Transporters as information processors in bacterial signalling pathways

Transporters are essential players in bacterial growth and survival, since they are key for uptake of nutrients on the one hand, and for defence against endogenous and environmental stresses on the other hand. Remarkably, in addition to their primary role in substrate translocation, it has become clear that some transporters have acquired a secondary function as sensors and information processors in signalling pathways. In this review, we describe recent advances in our understanding of the role of transporters in such signalling cascades, and discuss some of the emergent dynamic behaviour found in hallmark examples. A particular focus is placed on new insights into mechanistic details of information transfer between transporters and regulatory proteins. Quantitative considerations reveal that these signalling complexes can implement a remarkable diversity of regulatory logic functions, where the transporter can act as activity switch, as positive or negative reporter of transport flux, or as a signalling hub for the integration of multiple inputs. Such a dual use of transport proteins not only enables efficient substrate translocation but is also an elegant strategy to integrate important information about the cell's external conditions with its current physiological state.