Exchangeability of N termini in the ligand-gated porins of Escherichia coli

The ferric siderophore transporters of the Gram-negative bacterial outer me mbrane manifest a unique architecture: Their N termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin p-barrel formed by their C termini, We exchanged and deleted the N termini of two such siderophore receptors, FepA and FhuA, which recognize and tran sport ferric enterobactin and ferrichrome, respectively. The resultant chim eric proteins and empty beta -barrels avidly bound appropriate ligands, inc luding iron complexes, protein toxins, and viruses. Thus, the ability to re cognize and discriminate these molecules fully originates in the transmembr ane beta -barrel domain. Both the hybrid and the deletion proteins also tra nsported the ferric siderophore that they bound. The FepA constructs showed less transport activity than wild type receptor protein, but the FhuA cons tructs functioned with turnover numbers that were equivalent to wild type. The mutant proteins displayed the full range of transport functionalities, despite their aberrant or missing N termini, confirming (Braun, M., Killman n, H;, and Braun, V, (1999) Mel. Microbiol. 33, 1037-1049) that the globula r domain within the pore is dispensable to the siderophore internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest a transport process in which siderophore recep tors undergo multiple conformational states that ultimately expel the N ter minus from the channel concomitant with solute internalization.