The ion coupling and organic substrate specificities of osmoregulatory transporter ProP in Escherichia coli

Abstract

Transporter ProP of Escherichia coli, a member of the major facilitator superfamily, mediates osmoprotective proline or glycine betaine accumulation by bacteria exposed to high osmolality environments. Morpholinopropane sulfonic acid, a common constituent of microbiological media, accumulates in osmoadapting E. coli cells but it is not osmoprotective and it did not influence proP transcription or ProP activity. The apparent K m for proline uptake via ProP increased with decreasing pH in the range 7.5–4. ProP-dependent proline uptake by de-energized bacteria was associated with alkalinization of the external medium. Thus ProP mediates cotransport of H + and zwitterionic proline and a transporter functional group with a p K a of 5–6 is implicated in catalysis. Exogenous proline or glycine betaine elicits K + release from osmoadapting E. coli cells and ProP activity is stimulated by exogenous K +. However, uptake of proline or glycine betaine stimulated K + efflux from K +-loaded bacteria which expressed either ProP or alternative, osmoregulatory transporter ProU. This indicated that ProP was unlikely to mediate K + efflux. Zwitterions ectoine, pipecolate, proline betaine, N, N-dimethylglycine, carnitine and 1-carboxymethylpyridinium were identified as alternative ProP substrates. Choline, a cation and a structural analogue of glycine betaine, was a low affinity inhibitor but not a substrate of ProP.