Calcium Ions Modulate Amino Acid Sensing of the Chemoreceptor Mlp24 of Vibrio cholerae

Mlp24 and Mlp37 are homologous chemoreceptors ofVibrio cholerae that bind various amino acids. Although most of the residues involved in ligand interaction are conserved, these chemoreceptors show different affinities for the same ligand and play different cellular roles. A series of ligand complex structures of the periplasmic region of Mlp24 (Mlp24p) and following ITC analysis revealed that Ca2+ binds to the loop of Mlp24p and modulates the ligand binding affinity of Mlp24p. Moreover, Ca2+ changes the chemotactic behaviors mediated by Mlp24. We propose that Ca2+ acts as a cosignal that modulates the affinity of Mlp24 for the primary signal, thereby changing the chemotactic behavior ofV. cholerae .