c-di-AMP, a likely master regulator of bacterial K⁺ homeostasis machinery, activates a K⁺ exporter

Area of Science:

• Bacterial physiology and molecular biology
• Second messenger signaling
• Ion transport and homeostasis

Background:

• Bis-(3',5')-cyclic diadenosine monophosphate (c-di-AMP) is a crucial bacterial second messenger.
• c-di-AMP is implicated in regulating virulence, cell wall synthesis, and DNA integrity.
• Its role in coordinating potassium (K+) homeostasis machinery, including K+ transport, requires further quantitative evidence.

Purpose of the Study:

• To investigate the direct impact of c-di-AMP on the K+/H+ antiporter KhtTU in *B. subtilis*.
• To elucidate the functional, biochemical, and structural basis of c-di-AMP regulation of KhtTU.
• To provide a physiological context for c-di-AMP's role in bacterial K+ homeostasis.

Main Methods:

• Biochemical assays to confirm c-di-AMP binding to KhtT.
• Crystal structure determination of the c-di-AMP-KhtT complex.
• In vitro characterization of KhtTU and KhtU activity, assessing the effects of c-di-AMP and pH.
• Analysis of KhtTU function in *B. subtilis*.

Main Results:

• Confirmed direct binding of c-di-AMP to KhtT and determined the complex's crystal structure.
• Demonstrated that c-di-AMP activates the KhtTU transporter in vitro.
• Showed that pH modulates KhtTU's sensitivity to c-di-AMP, suggesting a regulatory mechanism.
• Provided in vivo evidence for KhtTU's role in *B. subtilis* physiology.

Conclusions:

• c-di-AMP directly activates the K+/H+ antiporter KhtTU.
• pH acts as a crucial modulator of c-di-AMP's regulatory effect on KhtTU.
• These findings support c-di-AMP as a master regulator of bacterial K+ homeostasis.
• The study provides a mechanistic understanding of c-di-AMP-mediated ion transport regulation.