The
dopamine transporter (DAT) operates via facilitated diffusion, harnessing an inward Na(+) gradient to drive
dopamine from the extracellular synaptic cleft to the neuron interior. The DAT is relevant to
central nervous system disorders such as
Parkinson disease and
attention-deficit hyperactivity disorder and is the primary site of action for the abused psychostimulants
cocaine and
amphetamines. Crystallization of a DAT homolog, the bacterial
leucine transporter LeuT, provided the first reliable 3-D DAT template. Here, the LeuT crystal structure and the DAT molecular model have been combined with their respective substrates,
leucine and
dopamine, in
lipid bilayer molecular dynamics simulations toward tracking substrate movement along the
protein's substrate/ion permeation pathway. Specifically, movement of residue pairs that comprise the "external gate" was followed as a function of substrate presence. The transmembrane (
TM) 1 arginine-TM 10
aspartate strut formed less readily in DAT compared with LeuT, with or without substrate present. For LeuT but not DAT, the addition of substrate enhanced the chances of forming the
TM 1-10 bridge. Also, movement of the fourth extracellular loop EL-4 in the presence of substrate was more pronounced for DAT, the EL-4 unwinding to a degree. The overall similarity between the LeuT and DAT molecular dynamics simulations indicated that LeuT was a legitimate model to guide DAT structure-function predictions. There were, nevertheless, differences significant enough to allow for DAT-unique insights, which may include how
cocaine,
methylphenidate (
Ritalin,
NIDA Drug Supply, Rockville, MD), and other DAT blockers are not recognized as substrates even though they can access the primary substrate binding pocket.
Proteins 2010. (c) 2009 Wiley-Liss, Inc.