Selectivity Profiling of the Human Monoamine Transporters: Investigation of the Serotonin Transporter Mechanism

Abstract

The human monoamine transporters play an important role in the central and peripheral nervous system by regulating signals among neurons. Numerous compound classes have been identified to interact with these proteins, either used in a therapeutic setting or abused as illicit drugs. However, based on the physicochemical properties of these ligands alone, the mechanistic distinction between transporter substrates and inhibitors is not always clear. For example, it remains to be established whether the closure of the extracellular pathway facilitated by substrate can also occur when such inhibitors are bound. It therefore seems necessary to pinpoint the molecular features of the substrate required for the transport-related conformational change, and in particular, the outward-open to outward-occluded transition. A recently-reported structure of the outward-open state of human serotonin transporter (hSERT), provides a starting point for molecular dynamics (MD) simulations of this transition. Initial MD simulations revealed stability and convergence of this conformation over time. However, the endpoint of this pathway closure event is not yet known. Therefore, we present a carefully-refined homology model of hSERT based on a combination of two templates (hSERT, outward-open and the bacterial leucine transporter LeuT, outward-occluded) in an outward-occluded state. This homology model will be used as the endpoint of enhanced sampling simulations of the transition between the outward-open and the outward-occluded state of hSERT. Combined with knowledge about substrates and inhibitors sharing the same scaffold interacting with this transporter, our approaches will lead to a better understanding of the transporter mechanism.