Amino Acid-binding Protein Released from Escherichia coli by Osmotic Shock

The ability of Escherichia coli K-12 to take up leucine, isoleucine, or valine against apparent concentration gradients is considerably reduced when the cells are subjected to osmotic shock in the cold. When the lyophilized supernatant fluid derived from that treatment was dialyzed against labeled amino acids, binding of leucine, isoleucine, or valine was observed. With leucine or isoleucine binding as an assay, a protein was isolated and highly purified. The dissociation constants for the leucine and isoleucine complexes were found to be indistinguishable from their respective Km values for cellular uptake. The results suggest that the isolated protein may well be part of an amino acid transport system of E. coli.