Substrate Binding and Conformational Dynamics of the Monoamine Transporters

The monoamine transporters move substrates across the plasma membrane by an alternating-access mechanism, in which a central substrate-binding site is alternately exposed to either the extracellular milieu or the cytoplasm at any given time. This process is driven by co-transport of sodium ions along the inwardly directed sodium gradient. Alternating access to the central substrate-binding site is facilitated by a stepwise series of changes to the transporter conformation, referred to as the transport cycle. The focus of this chapter is to discuss the conformational dynamics of the monoamine transporters that are orchestrated by the binding of substrate and ions, as part of the transport cycle. Firstly, we describe the substrate-binding event, and how it is fine-tuned to induce the conformational flexibility needed to initiate transport. Secondly, we discuss how sodium fuels the substrate transport as well as how it is aided by potassium and chloride. We also provide a mechanistic description of the cooperativity of the two sodium-binding sites and how they couple allosterically to the intracellular gating mechanism. Thirdly, we go over the amino acid residues of the intra- and extracellular gates and how they affect the transporter conformation.