Molecular characterization of tlyA gene product, Rv1694 of Mycobacterium tuberculosis: a non-conventional hemolysin and a ribosomal RNA methyl transferase.

BACKGROUND: Mycobacterium tuberculosis is a virulent bacillus causing tuberculosis, a disease responsible for million deaths each year worldwide. In order to understand its mechanism of pathogenesis in humans and to help control tuberculosis, functions of numerous Mycobacterium tuberculosis genes are being characterized. In this study we report the dual functionality of tlyA gene product of Mycobacterium tuberculosis annotated as Rv1694, a 268 amino acid long basic protein. RESULTS: The recombinant purified Rv1694 protein was found to exhibit hemolytic activity in vitro. It showed concentration and time-dependent hemolysis of rabbit and human erythrocytes. Multiple oligomeric forms (dimers to heptamers) of this protein were seen on the membranes of the lysed erythrocytes. Like the oligomers of conventional, well-known, pore-forming toxins, the oligomers of Rv1694 were found to be resistant to heat and SDS, but were susceptible to reducing agents like ß-mercaptoethanol as it had abolished the hemolytic activity of Rv1694 indicating the role of disulfide bond(s). The Rv1694 generated de novo by in vitro transcription and translation also exhibited unambiguous hemolysis confirming the self assembly and oligomerization properties of this protein. Limited proteolytic digestion of this protein has revealed that the amino terminus is susceptible while in solution but is protected in presence of membrane. Striking feature of Rv1694 is its presence on the cell wall of E. coli as visualized by confocal microscopy. The surface expression is consistent with the contact dependent haemolytic ability of E. coli expressing this protein. Also, immune serum specific to this protein inhibits the contact dependent hemolysis. Moreover, Rv1694 protein binds to and forms stable oligomers on the macrophage phagosomal membranes. In addition to all these properties, E. coli expressing Rv1694 was found to be susceptible to the antibiotic capreomycin as its growth was significantly slower than mock vector transformed E. coli. The S30 extract of E. coli expressing the Rv1694 had poor translational activity in presence of capreomycin, further confirming its methylation activity. Finally, incorporation of methyl group of [3H]-S-adenosylmethionine in isolated ribosomes also confirmed its methylation activity. CONCLUSIONS: The Rv1694 has an unusual dual activity. It appears to contain two diverse functions such as haemolytic activity and ribosomal RNA methylation activity. It is possible that the haemolytic activity might be relevant to intra-cellular compartments such as phagosomes rather than cell lysis of erythrocytes and the self-assembly trait may have a potential role after successful entry into macrophages by Mycobacterium tuberculosis.

Modulation of PP2A activity by Jacalin: is it through caveolae and ER chaperones?

Plant lectins have been reported to affect the proliferation of different human cancer cell line probably by binding to the specific carbohydrate moieties. In the present study, Badan labeled single cysteine mutant (present in the caveolin-1 binding motif) of jacalin (rJacalin) was found to penetrate the target membrane, indicating a protein-protein or protein-membrane interaction apart from its primary mode of binding i.e. protein-carbohydrate interaction. Further, Jacalin treatment has resulted in the movement of the GFP-Caveolin-1 predominantly at the cell-cell contact region with much restricted dynamics. Jacalin treatment has resulted in the perinuclear accumulation of PP2A and dissociation of the PHAP1/PP2A complex. PP2A was found to act as a negative regulator of ERK signaling and a significant decrease in the phosphorylation level of MEK and AKT (T308) in A431. In addition, we have also identified several ER resident proteins including molecular chaperones like ORP150, Hsp70, Grp78, BiP of A431 cells, which were bound to the Jacalin-sepharose column. Among various ER chaperones that were identified, ORP150 was found to present on the cell surface of A431 cells.

Mycobacterial tlyA gene product is localized to the cell-wall without signal sequence.

The mycobacterial tlyA gene product, Rv1694 (MtbTlyA), has been annotated as "hemolysin" which was re-annotated as 2'-O rRNA methyl transferase. In order to function as a hemolysin, it must reach the extracellular milieu with the help of signal sequence(s) and/or transmembrane segment(s). However, the MtbTlyA neither has classical signals sequences that signify general/Sec/Tat pathways nor transmembrane segments. Interestingly, the tlyA gene appears to be restricted to pathogenic strains such as H37Rv, M. marinum, M. leprae, than M. smegmatis, M. vaccae, M. kansasii etc., which highlights the need for a detailed investigation to understand its functions. In this study, we have provided several evidences which highlight the presence of TlyA on the surface of M. marinum (native host) and upon expression in M. smegmatis (surrogate host) and E. coli (heterologous host). The TlyA was visualized at the bacterial-surface by confocal microscopy and accessible to Proteinase K. In addition, sub-cellular fractionation has revealed the presence of TlyA in the membrane fractions and this sequestration is not dependent on TatA, TatC or SecA2 pathways. As a consequence of expression, the recombinant bacteria exhibit distinct hemolysis. Interestingly, the MtbTlyA was also detected in both membrane vesicles secreted by M. smegmatis and outer membrane vesicles secreted by E. coli. Our experimental evidences unambiguously confirm that the mycobacterial TlyA can reach the extra cellular milieu without any signal sequence. Hence, the localization of TlyA class of proteins at the bacterial surface may highlight the existence of non-classical bacterial secretion mechanisms.

Modulation of phagolysosome maturation by bacterial tlyA gene product.

The pathogenic traits of TlyA proteins of Mycobacterium tuberculosis are not known. Expressions of TlyA in bacteria that do not express endogenous TlyA adhere better to RAW264.7 macrophages and get phagocytosed efficiently. The internalized bacteria avoid acidification to the extent of greater than 65 percent in the case of both TlyA-expressing E. coli and M. smegmatis. Consistent with this observation, we have observed decreased co-localizaton of Lysosomal Membrane Associated Protein-1 (approx. 35 percent), Early Endosomal Antigen-1 (approx. 34 percent), Rab5 (approx. 30 percent) and Rab7 (approx. 35 percent) and enhanced colocalizaton of Rab14 (approx. 80 percent) on both TlyA-expressing bacteria as well as on TlyA-coated latex beads. These results suggest that the mycobacterial TlyA, in general, can modulate phagolysosome maturation pathway immediately after entry into macrophages, while other important molecules may aid the bacterium for long-term, intracellular survival at later point of time.

Mycobacterium tuberculosis subverts the TLR-2-MyD88 pathway to facilitate its translocation into the cytosol.

Mycobacterium tuberculosis (M.tb) has evolved mechanisms to evade its destruction in phagolysosomes, where it successfully survives and replicates within phagocytes. Recent studies have shown that virulent strains of M.tb can translocate from the phagosome into the cytosol of dendritic cells (DC). The molecular mechanisms by which virulent M.tb strains can escape the phagosome remain unknown. Here we show that the virulent M.tb strain H37Rv, but not the vaccine strain Bacille Calmette-Guérin (BCG), escapes from the phagolysosome and enters the cytosol by interfering with the TLR-2-MyD88 signaling pathway. Using H37Rv mutants, we further demonstrate that the region of difference-1 (RD-1) locus and ESAT-6, a gene within the RD-1 locus, play an important role in the capacity of M.tb to migrate from the phagosome to the cytosol of macrophages. H37Rv, BCG, H37RvΔRD1, and H37RvΔESAT6 were able to translocate to the cytosol in macrophages derived from TLR-2- and MyD88-deficient animals, whereas only virulent H37Rv was able to enter the cytosol in macrophages from wild type mice. Therefore, signaling through the TLR-2-MyD88 pathway in macrophages plays an important role in confining M.tb within phagolysomes. Virulent strains of M.tb have evolved mechanisms to subvert this pathway, thus facilitating their translocation to the cytosol and to escape the toxic microenvironment of the phagosome or phagolysosome.

Membrane bound monomer of Staphylococcal alpha-hemolysin induces caspase activation and apoptotic cell death despite initiation of membrane repair pathway.

BACKGROUND: Wild type Staphylococcal alpha-hemolysin (alpha-HL) assembly on target mammalian cells usually results in necrotic form of cell death; however, caspase activation also occurs. The pathways of caspase activation due to binding/partial assembly by alpha-HL are unknown till date. RESULTS: Cells treated with H35N (a mutant of alpha-HL that remains as membrane bound monomer), have been shown to accumulate hypodiploid nuclei, activate caspases and induce intrinsic mitochondrial apoptotic pathway. We have earlier shown that the binding and assembly of alpha-HL requires functional form of Caveolin-1 which is an integral part of caveolae. In this report, we show that the caveolae of mammalian cells, which undergo a continuous cycle of 'kiss and run' dynamics with the plasma membrane, have become immobile upon the binding of the monomer. The cells treated with H35N were unable to recover despite activation of membrane repair mechanism involving caspase-1 dependent activation of sterol regulatory element binding protein-1. CONCLUSIONS: This is for the first time we show the range of cellular changes and responses that take place immediately after the binding of the monomeric form of staphylococcal alpha-hemolysin.