In vitro and in vivo anti-tumor effect of Trichobakin fused with urokinase-type plasminogen activator ATF-TBK.

BACKGROUND: Trichobakin (TBK), a member of type I ribosome-inactivating proteins (RIPs), was first successfully cloned from Trichosanthes sp Bac Kan 8-98 in Vietnam. Previous study has shown that TBK acts as a potential protein synthesis inhibitor; however, the inhibition efficiency and specificity of TBK on cancer cells remain to be fully elucidated. METHODS AND RESULTS: In this work, we employed TBK and TBK conjugated with a part of the amino-terminal fragment (ATF) of the urokinase-type plasminogen activator (uPA), which contains the Ω-loop that primarily interacts with urokinase-type plasminogen activator receptor, and can be a powerful carrier in the drug delivery to cancer cells. Four different human tumor cell lines and BALB/c mice bearing Lewis lung carcinoma cells (LLC) were used to evaluate the role of TBK and ATF-TBK in the inhibition of tumor growth. Here we showed that the obtained ligand fused RIP (ATF-TBK) reduced the growth of four human cancer cell lines in vitro in the uPA receptor level-dependent manner, including the breast adenocarcinoma MDA-MB 231 cells and MCF7 cells, the prostate carcinoma LNCaP cells and the hepatocellular carcinoma HepG2 cells. Furthermore, the conjugate showed anti-tumor activity and prolonged the survival time of tumor-bearing mice. The ATF-TBK also did not cause the death of mice with doses up to 48 mg/kg, and they were not significantly distinct on parameters of hematology and serum biochemistry between the control and experiment groups. CONCLUSIONS: In conclusion, ATF-TBK reduced the growth of four different human tumor cell lines and inhibited lung tumor growth in a mouse model with little side effects. Hence, the ATF-TBK may be a target to consider as an anti-cancer agent for clinical trials.

Crystal Structure of Inhibitor-Bound Bacterial Oligopeptidase B in the Closed State: Similarity and Difference between Protozoan and Bacterial Enzymes.

The crystal structure of bacterial oligopeptidase B from Serratia proteamaculans (SpOpB) in complex with a chloromethyl ketone inhibitor was determined at 2.2 Å resolution. SpOpB was crystallized in a closed (catalytically active) conformation. A single inhibitor molecule bound simultaneously to the catalytic residues S532 and H652 mimicked a tetrahedral intermediate of the catalytic reaction. A comparative analysis of the obtained structure and the structure of OpB from Trypanosoma brucei (TbOpB) in a closed conformation showed that in both enzymes, the stabilization of the D-loop (carrying the catalytic D) in a position favorable for the formation of a tetrahedral complex occurs due to interaction with the neighboring loop from the ß-propeller. However, the modes of interdomain interactions were significantly different for bacterial and protozoan OpBs. Instead of a salt bridge (as in TbOpB), in SpOpB, a pair of polar residues following the catalytic D617 and a pair of neighboring arginine residues from the ß-propeller domain formed complementary oppositely charged surfaces. Bioinformatics analysis and structural modeling show that all bacterial OpBs can be divided into two large groups according to these two modes of D-loop stabilization in closed conformations.

Unusual Cytochrome c552 from Thioalkalivibrio paradoxus: Solution NMR Structure and Interaction with Thiocyanate Dehydrogenase.

The search of a putative physiological electron acceptor for thiocyanate dehydrogenase (TcDH) newly discovered in the thiocyanate-oxidizing bacteria Thioalkalivibrio paradoxus revealed an unusually large, single-heme cytochrome c (CytC552), which was co-purified with TcDH from the periplasm. Recombinant CytC552, produced in Escherichia coli as a mature protein without a signal peptide, has spectral properties similar to the endogenous protein and serves as an in vitro electron acceptor in the TcDH-catalyzed reaction. The CytC552 structure determined by NMR spectroscopy reveals significant differences compared to those of the typical class I bacterial cytochromes c: a high solvent accessible surface area for the heme group and so-called "intrinsically disordered" nature of the histidine-rich N- and C-terminal regions. Comparison of the signal splitting in the heteronuclear NMR spectra of oxidized, reduced, and TcDH-bound CytC552 reveals the heme axial methionine fluxionality. The TcDH binding site on the CytC552 surface was mapped using NMR chemical shift perturbations. Putative TcDH-CytC552 complexes were reconstructed by the information-driven docking approach and used for the analysis of effective electron transfer pathways. The best pathway includes the electron hopping through His528 and Tyr164 of TcDH, and His83 of CytC552 to the heme group in accordance with pH-dependence of TcDH activity with CytC552.

Differences in Medium-Induced Conformational Plasticity Presumably Underlie Different Cytotoxic Activity of Ricin and Viscumin.

Structurally similar catalytic subunits A of ricin (RTA) and viscumin (MLA) exhibit cytotoxic activity through ribosome inactivation. Ricin is more cytotoxic than viscumin, although the molecular mechanisms behind this difference are still poorly understood. To shed more light on this problem, we used a combined biochemical/molecular modeling approach to assess possible relationships between the activity of toxins and their structural/dynamic properties. Based on bioassay measurements, it was suggested that the differences in activity are associated with the ability of RTA and MLA to undergo structural/hydrophobic rearrangements during trafficking through the endoplasmic reticulum (ER) membrane. Molecular dynamics simulations and surface hydrophobicity mapping of both proteins in different media showed that RTA rearranges its structure in a membrane-like environment much more efficiently than MLA. Their refolded states also drastically differ in terms of hydrophobic organization. We assume that the higher conformational plasticity of RTA is favorable for the ER-mediated translocation pathway, which leads to a higher rate of toxin penetration into the cytoplasm.

First Crystal Structure of Bacterial Oligopeptidase B in an Intermediate State: The Roles of the Hinge Region Modification and Spermine.

Oligopeptidase B (OpB) is a two-domain, trypsin-like serine peptidase belonging to the S9 prolyloligopeptidase (POP) family. Two domains are linked by a hinge region that participates in the transition of the enzyme between two major states-closed and open-in which domains and residues of the catalytic triad are located close to each other and separated, respectively. In this study, we described, for the first time, a structure of OpB from bacteria obtained for an enzyme from Serratia proteomaculans with a modified hinge region (PSPmod). PSPmod was crystallized in a conformation characterized by a disruption of the catalytic triad together with a domain arrangement intermediate between open and closed states found in crystals of ligand-free and inhibitor-bound POP, respectively. Two additional derivatives of PSPmod were crystallized in the same conformation. Neither wild-type PSP nor its corresponding mutated variants were susceptible to crystallization, indicating that the hinge region modification was key in the crystallization process. The second key factor was suggested to be polyamine spermine since all crystals were grown in its presence. The influences of the hinge region modification and spermine on the conformational state of PSP in solution were evaluated by small-angle X-ray scattering. SAXS showed that, in solution, wild-type PSP adopted the open state, spermine caused the conformational transition to the intermediate state, and spermine-free PSPmod contained molecules in the open and intermediate conformations in dynamic equilibrium.

Backbone and side-chain chemical shift assignments for the ribosome-inactivating protein trichobakin (TBK).

Trichobakin (TBK) is a type-I ribosome-inactivating protein (RIP-I), acting as an extremely potent inhibitor of protein synthesis in the cell-free translation system of rabbit reticulocyte lysate (IC50: 3.5 pM). In this respect, TBK surpasses the well-studied highly homologous RIP-I trichosanthin (IC50: 20-27 pM), therefore creation of recombinant toxins based on it is of great interest. TBK needs to penetrate into cytosol through the cell membrane and specifically bind to α-sarcin/ricin loop of 28S ribosome RNA to perform the function of specific RNA depurination. At the moment, there is no detailed structural-dynamic information in solution about diverse states RIP-I can adopt at different stages on the way to protein synthesis inhibition. In this work, we report a near-complete assignment of 1H, 13C, and 15N TBK (27.3 kDa) resonances and analysis of the secondary structure based on the experimental chemical shifts data. This work will serve as a basis for further investigations of the structure, dynamics and interactions of the TBK with its molecular partners using NMR techniques.