Molecular cloning and functional characterization of the Paracoccus denitrificans porin

Bacterial porins facilitate the passive uptake of small solutes across the outer membrane of the cell. The channel properties and the primary structure of the porin from Paracoccus denitrificans were investigated. As judged from single-channel conductance experiments, this porin forms trimeric pores that show no ion selectivity in potassium chloride solution, which indicates that the charges within or near the channel are balanced. Based on peptide fragment sequence, the gene porG, which codes for this general pore protein, was cloned and analyzed. Its primary translation product contains a 20-residue signal sequence, followed by the 295 amino acids of the mature protein with a molecular mass of 31.9 kDa. Sequence alignments with porins from Rhodopseudomonas blastica and Rhodobacter capsulatus and secondary structure predictions suggest a typical rigid barrel structure consisting of 16 antiparallel beta-strands.