PT  - JOURNAL ARTICLE
AU  - Michael V. LeVine
AU  - Daniel S. Terry
AU  - George Khelashvili
AU  - Zarek S. Siegel
AU  - Matthias Quick
AU  - Jonathan A. Javitch
AU  - Scott C. Blanchard
AU  - Harel Weinstein
TI  - The allosteric mechanism of substrate-specific transport in SLC6 is mediated by a volumetric sensor
AID  - 10.1101/555565
DP  - 2019 Jan 01
TA  - bioRxiv
PG  - 555565
4099  - http://biorxiv.org/content/early/2019/02/20/555565.short
4100  - http://biorxiv.org/content/early/2019/02/20/555565.full
AB  - Neurotransmitter:sodium symporters (NSS) in the SLC6 family terminate neurotransmission by coupling the thermodynamically favorable transport of ions to the thermodynamically unfavorable transport of neurotransmitter back into presynaptic neurons. While a combination of structural, functional, and computational studies on LeuT, a bacterial NSS homolog, has provided critical insight into the mechanism of sodium-coupled transport, the mechanism underlying substrate-specific transport rates is still not understood. We present a combination of MD simulations, single-molecule FRET imaging, and measurements of Na+ binding and substrate transport that reveal an allosteric mechanism in which residues F259 and I359 in the substrate binding pocket couple substrate binding to Na+ release from the Na2 site through allosteric modulation of the stability of a partially-open, inward-facing state. We propose a new model for transport selectivity in which the two residues act as a volumetric sensor that inhibits the transport of bulky amino acids.