The KupA and KupB proteins of Lactococcus lactis IL1403 are novel c-di-AMP receptor proteins responsible for potassium uptake.

Cyclic di-adenosine monophosphate (c-di-AMP) is a second messenger involved in diverse metabolic processes including osmolyte uptake, cell wall homeostasis, as well as antibiotic and heat resistance. In Lactococcus lactis , a lactic acid bacterium which is used in the dairy industry and as a cell factory in biotechnological processes, the only reported interaction partners of c-di-AMP are the pyruvate carboxylase and BusR, the transcription regulator of the busAB operon for glycine betaine uptake. However, recent studies uncovered a major role of c-di-AMP in the control of potassium homeostasis, and potassium as the signal that triggers c-di-AMP synthesis. In this study, we have identified KupA and KupB, which belong to the Kup/HAK/KT family, as novel c-di-AMP binding proteins. Both proteins are high-affinity potassium transporters, and their transport activity is inhibited by binding of c-di-AMP. Thus, in addition to the well-studied Ktr/Trk potassium channels, KupA and KupB represent a second class of potassium transporters that are subject to inhibition by c-di-AMP. Importance Potassium is an essential ion in every living cell. Even though potassium is the most abundant cation in cells, accumulation can be toxic. Therefore, the level of potassium have to be tightly controlled. In many Gram-positive bacteria, the second messenger cyclic di-AMP plays a key role in the control of potassium homeostasis by binding to potassium transporters and regulatory proteins and RNA molecules. In the lactic acid bacterium Lactococcus lactis , none of these conserved c-di-AMP-responsive molecules is present. this study, we demonstrate KupA and KupB proteins of L. lactis IL1403 are high affinity potassium transporters, and that their transport activity is inhibited by the second messenger c-di-AMP.