Crystal structure of Omp32, the anion-selective porin from Comamonas acidovorans, in complex with a periplasmic peptide at 2.1 angstrom resolution

Background: Porins provide diffusion channels for salts and small organic m olecules in the outer membrane of bacteria. In OmpF from Escherichia coli a nd related porins, an electrostatic field across the channel and a potentia l, originating from a surplus of negative charges, create moderate cation s electivity. Here, we investigate the strongly anion-selective porin Omp32 f rom Comamonas acidovorans, which is closely homologous to the porins of pat hogenic Bordetella and Neisseria species. Results: The crystal structure of Omp32 was determined to a resolution of 2 .1 Angstrom using single isomorphous replacement with anomalous scattering (SIRAS). The porin consists of a 16-stranded beta barrel with eight externa l loops and seven periplasmic turns. Loops 3 and 8, together with a protrus ion located within beta-strand 2, narrow the cross-section of the pore cons iderably. Arginine residues create a charge filter in the constriction zone and a positive surface potential at the external and periplasmic faces. On e sulfate ion was bound to Arg38 in the channel constriction zone. A peptid e of 5.8 kDa appeared bound to Omp32 in a 1:1 stoichiometry on the periplas mic side close to the symmetry axis of the trimer. Eight amino acids of thi s peptide could be identified, revealing specific interactions with beta-st rand 1 of the porin. Conclusions: The Omp32 structure explains the strong anion selectivity of t his porin, Selectivity is conferred by a positive potential, which is not a ttenuated by negative charges inside the channel, and by an extremely narro w constriction zone. Moreover, Omp32 represents the anchor molecule for a p eptide which is homologous to proteins that link the outer membrane to the cell wall peptidoglycan.