Development of small molecule inhibitors and stabilizers of dimerization of tRNA-guanine transglycosylase to treat Shigellosis

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Interference with protein-protein interfaces (PPI) is a new option for therapeutic intervention. Inhibitors either perturbing or stabilizing PPI formation can modulate protein function and will arrest the protein in a non-functional state. From an experimental point of view, the study of homodimers is particularly challenging, as manipulations requiring separate monomer units are impossible. We want to study a tRNA-modifying enzyme, target to fight Shigellosis, which is only functional as homodimer. In solution, the protein is a stable dimer, which exchanges monomer units slowly over many hours. Via mutagenesis, we could produce interface variants with partial to complete dissociation in solution, remarkably in the crystalline state all variants still assemble in C2-symmetrical homodimer packing. This prevents access to structural information about the interface in monomeric state and makes development of modulators breaking-up the homodimer difficult. A Tyr/Cys interface variant results in monomerization in solution, in the crystal the customary homodimer packing is formed. However, upon oxidation the introduced cysteine residues undergo disulfide linkage and merge the monomer units together in a completely different packing arrangement, now exposing a crucial loop-helix motif of the old interface in a new conformation. Importantly, this loop is known to trigger dimer formation and it now occurs in a geometry incompatible with the original dimer packing. Below this loop, a small binding pocket is opened; ready to accommodate a small-molecule modulator to stabilize its geometry in the state incompatible with homodimerization. In the proposed project, we want to develop such allosteric surface binders capable to prevent protein-protein formation. By chemical expansion of our active-site inhibitors, we succeeded to induce a transition of the customary homodimer into an alternative, structurally completely different homodimer packing characterized by an equally large contact interface. The novel interface is incompetent to recognize a tRNA molecule, which is enzymatically only processed by the original homodimer. Thus, the novel dimer arrangement arrests the enzyme in a catalytically inactive state. Via modifications of our active-site inhibitors, we want to develop stabilizers freezing the newly discovered dimer in the functionally inactive state. The development of such protein-protein interface stabilizers is an alternative concept to block enzyme function. The project is accomplished by site-directed mutagenesis, protein crystallography, fragment-based lead discovery, chemical synthesis and biophysical characterization by mass spectrometry, ESR-spin resonance and isothermal titration calorimetry. The experiments will be supported by computer simulations. Collaborations with the groups of F. Diederich (synthesis, ETH Zurich), S. Cianferani (mass spectrometry, Univ. Strasbourg) and J. Klare (ESR, Univ. Osnabrück) are planned.