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 primar y structure of the porin from Paracoccus denitrificans were investigat ed. As judged from single-channel conductance experiments, this porin forms trimeric pores that show no ion selectivity in potassium chlorid e solution, which indicates that the charges within or near the channe l are balanced. Based on peptide fragment sequence, the gene porG, whi ch codes for this general pare protein, was cloned and analyzed. Its p rimary translation product contains a 20-residue signal sequence, foll owed by the 295 amino acids of the mature protein with a molecular mas s of 31.9 kDa. Sequence alignments with porins from Rhodopseudomonas b lastica and Rhodobacter capsulatus and secondary structure predictions suggest a typical rigid barrel structure consisting of 16 antiparalle l beta-strands.