Site-directed mutagenesis studies of the amino acid residue at position 412 of Escherichia coli TolC which is required for the activity

The Escherichia coli TolC acts as a channel-tunnel in the transport of molecules across the outer membrane. We previously showed that the region extending from the 50th to the 60th amino acid residues from the carboxy terminus is involved in the transport activity of TolC. To clarify which amino acids are important to the activity, we mutated the gene coding these residues and examined the activity of the mutant TolCs. The results showed that leucine at position 412, the 60th amino acid residue from the carboxy terminal end, is important. Further mutational research on the residue suggested that TolC required a nonpolar amino acid residue at position 412 to express its activity.