title: OccK channels from Pseudomonas aeruginosa exhibit diverse single-channel electrical signatures but conserved anion selectivity.
abstract: 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 previously expected, 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-open substate (OccK3), two-open substate (OccK4-OccK6), and three-open substate (OccK1, OccK2, and OccK7) 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.
authors.affiliation: Department of Physics, Syracuse University, Syracuse, New York 13244-1130, United States.
authors.forename: Jiaming
authors.initials: J
authors.lastname: Liu
authors.forename: Elif
authors.initials: E
authors.lastname: Eren
authors.forename: Jagamya
authors.initials: J
authors.lastname: Vijayaraghavan
authors.forename: Belete R
authors.initials: BR
authors.lastname: Cheneke
authors.forename: Mridhu
authors.initials: M
authors.lastname: Indic
authors.forename: Bert
authors.initials: B
authors.lastname: van den Berg
authors.forename: Liviu
authors.initials: L
authors.lastname: Movileanu
mesh_terms: D000838:Anions / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D015640:Ion Channel Gating; D007473:Ion Channels / Q000737:chemistry* / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D011550:Pseudomonas aeruginosa / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D000838:Anions; D001426:Bacterial Proteins; D007473:Ion Channels; D008565:Membrane Proteins