Pseudomonas Aerginosa Outer Membrane Carboxylate Channels Examined at the Single-Molecule Level Reveals Conserved Selectivity within each Subfamily

Pseudomonas aerginosa is a Gram-negative bacterium, which employs unique outer membrane proteins for the uptake of small, water-soluble molecules. Here, we explore the two subfamilies of the outer membrane carboxylate channels, OccK and OccD, using single-molecule electrophysiology to probe the specific signature of each of the members of these two families. The seven member OccK family displays a broad range of unitary conductance values which includes low (∼40−100 pS) and medium (∼100−380 pS) conductance. These values are broader than what was expected from structural studies alone. It was also found that the OccK subfamily displays conserved anion selectivity, coinciding with the location of a net pool of positive charges within the channels' constriction sites. Single-channel activities of 6 members of the OccD subfamily also displayed a broad range of unitary conductance values between 20 and 670 pS. Single-channel electrophysiology studies indicated that the OccD subfamily members are all cation selective. Together these findings lead to a better understanding of the diversity within the outer membrane carboxylate channels of Pseudomonas aerginosa.