Structural Studies of Potassium Transport Protein KtrA Regulator of Conductance of K+ (RCK) C domain in Complex with Cyclic Diadenosine Monophosphate (c-di-AMP)

Abstract Although it was only recently identified as a second messenger, c-di-AMP was found to have fundamental importance in numerous bacterial functions such as ion transport. The potassium transporter protein, KtrA, was identified as a c-di-AMP receptor. However, the cocrystallization of c-di-AMP with the protein has not been studied. Here, we determined the crystal structure of KtrA RCK_C domain in complex with c-di-AMP. The c-di-AMP nucleotide, which adopts a U-shaped conformation, is bound at the dimer interface of RCK_ C close to helices α3 and α4. c-di-AMP interacts with KtrA RCK_C mainly by forming hydrogen bonds and hydrophobic interactions. c-di-AMP binding induces the contraction of the dimer, bringing the two monomers of KtrA RCK_C into close proximity. The KtrA RCK_C was able to interact with only c-di-AMP, but not with c-di-GMP, 3'3-cGAMP, ATP and ADP. The structure of KtrA RCK_C domain and c-di-AMP complex would expand our understanding about the mechanism of inactivation in Ktr transporters governed by c-di-AMP.