STRUCTURAL AND FUNCTIONAL ALTERATIONS OF A COLICIN-RESISTANT MUTANT OF OMPF PORIN FROM ESCHERICHIA-COLI

A strain of Escherichia coli, selected on the basis of its resistance to colicin N, reveals distinct structural and functional alterations i n unspecific OmpF porin. A single mutation [Gly-119 --> Gsp (G119D)] w as identified in the internal loop L3 that contributes critically to t he formation of the constriction inside the lumen of the pore. X-ray s tructure analysis to a resolution of 3.0 Angstrom reveals a locally al tered peptide backbone, with the side chain of residue Asp-119 protrud ing into the channel, causing the area of the constriction (7 x 11 Ang strom in the wild type) to be subdivided into two intercommunicating s ubcompartments of 3-4 Angstrom in diameter. The functional consequence s of this structural modification consist of a reduction of the channe l conductance by about one-third, of altered ion selectivity and volta ge gating, and of a decrease of permeation rates of various sugars by factors of 2-12. The structural modification of the mutant protein aff ects neither the beta-barrel structure nor those regions of the molecu le that are exposed at the cell surface. Considering the colicin resis tance of the mutant, it is inferred that in vivo, colicin N traverses the outer membrane through the porin channel or that the dynamics of t he exposed loops are affected in the mutant such that these may impede the binding of the toxin.`