Haryadi Rajeswari

Molecular characterisation of ABC transporter type FtsE and FtsX proteins of Mycobacterium tuberculosis

Elicitation of drug resistance and various survival strategies inside host macrophages have been the hallmarks of Mycobacterium tuberculosis as a successful pathogen. ATP Binding Cassette (ABC) transporter type proteins are known to be involved in the efflux of drugs in bacterial and mammalian systems. FtsE, an ABC transporter type protein, in association with the integral membrane protein FtsX, is involved in the assembly of potassium ion transport proteins and probably of cell division proteins as well, both of which being relevant to tubercle bacillus. In this study, we cloned ftsE gene of M. tuberculosis, overexpressed and purified. The recombinant MtFtsE-6xHis protein and the native MtFtsE protein were found localized on the membrane of E. coli and M. tuberculosis cells, respectively. MtFtsE-6xHis protein showed ATP binding in vitro, for which the K42 residue in the Walker A motif was found essential. While MtFtsE-6xHis protein could partially complement growth defect of E. coli ftsE temperature-sensitive strain MFT1181, co-expression of MtFtsE and MtFtsX efficiently complemented the growth defect, indicating that the MtFtsE and MtFtsX proteins might be performing an associated function. MtFtsE and MtFtsX-6xHis proteins were found to exist as a complex on the membrane of E. coli cells co-expressing the two proteins.

Cloning, expression, purification, and biochemical characterisation of the FIC motif containing protein of Mycobacterium tuberculosis.

The role of FIC (Filamentation induced by cAMP)(2) domain containing proteins in the regulation of many vital pathways, mostly through the transfer of NMPs from NTPs to specific target proteins (NMPylation), in microorganisms, higher eukaryotes, and plants is emerging. The identity and function of FIC domain containing protein of the human pathogen, Mycobacterium tuberculosis, remains unknown. In this regard, M. tuberculosis fic gene (Mtfic) was cloned, overexpressed, and purified to homogeneity for its biochemical characterisation. It has the characteristic FIC motif, HPFREGNGRSTR (HPFxxGNGRxxR), spanning 144th to 155th residue. Neither the His-tagged nor the GST-tagged MtFic protein, overexpressed in Escherichia coli, nor expression of Mtfic in Mycobacterium smegmatis, yielded the protein in the soluble fraction. However, the maltose binding protein (MBP) tagged MtFic (MBP-MtFic) could be obtained partly in the soluble fraction. The cloned, overexpressed, and purified recombinant MBP-MtFic showed conversion of ATP, GTP, CTP, and UTP into AMP, GMP, CMP, and UMP, respectively. Sequence alignment with several FIC motif containing proteins, complemented with homology modeling on the FIC motif containing protein, VbhT of Bartonella schoenbuchensis as the template, showed conservation and interaction of residues constituting the FIC domain. Site-specific mutagenesis of the His144, or Glu148, or Asn150 of the FIC motif, or of Arg87 residue that constitutes the FIC domain, or complete deletion of the FIC motif, abolished the NTP to NMP conversion activity. The design of NMP formation assay using the recombinant, soluble MtFic would enable identification of its target substrate for NMPylation.

Mycobacterium tuberculosis Cell Division Protein, FtsE, is an ATPase in Dimeric Form

FtsE is one of the earliest cell division proteins that assembles along with FtsX at the mid-cell site during cell division in Escherichia coli. Both these proteins are highly conserved across diverse bacterial genera and are predicted to constitute an ABC transporter type complex, in which FtsE is predicted to bind ATP and hydrolyse it, and FtsX is predicted to be an integral membrane protein. We had earlier reported that the MtFtsE of the human pathogen, Mycobacterium tuberculosis, binds ATP and interacts with MtFtsX on the cell membrane of M. tuberculosis and E. coli. In this study, we demonstrate that MtFtsE is an ATPase, the active form of which is a dimer, wherein the participating monomers are held together by non-covalent interactions, with the Cys84 of each monomer present at the dimer interface. Under oxidising environment, the dimer gets stabilised by the formation of Cys84–Cys84 disulphide bond. While the recombinant MtFtsE forms a dimer on the membrane of E. coli, the native MtFtsE seems to be in a different conformation in the M. tuberculosis membrane. Although disulphide bridges were not observed on the cytoplasmic side (reducing environment) of the membrane, the two participating monomers could be isolated as dimers held together by non-covalent interactions. Taken together, these findings show that MtFtsE is an ATPase in the non-covalent dimer form, with the Cys84 of each monomer present in the reduced form at the dimer interface, without participating in the dimerisation or the catalytic activity of the protein.

In vitro polymerization of Mycobacterium leprae FtsZ OR Mycobacterium tuberculosis FtsZ is revived or abolished, respectively, by reciprocal mutation of a single residue

A single residue that dramatically influences polymerization of principal cell division protein FtsZ of Mycobacterium leprae (MlFtsZ) and Mycobacterium tuberculosis (MtFtsZ) has been identified. Soluble, recombinant MlFtsZ did not show polymerization in vitro, in contrast to MtFtsZ, which polymerised. Mutation of the lone non-conserved residue T172 in the N-terminal domain of MlFtsZ to A172, as it exists in MtFtsZ, showed dramatic polymerization of MlFtsZ-T172A in vitro. Reciprocal mutation of A172 in MtFtsZ to T172, as it exists in MlFtsZ, abolished polymerization of MtFtsZ-A172T in vitro. While T172A mutation enhanced weak GTPase activity of MlFtsZ, reciprocal A172T mutation marginally reduced GTPase activity of MtFtsZ in vitro. These observations demonstrate that the residue at position 172 plays critical role in the polymerization of MlFtsZ and MtFtsZ. A possible evolutionary correlation between the presence of polymerization-adversive or polymerization-favouring residue at position 172 in FtsZ and generation time of the respective bacterium are discussed.

NDK Interacts with FtsZ and Converts GDP to GTP to Trigger FtsZ Polymerisation - A Novel Role for NDK

Introduction Nucleoside diphosphate kinase (NDK), conserved across bacteria to humans, synthesises NTP from NDP and ATP. The eukaryotic homologue, the NDPK, uses ATP to phosphorylate the tubulin-bound GDP to GTP for tubulin polymerisation. The bacterial cytokinetic protein FtsZ, which is the tubulin homologue, also uses GTP for polymerisation. Therefore, we examined whether NDK can interact with FtsZ to convert FtsZ-bound GDP and/or free GDP to GTP to trigger FtsZ polymerisation. Methods Recombinant and native NDK and FtsZ proteins of Mycobacterium smegmatis and Mycobacterium tuberculosis were used as the experimental samples. FtsZ polymersation was monitored using 90° light scattering and FtsZ polymer pelleting assays. The γ32P-GTP synthesised by NDK from GDP and γ32P-ATP was detected using thin layer chromatography and quantitated using phosphorimager. The FtsZ bound 32P-GTP was quantitated using phosphorimager, after UV-crosslinking, followed by SDS-PAGE. The NDK-FtsZ interaction was determined using Ni2+-NTA-pulldown assay and co-immunoprecipitation of the recombinant and native proteins in vitro and ex vivo, respectively. Results NDK triggered instantaneous polymerisation of GDP-precharged recombinant FtsZ in the presence of ATP, similar to the polymerisation of recombinant FtsZ (not GDP-precharged) upon the direct addition of GTP. Similarly, NDK triggered polymerisation of recombinant FtsZ (not GDP-precharged) in the presence of free GDP and ATP as well. Mutant NDK, partially deficient in GTP synthesis from ATP and GDP, triggered low level of polymerisation of MsFtsZ, but not of MtFtsZ. As characteristic of NDK’s NTP substrate non-specificity, it used CTP, TTP, and UTP also to convert GDP to GTP, to trigger FtsZ polymerisation. The NDK of one mycobacterial species could trigger the polymerisation of the FtsZ of another mycobacterial species. Both the recombinant and the native NDK and FtsZ showed interaction with each other in vitro and ex vivo, alluding to the possibility of direct phosphorylation of FtsZ-bound GDP by NDK. Conclusion Irrespective of the bacterial species, NDK interacts with FtsZ in vitro and ex vivo and, through the synthesis of GTP from FtsZ-bound GDP and/or free GDP, and ATP (CTP/TTP/UTP), triggers FtsZ polymerisation. The possible biological context of this novel activity of NDK is presented.

NUCLEOSIDE DIPHOSPHATE KINASE GENE IS EXPRESSED THROUGH MULTIPLE TRANSCRIPTS IN Mycobacterium smegmatis

Nucleoside diphosphate kinase, NDK, plays a vital role in maintaining pools of nucleoside triphosphates and their respective deoxynucleoside triphosphates for the synthesis of RNA and DNA. Transcriptional regulation of ndk in mycoacteria remains unknown, although modulation of ndk expression under stress conditions involving DNA and, RNA synthesis arrest and cell division arrest had been studied in several bacterial systems. Therefore, in the present study, the start sites of transcription of ndk of Mycobacterium smegmatis (Msmndk) were identified and putative promoter regions were predicted. Using transcriptional fusions of the cloned putative promoter regions to mycobacterial codon-optimised reporter gene, gfpm, promoter activity was examined under active phase of growth, nutrient starvation and other stress conditions involving DNA replication inhibition and cell division arrest. Msmndk was found to be expressed through two transcripts, T1 and T2, arising from P1 and P2 promoters, respectively. Both the promoters belonged to C group of mycobacterial promoters, which do not possess consensus to any known canonical sigma factor recognition sequences. The levels of T2, but not of T1, were found to be low under the different stress conditions studied. The data documents modulation of ndk transcripts in mycobacteria. Key wordsNucleoside diphosphate kinase transcripts, Mycobacterium smegmatis, Transcription start site, Primer extension, Hydroxyurea, Phenethyl alcohol, nutrient-depleted stationary phase. International Journal of Microbiology Research ISSN: 0975-5276 & E-ISSN:0975-9174, Volume 4, Issue 4, 2012 Introduction Nucleoside diphosphate kinase (NDK), originally discovered by Krebs and Hems [1] and Berg and Joklik [2], catalyses the transfer of a 5’ terminal phosphate from ATP or GTP to a nucleoside diphosphate (NDP), via a phosphohistidine enzyme intermediate [3]. The primary role of NDK is to maintain nucleoside triphosphates (NTPs) and their deoxy derivatives (dNTPs) pool for the synthesis of RNA and DNA and for other biosynthetic processes in bacteria to humans [4-6]. Besides this fundamental role in nucleotide metabolism, NDK has been implicated in many other cellular functions. The ndk transcription gets downregulated by the phosphorylated ArcA under redox conditions in Escherichia coli [7], in the classical biotype O1 of Vibrio cholerae but not in O1 El Tor biotype [8], and in the bacterial pathogen, Chlamydia trachomatis, during the developmental cycle [9]. Similarly, E. coli ndk (Econdk) is repressed by the DNA-bending protein, HU, under aerobic conditions [10], by the global regulator, FNR, under anaerobic conditions [11], and by cyclic AMP receptor protein (CRP) in vitro [12]. Repression of Salmonella enterica serovar Typhimurium ndk by the DNA-binding and transcription regulatory protein, Fis [13] lowers GTP levels, thereby regulating pppGpp and ppGpp pool size. On the contrary, in Pseudomonas aeruginosa, AlgQ, which modulates the levels of alginate [14], upregulates ndk, thereby positively regulates the levels of GTP, ppGpp, and inorganic polyphosphate (polyp) [15, 16]. Similarly, under osmotic and oxidative stress conditions, ndk is upregulated in order to restore cell homeostasis in Sinorhizobium meliloti, carrying a mutation in the outer membrane protein, TolC [17]. Citation: Muthu Arumugam, et al. (2012) Nucleoside diphosphate kinase gene is expressed through multiple transcripts in Mycobacterium smegmatis. International Journal of Microbiology Research, ISSN: 0975-5276 & E-ISSN:0975-9174, Volume 4, Issue 4, pp.-201-210. Copyright: Copyright©2012 Muthu Arumugam, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.