Repurposing p97 inhibitors for chemical modulation of the bacterial ClpB–DnaK bichaperone system

The ClpB–DnaK bichaperone system reactivates aggregated cellular proteins and is essential for survival of bacteria, fungi, protozoa, and plants under stress. AAA+ ATPase ClpB is a promising target for the development of antimicrobials because a loss of its activity is detrimental for survival of many pathogens and no apparent ClpB orthologs are found in metazoans. We investigated ClpB activity in the presence of several compounds that were previously described as inhibitor leads for the human AAA+ ATPase p97, an antitumor target. We discovered that N2,N4-dibenzylquinazoline-2,4-diamine (DBeQ), the least potent among the tested p97 inhibitors, binds to ClpB with a Kd∼60 μM and inhibits the casein-activated, but not the basal, ATPase activity of ClpB with an IC50∼5 μM. The remaining p97 ligands, which displayed a higher affinity toward p97, did not affect the ClpB ATPase. DBeQ also interacted with DnaK with a Kd∼100 μM and did not affect the DnaK ATPase but inhibited the DnaK chaperone activity in vitro. DBeQ inhibited the reactivation of aggregated proteins by the ClpB–DnaK bichaperone system in vitro with an IC50∼5 μM and suppressed the growth of cultured Escherichia coli. The DBeQ-induced loss of E. coli proliferation was exacerbated by heat shock but was nearly eliminated in a ClpB-deficient E. coli strain, which demonstrates a significant selectivity of DBeQ toward ClpB in cells. Our results provide chemical validation of ClpB as a target for developing novel antimicrobials. We identified DBeQ as a promising lead compound for structural optimization aimed at selective targeting of ClpB and/or DnaK

Cryo-EM structures of CusA reveal a mechanism of metal-ion export

Gram-negative bacteria utilize the resistance-nodulation-cell division (RND) superfamily of efflux pumps to expel a variety of toxic compounds from the cell. The CusA membrane protein, which recognizes and extrudes biocidal Cu(I) and Ag(I) ions, belongs to the heavy-metal efflux (HME) subfamily of RND efflux pumps. We here report four structures of the trimeric CusA heavy-metal efflux pump in the presence of Cu(I) using single-particle cryo-electron microscopy (cryo-EM). We discover that different CusA protomers within the trimer are able to bind Cu(I) ions simultaneously. Our structural data combined with molecular dynamics (MD) simulations allow us to propose a mechanism for ion transport where each CusA protomer functions independently within the trimer. The bacterial RND superfamily of efflux pumps mediate resistance to a variety of biocides, including Cu(I) and Ag(I) ions. Here we report four cryo-EM structures of the trimeric CusA pump in the presence of Cu(I). Combined with MD simulations, our data indicate that each CusA protomer within the trimer recognizes and extrudes Cu(I) independently. [Abstract copyright: Copyright © 2021 Moseng et al.

Structural and Functional Analysis of Human HtrA3 Protease and Its Subdomains.

Human HtrA3 protease, which induces mitochondria-mediated apoptosis, can be a tumor suppressor and a potential therapeutic target in the treatment of cancer. However, there is little information about its structure and biochemical properties. HtrA3 is composed of an N-terminal domain not required for proteolytic activity, a central serine protease domain and a C-terminal PDZ domain. HtrA3S, its short natural isoform, lacks the PDZ domain which is substituted by a stretch of 7 C-terminal amino acid residues, unique for this isoform. This paper presents the crystal structure of the HtrA3 protease domain together with the PDZ domain (ΔN-HtrA3), showing that the protein forms a trimer whose protease domains are similar to those of human HtrA1 and HtrA2. The ΔN-HtrA3 PDZ domains are placed in a position intermediate between that in the flat saucer-like HtrA1 SAXS structure and the compact pyramidal HtrA2 X-ray structure. The PDZ domain interacts closely with the LB loop of the protease domain in a way not found in other human HtrAs. ΔN-HtrA3 with the PDZ removed (ΔN-HtrA3-ΔPDZ) and an N-terminally truncated HtrA3S (ΔN-HtrA3S) were fully active at a wide range of temperatures and their substrate affinity was not impaired. This indicates that the PDZ domain is dispensable for HtrA3 activity. As determined by size exclusion chromatography, ΔN-HtrA3 formed stable trimers while both ΔN-HtrA3-ΔPDZ and ΔN-HtrA3S were monomeric. This suggests that the presence of the PDZ domain, unlike in HtrA1 and HtrA2, influences HtrA3 trimer formation. The unique C-terminal sequence of ΔN-HtrA3S appeared to have little effect on activity and oligomerization. Additionally, we examined the cleavage specificity of ΔN-HtrA3. Results reported in this paper provide new insights into the structure and function of ΔN-HtrA3, which seems to have a unique combination of features among human HtrA proteases

Temperature-induced changes of HtrA2(Omi) protease activity and structure

HtrA2(Omi), belonging to the high-temperature requirement A (HtrA) family of stress proteins, is involved in the maintenance of mitochondrial homeostasis and in the stimulation of apoptosis, as well as in cancer and neurodegenerative disorders. The protein comprises a serine protease domain and a postsynaptic density of 95 kDa, disk large, and zonula occludens 1 (PDZ) regulatory domain and functions both as a protease and a chaperone. Based on the crystal structure of the HtrA2 inactive trimer, it has been proposed that PDZ domains restrict substrate access to the protease domain and that during protease activation there is a significant conformational change at the PDZ–protease interface, which removes the inhibitory effect of PDZ from the active site. The crystal structure of the HtrA2 active form is not available yet. HtrA2 activity markedly increases with temperature. To understand the molecular basis of this increase in activity, we monitored the temperature-induced structural changes using a set of single-Trp HtrA2 mutants with Trps located at the PDZ–protease interface. The accessibility of each Trp to aqueous medium was assessed by fluorescence quenching, and these results, in combination with mean fluorescence lifetimes and wavelength emission maxima, indicate that upon an increase in temperature the HtrA2 structure relaxes, the PDZ–protease interface becomes more exposed to the solvent, and significant conformational changes involving both domains occur at and above 30 °C. This conclusion correlates well with temperature-dependent changes of HtrA2 proteolytic activity and the effect of amino acid substitutions (V226K and R432L) located at the domain interface, on HtrA2 activity. Our results experimentally support the model of HtrA2 activation and provide an insight into the mechanism of temperature-induced changes in HtrA2 structure. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s12192-012-0355-1) contains supplementary material, which is available to authorized users

Size exclusion chromatography of HtrA3 protein variants.

<p>Calculated molecular mass is an average of at least three independent experiments. ΔN-HtrA3 to β-casein molar ratio was calculated for ΔN-HtrA3 trimers. All HtrA3 variants had the S305A substitution and were proteolytically inactive.</p><p>Size exclusion chromatography of HtrA3 protein variants.</p

Structure refinement statistics.

<p>Structure refinement statistics.</p

Secondary structure of the ΔN-HtrA3 protein chain A as defined by the PDBSum server (http://www.ebi.ac.uk/pdbsum/).

<p>α-helices are labeled H1, H2, …, H8 and β-strands by their sheets as A, B, C, and D. Structural motifs β-turns, γ-turns, and β-hairpins are marked as β, γ, and ⊃, respectively. The catalytic residues H191, D227 and S305A are marked in green, phenylalanine residues involved in trimerization are orange, and residues involved in LB loop – PDZ interactions are blue. C-terminal amino acids (residues 351–357) of the ΔN-HtrA3S isoform are shown below the ΔN-HtrA3 sequence (underlined). The amino acid sequence 1–350 of HtrA3S is identical to that of HtrA3L.</p

Comparison of the HtrA3 structure with the structures of HtrA1 and HtrA2.

<p>(A) Superposition of ΔN-HtrA3 (green) and HtrA2 (gray) (PDB entry 1LCY) monomers. The structures were aligned via the Cα atoms of the PDs. The HtrA2 structure has a closed conformation whereby the PDZ domain has rotated ~60⁰ around the interdomain linker (direction indicated by the arrow). The subunits of the ΔN-HtrA3 trimer are shown in green, magenta and blue. (B) Superposition of the ΔN-HtrA3_S305A (green) and HtrA1_S328A (PDB entry 3TJO, gray) catalytic sites. The residues of the catalytic triad are shown as sticks. The residue numbers correspond to the HtrA3 protein. The ΔN-HtrA3 catalytic triad is not properly positioned, with S305A too distant from H191 (9.5 Å) for proton transfer. The residue L322 occludes the S1 specificity pocket.</p