Identification of two porins in Pelobacter venetianus fermenting high-molecular-mass polyethylene glycols

Porins were purified from cells of the anaerobic gram-negative bacterium Pelobacter venetianus grown with 20-kDa polyethylene glycol. After treatment of the cell envelope fraction with sodium dodecyl sulfate-containing solutions, the murein contained only two major peptidoglycan-associated proteins of 14 and 23 kDa. Both proteins were released from the peptidoglycan by the detergent Triton X-100. Genapol X-80 released only the 23-kDa protein. This protein was purified by chromatography on a hydroxyapatite column. It did not form sodium dodecyl sulfate-resistant oligomers. Reconstituted in lipid bilayer membranes, the 23-kDa protein formed cation-selective channels with a single-channel conductance of 230 pS in 1 M KCl. The channel is not a general-diffusion pore, since its conductance depends only moderately on the salt concentration. The channel conducted ammonium much better than potassium or rubidium ions, suggesting that it is probably involved in ammonium uptake. The outer membrane of P. venetianus contains a further, non-murein-associated pore with an unknown molecular mass. It is also cationically selective and has a single-channel conductance of 1.6 nS in 1 M KCl, which suggests that its effective diameter is similar to that of porins from enteric bacteria.