The second messenger c-di-AMP inhibits the osmolyte uptake system OpuC in Staphylococcus aureus

Staphylococcus aureus is an important opportunistic human pathogen that is highly resistant to osmotic stresses. In order to survive an increase in osmolarity, bacteria immediately take up potassium and small organic compounds, also referred to as compatible solutes. The second messenger c-di-AMP binds to several receptor proteins, most of which are involved in ion and potassium uptake, that help bacteria cope with osmotic stress. In this study, we identified OpuCA, the ATPase component of an uptake system for the compatible solute carnitine, as a cdi-AMP target protein in S. aureus and found that a strain overproducing c-di-AMP showed reduced carnitine uptake. The CBS domains of OpuCA bound to c-di-AMP, and a crystal structure revealed a putative binding pocket for c-di-AMP in the cleft between the two CBS domains. Thus, c-di-AMP is involved in regulating both branches of osmoprotection (potassium uptake and compatible solute uptake), suggesting that c-di-AMP is a general osmotic stress regulator