Characterization of a large family of outer membrane channels from gram-negative bacteria suggest how they can thrive in nutrient-poor environments and how channel inactivation can contribute to antibiotic resistance.

Pseudomonas aeruginosa is a Gram-negative bacterium that utilizes substrate-specific outer membrane (OM) proteins for the uptake of small, water-soluble nutrients employed in the growth and function of the cell. In this paper, we present for the first time a comprehensive single-channel examination of seven members of the OM carboxylate channel K (OccK) subfamily. Recent biochemical, functional and structural characterization of the OccK proteins revealed their common features, such as a closely related, monomeric, 18-stranded β-barrel conformation with a kidney-shaped transmembrane pore and the presence of a basic ladder within the channel lumen. Here, we report that the OccK proteins exhibited fairly distinct unitary conductance values, in a much broader range than earlier expectations, which includes low (~40–100 pS) and medium (~100–380 pS) conductance. These proteins showed diverse single-channel dynamics of current gating transitions, revealing one (OccK3)-, two (OccK4, OccK5 and OccK6)- and three (OccK1, OccK2 and OccK7)-open sub-state kinetics with functionally distinct conformations. Interestingly, we discovered that anion selectivity is a conserved trait among the members of the OccK subfamily, confirming the presence of a net pool of positively charged residues within their central constriction. Moreover, these results are in accord with an increased specificity and selectivity of these protein channels for negatively charged, carboxylate-containing substrates. Our findings might ignite future functional examinations and full-atomistic computational studies for unraveling a mechanistic understanding of the passage of small molecules across the lumen of substrate-specific, β-barrel OM proteins.

The bacterial soluble lytic transglycosylase (LT) breaks down the peptidoglycan (PG) layer during processes such as cell division. We present here crystal structures of the soluble LT Cj0843 from Campylobacter jejuni with and without bulgecin A inhibitor in the active site. Cj0843 has a doughnut shape similar but not identical to that of E. coli SLT70. The C-terminal catalytic domain is preceded by an L-domain, a large helical U-domain, a flexible linker, and a small N-terminal NU-domain. The flexible linker allows the NU-domain to reach over and complete the circular shape, using residues conserved in the Epsilonproteobacteria LT family. The inner surface of the Cj0843 doughnut is mostly positively charged including a pocket that has 8 Arg/Lys residues. Molecular dynamics simulations with PG strands revealed a potential functional role for this pocket in anchoring the negatively charged terminal tetrapeptide of the PG during several steps in the reaction including homing and aligning the PG strand for exolytic cleavage, and subsequent ratcheting of the PG strand to enhance processivity in degrading PG strands.

To achieve the uptake of small, water-soluble nutrients, Pseudomonas aeruginosa, a pathogenic Gram-negative bacterium, employs substrate-specific channels located within its outer membrane (OM). In this paper, we present a detailed description of the single-channel characteristics of six members of the OM carboxylate channel D (OccD) subfamily. Recent structural studies showed that the OccD proteins share common features, such as a closely related, monomeric, 18-stranded β-barrel conformation and large extracellular loops, which are folded back into the channel lumen. Here, we report that the OccD proteins displayed single-channel activity with a unitary conductance covering an unusually broad range, between 20 and 670 pS, as well as a diverse gating dynamics. Interestingly, we found that cation selectivity is a conserved trait among all members of the OccD subfamily, bringing a new distinction between the members of the OccD subfamily and the anion-selective OccK channels. Conserved cation selectivity of the OccD channels is in accord with an increased specificity and selectivity of these proteins for positively charged, carboxylate-containing substrates.

Soluble guanylyl cyclase (sGC) plays a crucial role in cyclic nucleotide signaling that regulates numerous important physiological processes. To identify new sGC inhibitors that may prevent the formation of the active catalytic domain conformation, we carried out an in silico docking screen targeting a ‘backside pocket’ of the inactive sGC catalytic domain structure. Compounds 1 and 2 were discovered to inhibit sGC even at high/saturating nitric oxide concentrations. Both compounds also inhibit the BAY 58-2667-activated sGC as well as BAY 41-2272-stimulated sGC activity. Additional biochemical analyses showed that compound 2 also inhibits the isolated catalytic domain, thus demonstrating functional binding to this domain. Both compounds have micromolar affinity for sGC and are potential leads to develop more potent sGC inhibitors.