Mediates chemotaxis towards five attractant amino acids (leucine, isoleucine, valine, methionine and cysteine). May function as a receptor that binds the amino acids and transduces a signal to BasT. Has probably no additional role in transport. Probably anchored to the membrane by lipids. Mutants completely lose the chemotactic response towards leucine, isoleucine, valine, methionine and cysteine, but they still respond to arginine.