Molecular and conformational basis of a specific and high-affinity interaction between AlbA and albicidin phytotoxin

The albA gene of Klebsiella oxytoca encodes a protein of 221 amino acids that binds the albicidin phytotoxin with a high affinity (dissociation constant = 6.4 x 10(-8) M). For this study, circular dichroism (CD) spectrometry and an alanine scanning mutagenesis approach were used in combination to investigate the molecular and conformational mechanisms of this high-affinity protein-ligand interaction. CD analysis revealed that AlbA contains a high-affinity binding site, and binding of the albicidin ligand to AlbA in a low-ionic-strength environment induced significant conformational changes. The ligand-dependent conformational changes of AlbA were specific and rapid and reached a stable plateau within seconds after the addition of the antibiotic. However, such conformational changes were not detected when AlbA and albicidin were mixed in the high-ionic-strength buffer that is required for maximal binding activity. Based on the conceptual model of protein-ligand interaction, we propose that a threshold ion strength allows AlbA to complete its conformational rearrangement and resume its original stable structure for accommodation of the bound albicidin. Mutagenesis analysis showed that the replacement of Lys106, Trp110, Tyr113, Leu114, Tyr126, Pro134, and Trp162 with alanine did not change the overall conformational structure of AlbA but decreased the albicidin binding activity about 30 to 60%. We conclude that these residues, together with the previously identified essential residue His125, constitute a high-affinity binding pocket for the ligand albicidin. The results also suggest that hydrophobic and electrostatic potentials of these key amino acid residues may play important roles in the AlbA-albicidin interaction.