Topological investigations of the FomA porin from Fusobacterium nucleatum and identification of the constriction loop L6

Porin FomA in the outer membrane of Fusobacterium nucleatum is a trimeric p rotein, which exhibits permeability properties similar to that of the well- known enterobacterial diffusion porins. The proposed topology model of the FomA monomer depicts the beta -barrel motif typical of diffusion porins, co nsisting of 16 antiparallel beta -strands. To investigate the accuracy of t he FomA model and assess the topological relationship with other porins, in dividual deletions of variable size in seven of the eight surface-exposed r egions of the porin were genetically engineered. Deletions in the predicted loops L1 to L7 were tolerated by the FomA porins, as judged by a normal as sembly in the outer membrane of Escherichia coli and a sustained pore-formi ng ability. Deletions in the largest proposed external region, loop L6, mad e the FomA porins considerably more permeable to antibiotics, indicating la rger pore channels. The distinctly increased uptake rates and size exclusio n limits displayed by the L6 deletion mutant porins, suggest that loop L6 f olds back into the beta -barrel thereby constricting the native FomA channe l. Thus, the position of the channel constriction loop appears to be shifte d towards the C terminus in the FomA porin, as compared to the crystal stru ctures of five nonspecific diffusion porins.