The active transport systems of proline and potassium in Escherichia coli
The transport systems for proline and potassium represent two of the active transport systems in Escherichia coli. They have further similarities that their transport may be utilized as a response to osmotic perturbations in the environment. The exact mechanism of transport had not been totally elucidated. The transport of proline had been assumed to operate as a proton symport and as such had been used as a model system when other transport systems were being investigated. This study has demonstrated that the major route of proline uptake through the proline permease 1 (PP1), operates as a Na+ - proline cotransport which may accept Li+ in the place of Na+. Unusually, Na+ stimulates the Vmax of transport with little or no effect on the Km. In addition to this transport system, there are two other proline uptake systems which function primarily for the transport of betaine. The transport of K + is also facilitated by more than one system. The Kdp system is a K+ transporting ATPase; the TrkF system is a low rate transport system which may represent leak through another pathway. The TrkA transport system is the major system but the mechanism is not known. Transport through the system is energised by ATP and a pmf, while exchange through the system requires only ATP. The role of ATP was investigated in this study by the use of metabolic inhibitors and vesicles. It was determined that the availability of ATP affected the steady state level of potassium in the cells rather than the rate of potassium upake. It was speculated that ATP would act as a regulator of the system which would be driven by the pmf. ATP may regulate TrkA through phosphorylation or by allosteric modification of the carrier.