Regulatory role of charged clusters in the N-terminal domain of BetP from Corynebacterium glutamicum

The trimeric transporter BetP counteracts hyper-osmotic stress by a fast increase in transport rate in order to accumulate the compatible solute betaine. The positively charged a-helical C-terminal domain acts as an osmosensor perceiving the increase in the internal potassium (K+) concentration. A second, still unidentified stimulus originates from stress-induced changes in the physical state of the membrane and depends on the amount of negatively charged lipids. BetP possesses a 60-amino acid (aa)-long negatively charged N-terminal domain, which is predicted to adopt a partly helical fold affecting osmoregulation by an unknown mechanism. It is assumed that the C-terminal domain, the N-terminal domain, and negatively charged lipids interact during stress sensing and regulation. Here, we have investigated the regulatory role of negatively charged clusters in the N-terminal domain. We identified one cluster, Glu24Glu25, to be crucial for osmoregulation. Cross-linking studies revealed an interaction between the C-and N-terminal domains of adjacent protomers modulating transport activation. A regulatory partner-switching mechanism emerges in which the C-terminal domain changes its interaction with the N-terminal domain of its own promoter and negatively charged lipids to an interaction with the N-terminal domain of an adjacent protomer and lipids bound to the central cavity of the BetP trimer.