Chapter 21 K+ circulation across the prokaryotic cell membrane: K+-uptake systems

Publisher Summary All living cells contain relatively high concentrations of K + in their cytoplasm. In prokaryotes, the main function of this cell K + lies in its role in turgor-pressure homeostasis (with related processes, such as osmoadaptation, osmoregulation and volume regulation, and in pH homeostasis. For this purpose, the cells transport K + across the cell membrane via a variety of both K + -uptake and K + -efflux systems. Currently, three different types of K + -accepting channels may exist in the cell membrane of prokaryotes. The activity of these transport systems and channels is coordinated in such a manner, that the basal rate of K + cycling across the cell membrane is low. Thereby, the cells avoid unnecessary loss of energy due to futile K + circulation. Under extreme conditions—for example, after osmotic shock or a sudden change in pH of the medium—the cells rapidly activate one or more of their K + -transport systems and depending on the conditions this can lead to either net K + uptake or net K + exit. This chapter describes the K + -transport genes and K + -transport systems that are identified in prokaryotes, the roles K + and K + transport play in the physiology of these organisms, and all that is known about the various K + -uptake systems.