Dynamics of the Outward and Inward Facing States of the Serotonin Transporter

Abstract

The serotonin transporter (SERT), a member of the neurotransmitter:sodium symporter (NSS) family, is responsible for clearing neuronal synapses of serotonin, and a major drug target in psychopharmacology. Since no structures have been reported for SERT, structural information is deduced from the crystal structure of LeuT, a prokaryotic NSS homologue. SERT is believed to function by alternating between outward-facing (OF) and inward-facing (IF) states, hence a description of these and intermediate states is vital to the understanding of its transport cycle. To date, studies of SERT structure and dynamics have been restricted to models of the OF state, that in which LeuT is crystallized. We have generated models of substrate/ion-bound as well as free states of SERT, using the OF crystal structure, and our recently published IF models of LeuT. We performed comparative modeling for model generation, docking to position the substrate and extensive optimization of the protein sidechains to generate these models. These SERT models were then simulated in the presence of membrane, water and ions, to characterize the dynamics of the transporter in different states. The simulations reveal differential water permeation behavior among the OF and IF states, and are discussed in view of the known channel-like behavior in SERT. Ion binding patterns hint at a putative binding site for K+, which is known to assist in SERT function. We describe major differences in local and global conformational behavior between the OF and IF states, which provide clues to the transition between these states, and the transport cycle. To our knowledge, these studies provide the first description of dynamics of SERT in the IF state, and a comparative view of the dynamics of the OF and IF as well as substrate/ion bound and free states of SERT.