assignment of the disulfide connectivity in Mo3964, given its novel cysteine framework. The structure of Mo3964 was calculated using a total of 549 NOE distance restraints, 84 dihedral angle restraints and 28 hydrogen bond distance restraints. The tertiary structure was constructed from the disulfide connectivity pattern 1–3, 2–5 and 4–6, that is hitherto undescribed for the M–superfamily conotoxins. The ensemble of structures showed a backbone Root Mean Square Deviation of 0.68 ± 0.18 Å, with 87% and 13% of the backbone dihedral (φ, ψ) angles lying in the most favored and additional allowed regions of the Ramachandran map. The remarkable stability and anomalous spectral properties exhibited by Mo3964 could be rationalized using the disulfide connectivity and the tertiary structure. The tertiary structural fold has not been described for any of the known Conus peptides. Further, a search for structures similar to that of Mo3964 using the web server DALI returned no hits indicating that the peptide scaffold of Mo3964 has no structural homologues. Hence, the conotoxin Mo3964 represents a new bioactive peptide fold that is stabilized by disulfide bonds and adds to the existing repertoire of scaffolds that can be used to design stable bioactive peptide molecules. The structure of Mo3964 was submitted to the Protein Data Bank (PDB ID: 2MW7)[1]. Chapter 4 describes the structural studies of a 17 residue, single disulfide containing conopeptide Mo1853. The samples for structural studies were obtained either by chemical synthesis or by recombinant expression methods. Structural studies using homonuclear solution NMR methods revealed that Mo1853 exists as two equally populated cis and trans X–Pro conformers which are in slow exchange regime, compared to the chemical shift timescale. Sequence specific assignments were obtained for both the conformers by analysis of homonuclear 2D 1 H,1H–DQF–COSY,1H,1 H–TOCSY, 1H,1 H–NOESY and 1H,1 H–ROESY spectra. Temperature dependence of chemical shifts was measured and coalescence was observed for two amide protons at 318 K. At this temperature, the rate of exchange and the free energy of activation were determined to be 59 Hz and ≈ 67.2 kJ mol−1 respectively. The evidence for this conformational equilibrium was also observed as exchange correlation peaks in the 2D- NOESY and ROESY spectra. Tertiary structures of both the cis and trans conformers were determined using distance restraints, backbone dihedral angle restraints, the disulfide bond restraint and the cis or trans conformation of the X–Pro peptide bond. Tertiary structures of both the conformers consist of a 29-membered macro-cyclic ring formed by 9 amino acid residues which are cyclized by side chain to side chain disulfide bond. The conformation of the X–Pro peptide bond which is located within this macro-cyclic ring causes the cis structure to be compact and the trans structure to be in an extended form. Analysis of the tertiary structures indicated that the trans conformer is stabilized by hydrogen bonds while the cis conformer is likely to be stabilized by hydrophobic interactions. This was further corroborated by the fact that at lower temperatures, the hydrophobic interactions became weaker reducing the population of the cis conformer with respect to that of the trans conformer. Preliminary electrophysiological studies carried out on rat DRG neurons indicate that Mo1853 transiently reduces late outward potassium currents. Part II: Structural studies of Small MutS Related (Smr) domain from Helicobacter pylori MutS2 Chapter 5 presents the solution NMR studies of the Smr domain from MutS2 of H. pylori , henceforth called as HpSmr. In H. pylori , MutS2 is involved in suppression of homologous recombination and its Smr domain was shown to be necessary for this activity. As of date, in spite of the availability of structural information for the Smr domain, unambiguous identification of the residues involved in metal binding, DNA binding and catalysis remains elusive. Structural studies were carried out on two different constructs of HpSmr viz., HpSmr– (His)6 and GSHM–HpSmr, with and without the hexahistidine tag respectively. Se- quence specific assignments of HpSmr–(His)6 were obtained at two different sample pH conditions viz., pH 8.0 and pH 5.35 using the standard suite of triple resonance NMR experiments. Since, valines and leucines constitute about 25% of the total number of amino acid residues in HpSmr–(His)6 , stereospecific assignments were obtained for di- astereotopic methyl groups of these residues by preparing a fractionally 13C labeled sample of HpSmr–(His)6 . Solution structure of HpSmr–(His)6 at pH 8.0 was determined using 766 NOE restraints, 170 backbone dihedral angle restraints and 70 hydrogen bond distance restraints. The tertiary structure exhibits the canonical α/β sandwich fold ex- hibited by all the other known structures of Smr domains. Further, NMR studies and analytical gel filtration studies indicated the presence of pH dependent conformational exchange in HpSmr that involves strand to coil transition in the C-terminal β-strand. In order ascertain that the conformational equilibrium is not at an artifact caused by the C-terminal hexa-histidine-tag, HpSmr protein construct GSHM–HpSmr, which does not have the hexa-histidine-tag, was prepared. Conformational exchange was observed in this construct as well. The preliminary NMR evidence suggests that the conformational exchange is caused by pH dependent cis–trans isomerization of a semi-conserved Proline residue Pro66 . We have hypothesized that the pH dependent modulation of the activity of Smr domain of MutS2 can be advantageous to H. pylori . Such a regulation could help the bacteria to achieve optimal rate of homologous recombination in response to changes in pH, which is necessary for maintaining homeostasis and tiding over stress conditions. Part III: Appendix Appendix A describes an NMR pulse program LRCC_CH2 that was designed with the aim of determining disulfide connectivity using long-range 13C–13 C (C β –C β ′ ) couplings across the disulfide bond. This experiment is a modification of an earlier experiment pub- lished by Bax and co-workers designed to measure the side-chain χ3 dihedral angle in me- thionines. The experiment described here is optimized for the detection of 3 bond scalar coupled methylene carbons. The details of modifications introduced in LRCC_CH2, its product operator analysis, a representative spectrum acquired on [U-13C,15 N]–Mo3964, short-comings and future directions are described. The C programming code that was used to implement the pulse program is also included in the appendix. Appendices B, C and D contain the supplementary information (acquisition pa- rameters for the NMR experiments and chemical shifts) for the structural studies carried out on Mo3964, Mo1853 and HpSmr.