Structural characterization of the fusobacterial non-specific porin FomA suggests a 14- stranded topology, unlike the classical porins

Native and recombinant FomA proteins were extracted by detergent from the cell envelopes of Fusobacterium nucleatum and Escherichia coli, and purified to near homogeneity by chromatography. Circular dichroism analysis revealed that the FomA protein consists predominantly of b-sheets, in line with the previously proposed 16-stranded b-barrel topology model. Results obtained by trypsin treatment of intact cells and cell envelopes of F. nucleatum, and from limited proteolysis of purified FomA protein, indicated that the N-terminal part of the FomA protein is not an integral part of the b-barrel, but forms a periplasmic domain. Based on these results a new topology model is proposed for the FomA protein, where the C-terminal part forms a 14-stranded b-barrel separate from the periplasmic N-terminal domain.