Structure and function of the betaine uptake system BetP of Corynebacterium glutamicum: strategies to sense osmotic and chill stress

The soil bacterium Corynebacterium glutamicum has to cope with frequent fluctuations of the external osmolarity and temperature. The consequences of hyperosmotic and chill stress seem to differ, either causing dehydration of the cytoplasm or leading to impairment of cellular functions due to low temperature. Nevertheless, a particular type of regulatory response, namely the accumulation of so-called compatible solutes, is induced under both conditions. Compatible solutes are known to stabilize the native conformation of enzymes, which may be affected by osmotic and chill stress. BetP is a high-affinity uptake carrier for the compatible solute glycine betaine in C. glutamicum. BetP includes, besides its catalytic function, the ability to sense hyperosmotic conditions and chill stress. As a consequence, the carrier is activated in dependence of the extent of these types of stress. The signal input related to these changes of the environmental conditions is based on at least two different mechanisms. In case of hyperosmotic stress, BetP responds to the internal potassium concentration as a measure for hypertonicity, whereas chill stress is detected by an independent signal, most probably changes of the physical state of the membrane.