Structural basis and synergism of ATP and Na⁺ activation in bacterial K⁺ uptake system KtrAB

The K⁺ uptake system KtrAB is essential for bacterial survival in low K⁺ environments. The activity of KtrAB is regulated by nucleotides and Na⁺. Previous studies proposed a putative gating mechanism of KtrB regulated by KtrA upon binding to ATP or ADP. However, how Na⁺ activates KtrAB and the Na⁺ binding site remain unknown. Here we present the cryo-EM structures of ATP- and ADP-bound KtrAB from Bacillus subtilis (BsKtrAB) both solved at 2.8 Å. A cryo-EM density at the intra-dimer interface of ATP-KtrA was identified as Na⁺, as supported by X-ray crystallography and ICP-MS. Thermostability assays and functional studies demonstrated that Na⁺ binding stabilizes the ATP-bound BsKtrAB complex and enhances its K⁺ flux activity. Comparing ATP- and ADP-BsKtrAB structures suggests that BsKtrB Arg417 and Phe91 serve as a channel gate. The synergism of ATP and Na⁺ in activating BsKtrAB is likely applicable to Na⁺-activated K⁺ channels in central nervous system.