BetP of Corynebacterium glutamicum, a transporter with three different functions: betaine transport, osmosensing, and osmoregulation

In order to circumvent deleterious effects of hypo- and hyperosmotic conditions in its environment, Corynebacterium glutamicum has developed a number of mechanisms to counteract osmotic stress. The first response to an osmotic upshift is the activation of uptake mechanisms for the compatible solutes betaine, proline, or ectoine, namely BetP, EctP, ProP, LcoP and PutP. BetP, the most important uptake system responds to osmotic stress by regulation at the level of both protein activity and gene expression. BetP was shown to harbor three different properties, i.e. catalytic activity (betaine transport), sensing of appropriate stimuli (osmosensing) and signal transduction to the catalytic part of the carrier protein which adapts its activity to the extent of osmotic stress (osmoregulation). BetP is comprised of 12 transmembrane segments and carries N- and C-terminal domains, which are involved in osmosensing and/or osmoregulation. Recent results on molecular properties of these domains indicate the significance of particular amino acids within the terminal 25 amino acids of the C-terminal domain of BetP for the process of osmosensing and osmoregulation.