Location of acidic residues on the extracellular loops of a monomer of the E. coli porin OmpF, viewed from the extracellular side (top) or in the plane of the membrane (bottom). Acidic residues are shown as CPK models in magenta for loops L2, L4, L5 and L6, and in turquoise for loops L1, L7 and L8. The deletion of loop L1, or L7 or L8 eliminates the sensitivity of the channel to acidic pH, possibly due to disruption of the cluster of protonatable charges formed by these loops. For further details, see Basle et al., pages 665-672.
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