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Physiological contribution of the Pseudomonas aeruginosa OprD family of porins Tamber, Sandeep


To circumvent the permeability barrier of its outer membrane, Pseudomonas aeruginosa has evolved a series of specific porins to facilitate nutrient uptake. These channels have substrate-specific binding sites that selectively permit the passage of related classes of molecules. In this study, the identification of a novel 19 member family of porins is reported. The members of this family share a considerable degree of protein sequence conservation (46% to 57%) and fall into one of two phylogenetically distinct clusters; one bearing high similarity to the basic amino acid specific porin OprD and the other being most similar to the phenylacetic acid uptake porin, PhaK, of P. putida . The physiological contribution of this family was investigated by predicting substrates for of each homologue based on the genomic context of their genes. These predictions were then tested by performing growth curves with porin deficient mutants in minimal media containing the proposed substrate as the sole carbon source. The following substrates were identified for 7 of the 18 novel homologues: OpdB - proline, OpdC - histidine, OpdP - glycine-glutamate, OpdT - tyrosine, OpdH - cis-aconitate, OpdK--vanillate and OpdO--pyroglutamate. Functional overlap was observed between the basic amino acid specific porin, OprD, and the glycine-glutamate specific porin, OpdP, with respect to arginine transport. Thus, members of this family had diverged to take up unique substrates but had also retained some redundancy which may allow them to compensate for one another in the event of a gene loss or mutation. To gain insights regarding the evolution of this family, one homologue, OpdH, was characterized in detail. This porin was specifically induced by citrate, isocitrate and cis-aconitate through the action of the PA0756-757 two-component regulatory system and was involved in the uptake of the latter two compounds. The channel properties of OpdH differed significantly from those of OprD. OpdH demonstrated an average single channel conductance of 0.7 nS in 1 M KCl. The channel was cation selective and did not harbour a tricarboxylate specific binding site. The structural and evolutionary implications of the differences between the two porins are discussed.

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